Add a function for type-checking arguments (#207)

- Define an expect(index, actual_value, types...) helper function which
   takes an argument index, value and list of permissable types and
   ensures the value is of one of those types.
   If not, it will produce an error message with the expected and actual
   type, as well as the argument number and (if available) the function
   name.
 - Expose expect in the global scope as _G["~expect"], hopefully making
   it clear it is internal.
 - Replace most manual type checks with this helper method.
 - Write tests to ensure this argument validation works as expected

Also fix a couple of bugs exposed by this refactor and the subsequent
tests:
 - Make rednet checks a little more strict - rednet.close(false) is no
   longer valid.
 - Error when attempting to redirect the terminal to itself 
   (term.redirect(term)).

src/main/resources/assets/computercraft/lua/bios.lua

@@ -1,5 +1,48 @@
+local native_select, native_type = select, type
+
+--- Expect an argument to have a specific type.
+--
+-- @tparam int index The 1-based argument index.
+-- @param value The argument's value.
+-- @tparam string ... The allowed types of the argument.
+-- @throws If the value is not one of the allowed types.
+local function expect(index, value, ...)
+    local t = native_type(value)
+    for i = 1, native_select("#", ...) do
+        if t == native_select(i, ...) then return true end
+    end
+
+    local types = table.pack(...)
+    for i = types.n, 1, -1 do
+        if types[i] == "nil" then table.remove(types, i) end
+    end
+
+    local type_names
+    if #types <= 1 then
+        type_names = tostring(...)
+    else
+        type_names = table.concat(types, ", ", 1, #types - 1) .. " or " .. types[#types]
+    end
+
+    -- If we can determine the function name with a high level of confidence, try to include it.
+    local name
+    if native_type(debug) == "table" and native_type(debug.getinfo) == "function" then
+        local ok, info = pcall(debug.getinfo, 3, "nS")
+        if ok and info.name and #info.name ~= "" and info.what ~= "C" then name = info.name end
+    end
+
+    if name then
+        error( ("bad argument #%d to '%s' (expected %s, got %s)"):format(index, name, type_names, t), 3 )
+    else
+        error( ("bad argument #%d (expected %s, got %s)"):format(index, type_names, t), 3 )
+    end
+end
+
+-- We expose expect in the global table as APIs need to access it, but give it
+-- a non-identifier name - meaning it does not show up in auto-completion.
+-- expect is an internal function, and should not be used by users.
+_G["~expect"] = expect
 
-local nativegetfenv = getfenv
 if _VERSION == "Lua 5.1" then
     -- If we're on Lua 5.1, install parts of the Lua 5.2/5.3 API so that programs can be written against it
     local type = type
@@ -20,18 +63,11 @@ if _VERSION == "Lua 5.1" then
     end
 
     function load( x, name, mode, env )
-        if type( x ) ~= "string" and type( x ) ~= "function" then
+        expect(1, x, "function", "string")
-            error( "bad argument #1 (expected string or function, got " .. type( x ) .. ")", 2 )
+        expect(2, name, "string", "nil")
-        end
+        expect(3, mode, "string", "nil")
-        if name ~= nil and type( name ) ~= "string" then
+        expect(4, env, "table", "nil")
-            error( "bad argument #2 (expected string, got " .. type( name ) .. ")", 2 )
+
-        end
-        if mode ~= nil and type( mode ) ~= "string" then
-            error( "bad argument #3 (expected string, got " .. type( mode ) .. ")", 2 )
-        end
-        if env ~= nil and type( env) ~= "table" then
-            error( "bad argument #4 (expected table, got " .. type( env ) .. ")", 2 )
-        end
         local ok, p1, p2 = pcall( function()
             if type(x) == "string" then
                 local result, err = nativeloadstring( x, name )
@@ -76,10 +112,9 @@ if _VERSION == "Lua 5.1" then
         math.log10 = nil
         table.maxn = nil
     else
-        loadstring = function(string, chunkname) return nativeloadstring(string, prefix( chunkname ))
+        loadstring = function(string, chunkname) return nativeloadstring(string, prefix( chunkname )) end
 
         -- Inject a stub for the old bit library
-        end
         _G.bit = {
             bnot = bit32.bnot,
             band = bit32.band,
@@ -175,9 +210,7 @@ end
 
 -- Install globals
 function sleep( nTime )
-    if nTime ~= nil and type( nTime ) ~= "number" then
+    expect(1, nTime, "number", "nil")
-        error( "bad argument #1 (expected number, got " .. type( nTime ) .. ")", 2 )
-    end
     local timer = os.startTimer( nTime or 0 )
     repeat
         local sEvent, param = os.pullEvent( "timer" )
@@ -185,9 +218,7 @@ function sleep( nTime )
 end
 
 function write( sText )
-    if type( sText ) ~= "string" and type( sText ) ~= "number" then
+    expect(1, sText, "string", "number")
-        error( "bad argument #1 (expected string or number, got " .. type( sText ) .. ")", 2 )
-    end
 
     local w,h = term.getSize()
     local x,y = term.getCursorPos()
@@ -275,18 +306,11 @@ function printError( ... )
 end
 
 function read( _sReplaceChar, _tHistory, _fnComplete, _sDefault )
-    if _sReplaceChar ~= nil and type( _sReplaceChar ) ~= "string" then
+    expect(1, _sReplaceChar, "string", "nil")
-        error( "bad argument #1 (expected string, got " .. type( _sReplaceChar ) .. ")", 2 )
+    expect(2, _tHistory, "table", "nil")
-    end
+    expect(3, _fnComplete, "function", "nil")
-    if _tHistory ~= nil and type( _tHistory ) ~= "table" then
+    expect(4, _sDefault, "string", "nil")
-        error( "bad argument #2 (expected table, got " .. type( _tHistory ) .. ")", 2 )
+
-    end
-    if _fnComplete ~= nil and type( _fnComplete ) ~= "function" then
-        error( "bad argument #3 (expected function, got " .. type( _fnComplete ) .. ")", 2 )
-    end
-    if _sDefault ~= nil and type( _sDefault ) ~= "string" then
-        error( "bad argument #4 (expected string, got " .. type( _sDefault ) .. ")", 2 )
-    end
     term.setCursorBlink( true )
 
     local sLine
@@ -544,13 +568,10 @@ function read( _sReplaceChar, _tHistory, _fnComplete, _sDefault )
     return sLine
 end
 
-loadfile = function( _sFile, _tEnv )
+function loadfile( _sFile, _tEnv )
-    if type( _sFile ) ~= "string" then
+    expect(1, _sFile, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sFile ) .. ")", 2 )
+    expect(2, _tEnv, "table", "nil")
-    end
+
-    if _tEnv ~= nil and type( _tEnv ) ~= "table" then
-        error( "bad argument #2 (expected table, got " .. type( _tEnv ) .. ")", 2 )
-    end
     local file = fs.open( _sFile, "r" )
     if file then
         local func, err = load( file.readAll(), "@" .. fs.getName( _sFile ), "t", _tEnv )
@@ -560,10 +581,9 @@ loadfile = function( _sFile, _tEnv )
     return nil, "File not found"
 end
 
-dofile = function( _sFile )
+function dofile( _sFile )
-    if type( _sFile ) ~= "string" then
+    expect(1, _sFile, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sFile ) .. ")", 2 )
+
-    end
     local fnFile, e = loadfile( _sFile, _G )
     if fnFile then
         return fnFile()
@@ -574,12 +594,9 @@ end
 
 -- Install the rest of the OS api
 function os.run( _tEnv, _sPath, ... )
-    if type( _tEnv ) ~= "table" then
+    expect(1, _tEnv, "table")
-        error( "bad argument #1 (expected table, got " .. type( _tEnv ) .. ")", 2 )
+    expect(2, _sPath, "string")
-    end
+
-    if type( _sPath ) ~= "string" then
-        error( "bad argument #2 (expected string, got " .. type( _sPath ) .. ")", 2 )
-    end
     local tArgs = table.pack( ... )
     local tEnv = _tEnv
     setmetatable( tEnv, { __index = _G } )
@@ -604,9 +621,7 @@ end
 
 local tAPIsLoading = {}
 function os.loadAPI( _sPath )
-    if type( _sPath ) ~= "string" then
+    expect(1, _sPath, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sPath ) .. ")", 2 )
-    end
     local sName = fs.getName( _sPath )
     if sName:sub(-4) == ".lua" then
         sName = sName:sub(1,-5)
@@ -644,9 +659,7 @@ function os.loadAPI( _sPath )
 end
 
 function os.unloadAPI( _sName )
-    if type( _sName ) ~= "string" then
+    expect(1, _sName, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sName ) .. ")", 2 )
-    end
     if _sName ~= "_G" and type(_G[_sName]) == "table" then
         _G[_sName] = nil
     end
@@ -692,9 +705,11 @@ if http then
 
     local function checkOptions( options, body )
         checkKey( options, "url", "string")
-        if body == false
+        if body == false then
-        then checkKey( options, "body", "nil" )
+          checkKey( options, "body", "nil" )
-        else checkKey( options, "body", "string", not body ) end
+        else
+          checkKey( options, "body", "string", not body )
+        end
         checkKey( options, "headers", "table", true )
         checkKey( options, "method", "string", true )
         checkKey( options, "redirect", "boolean", true )
@@ -725,15 +740,9 @@ if http then
             return wrapRequest( _url.url, _url )
         end
 
-        if type( _url ) ~= "string" then
+        expect(1, _url, "string")
-            error( "bad argument #1 (expected string, got " .. type( _url ) .. ")", 2 )
+        expect(2, _headers, "table", "nil")
-        end
+        expect(3, _binary, "boolean", "nil")
-        if _headers ~= nil and type( _headers ) ~= "table" then
-            error( "bad argument #2 (expected table, got " .. type( _headers ) .. ")", 2 )
-        end
-        if _binary ~= nil and type( _binary ) ~= "boolean" then
-            error( "bad argument #3 (expected boolean, got " .. type( _binary ) .. ")", 2 )
-        end
         return wrapRequest( _url, _url, nil, _headers, _binary )
     end
 
@@ -743,18 +752,10 @@ if http then
             return wrapRequest( _url.url, _url )
         end
 
-        if type( _url ) ~= "string" then
+        expect(1, _url, "string")
-            error( "bad argument #1 (expected string, got " .. type( _url ) .. ")", 2 )
+        expect(2, _post, "string")
-        end
+        expect(3, _headers, "table", "nil")
-        if type( _post ) ~= "string" then
+        expect(4, _binary, "boolean", "nil")
-            error( "bad argument #2 (expected string, got " .. type( _post ) .. ")", 2 )
-        end
-        if _headers ~= nil and type( _headers ) ~= "table" then
-            error( "bad argument #3 (expected table, got " .. type( _headers ) .. ")", 2 )
-        end
-        if _binary ~= nil and type( _binary ) ~= "boolean" then
-            error( "bad argument #4 (expected boolean, got " .. type( _binary ) .. ")", 2 )
-        end
         return wrapRequest( _url, _url, _post, _headers, _binary )
     end
 
@@ -764,19 +765,10 @@ if http then
             checkOptions( _url )
             url = _url.url
         else
-            if type( _url ) ~= "string" then
+            expect(1, _url, "string")
-                error( "bad argument #1 (expected string, got " .. type( _url ) .. ")", 2 )
+            expect(2, _post, "string", "nil")
-            end
+            expect(3, _headers, "table", "nil")
-            if _post ~= nil and type( _post ) ~= "string" then
+            expect(4, _binary, "boolean", "nil")
-                error( "bad argument #2 (expected string, got " .. type( _post ) .. ")", 2 )
-            end
-            if _headers ~= nil and type( _headers ) ~= "table" then
-                error( "bad argument #3 (expected table, got " .. type( _headers ) .. ")", 2 )
-            end
-            if _binary ~= nil and type( _binary ) ~= "boolean" then
-                error( "bad argument #4 (expected boolean, got " .. type( _binary ) .. ")", 2 )
-            end
-
             url = _url.url
         end
 
@@ -802,12 +794,9 @@ if http then
     local nativeWebsocket = http.websocket
     http.websocketAsync = nativeWebsocket
     http.websocket = function( _url, _headers )
-        if type( _url ) ~= "string" then
+        expect(1, _url, "string")
-            error( "bad argument #1 (expected string, got " .. type( _url ) .. ")", 2 )
+        expect(2, _headers, "table", "nil")
-        end
+
-        if _headers ~= nil and type( _headers ) ~= "table" then
-            error( "bad argument #2 (expected table, got " .. type( _headers ) .. ")", 2 )
-        end
         local ok, err = nativeWebsocket( _url, _headers )
         if not ok then return ok, err end
 
@@ -825,18 +814,11 @@ end
 -- Install the lua part of the FS api
 local tEmpty = {}
 function fs.complete( sPath, sLocation, bIncludeFiles, bIncludeDirs )
-    if type( sPath ) ~= "string" then
+    expect(1, sPath, "string")
-        error( "bad argument #1 (expected string, got " .. type( sPath ) .. ")", 2 )
+    expect(2, sLocation, "string")
-    end
+    expect(3, bIncludeFiles, "boolean", "nil")
-    if type( sLocation ) ~= "string" then
+    expect(4, bIncludeDirs, "boolean", "nil")
-        error( "bad argument #2 (expected string, got " .. type( sLocation ) .. ")", 2 )
+
-    end
-    if bIncludeFiles ~= nil and type( bIncludeFiles ) ~= "boolean" then
-        error( "bad argument #3 (expected boolean, got " .. type( bIncludeFiles ) .. ")", 2 )
-    end
-    if bIncludeDirs ~= nil and type( bIncludeDirs ) ~= "boolean" then
-        error( "bad argument #4 (expected boolean, got " .. type( bIncludeDirs ) .. ")", 2 )
-    end
     bIncludeFiles = (bIncludeFiles ~= false)
     bIncludeDirs = (bIncludeDirs ~= false)
     local sDir = sLocation

src/main/resources/assets/computercraft/lua/rom/apis/colors.lua

@@ -1,3 +1,5 @@
+local expect = _G["~expect"]
+
 -- Colors
 white = 1
 orange = 2
@@ -19,48 +21,32 @@ black = 32768
 function combine( ... )
     local r = 0
     for n,c in ipairs( { ... } ) do
-        if type( c ) ~= "number" then
+        expect(n, c, "number")
-            error( "bad argument #"..n.." (expected number, got " .. type( c ) .. ")", 2 )
-        end
         r = bit32.bor(r,c)
     end
     return r
 end
 
 function subtract( colors, ... )
-    if type( colors ) ~= "number" then
+    expect(1, colors, "number")
-        error( "bad argument #1 (expected number, got " .. type( colors ) .. ")", 2 )
-    end
     local r = colors
     for n,c in ipairs( { ... } ) do
-        if type( c ) ~= "number" then
+        expect(n + 1, c, "number")
-            error( "bad argument #"..tostring( n+1 ).." (expected number, got " .. type( c ) .. ")", 2 )
-        end
         r = bit32.band(r, bit32.bnot(c))
     end
     return r
 end
 
 function test( colors, color )
-    if type( colors ) ~= "number" then
+    expect(1, colors, "number")
-        error( "bad argument #1 (expected number, got " .. type( colors ) .. ")", 2 )
+    expect(2, color, "number")
-    end
-    if type( color ) ~= "number" then
-        error( "bad argument #2 (expected number, got " .. type( color ) .. ")", 2 )
-    end
     return bit32.band(colors, color) == color
 end
 
 function packRGB( r, g, b )
-    if type( r ) ~= "number" then
+    expect(1, r, "number")
-        error( "bad argument #1 (expected number, got " .. type( r ) .. ")", 2 )
+    expect(2, g, "number")
-    end
+    expect(3, b, "number")
-    if type( g ) ~= "number" then
-        error( "bad argument #2 (expected number, got " .. type( g ) .. ")", 2 )
-    end
-    if type( b ) ~= "number" then
-        error( "bad argument #3 (expected number, got " .. type( b ) .. ")", 2 )
-    end
     return
         bit32.band( r * 255, 0xFF ) * 2^16 +
         bit32.band( g * 255, 0xFF ) * 2^8 +
@@ -68,9 +54,7 @@ function packRGB( r, g, b )
 end
 
 function unpackRGB( rgb )
-    if type( rgb ) ~= "number" then
+    expect(1, rgb, "number")
-        error( "bad argument #1 (expected number, got " .. type( rgb ) .. ")", 2 )
-    end
     return
         bit32.band( bit32.rshift( rgb, 16 ), 0xFF ) / 255,
         bit32.band( bit32.rshift( rgb, 8 ), 0xFF ) / 255,

src/main/resources/assets/computercraft/lua/rom/apis/gps.lua

@@ -1,3 +1,5 @@
+local expect = _G["~expect"]
+
 CHANNEL_GPS = 65534
 
 local function trilaterate( A, B, C )
@@ -55,12 +57,8 @@ local function narrow( p1, p2, fix )
 end
 
 function locate( _nTimeout, _bDebug )
-    if _nTimeout ~= nil and type( _nTimeout ) ~= "number" then
+    expect(1, _nTimeout, "number", "nil")
-        error( "bad argument #1 (expected number, got " .. type( _nTimeout ) .. ")", 2 )
+    expect(2, _bDebug, "boolean", "nil")
-    end
-    if _bDebug ~= nil and type( _bDebug ) ~= "boolean" then
-        error( "bad argument #2 (expected boolean, got " .. type( _bDebug) .. ")", 2 )
-    end
     -- Let command computers use their magic fourth-wall-breaking special abilities
     if commands then
         return commands.getBlockPosition()

src/main/resources/assets/computercraft/lua/rom/apis/help.lua

@@ -1,3 +1,4 @@
+local expect = _G["~expect"]
 
 local sPath = "/rom/help"
 
@@ -6,16 +7,12 @@ function path()
 end
 
 function setPath( _sPath )
-    if type( _sPath ) ~= "string" then
+    expect(1, _sPath, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sPath ) .. ")", 2 )
-    end
     sPath = _sPath
 end
 
 function lookup( _sTopic )
-    if type( _sTopic ) ~= "string" then
+    expect(1, _sTopic, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sTopic ) .. ")", 2 )
-    end
     -- Look on the path variable
     for sPath in string.gmatch(sPath, "[^:]+") do
         sPath = fs.combine( sPath, _sTopic )
@@ -63,9 +60,7 @@ function topics()
 end
 
 function completeTopic( sText )
-    if type( sText ) ~= "string" then
+    expect(1, sText, "string")
-        error( "bad argument #1 (expected string, got " .. type( sText ) .. ")", 2 )
-    end
     local tTopics = topics()
     local tResults = {}
     for n=1,#tTopics do

src/main/resources/assets/computercraft/lua/rom/apis/io.lua

@@ -1,5 +1,6 @@
 -- Definition for the IO API
-local typeOf = _G.type
+
+local expect, typeOf = _G["~expect"], _G.type
 
 --- If we return nil then close the file, as we've reached the end.
 -- We use this weird wrapper function as we wish to preserve the varargs
@@ -65,7 +66,7 @@ handleMetatable = {
             local handle = self._handle
             if not handle.read and not handle.readLine then return nil, "Not opened for reading" end
 
-            local n = select('#', ...)
+            local n = select("#", ...)
             local output = {}
             for i = 1, n do
                 local arg = select(i, ...)
@@ -184,9 +185,7 @@ function input(_arg)
 end
 
 function lines(_sFileName)
-    if _sFileName ~= nil and typeOf(_sFileName) ~= "string" then
+    expect(1, _sFileName, "string", "nil")
-        error("bad argument #1 (expected string, got " .. typeOf(_sFileName) .. ")", 2)
-    end
     if _sFileName then
         local ok, err = open(_sFileName, "rb")
         if not ok then error(err, 2) end
@@ -201,12 +200,8 @@ function lines(_sFileName)
 end
 
 function open(_sPath, _sMode)
-    if typeOf(_sPath) ~= "string" then
+    expect(1, _sPath, "string")
-        error("bad argument #1 (expected string, got " .. typeOf(_sPath) .. ")", 2)
+    expect(2, _sMode, "string", "nil")
-    end
-    if _sMode ~= nil and typeOf(_sMode) ~= "string" then
-        error("bad argument #2 (expected string, got " .. typeOf(_sMode) .. ")", 2)
-    end
 
     local sMode = _sMode and _sMode:gsub("%+", "") or "rb"
     local file, err = fs.open(_sPath, sMode)

src/main/resources/assets/computercraft/lua/rom/apis/keys.lua

@@ -1,7 +1,8 @@
-
 -- Minecraft key code bindings
 -- See http://www.minecraftwiki.net/wiki/Key_codes for more info
 
+local expect = _G["~expect"]
+
 local tKeys = {
     nil,	 	"one", 		"two", 		"three", 	"four",			-- 1
     "five", 	"six", 		"seven", 	"eight", 	"nine",			-- 6
@@ -58,8 +59,6 @@ keys.scollLock = keys.scrollLock
 keys.cimcumflex = keys.circumflex
 
 function getName( _nKey )
-    if type( _nKey ) ~= "number" then
+    expect(1, _nKey, "number")
-        error( "bad argument #1 (expected number, got " .. type( _nKey ) .. ")", 2 )
-    end
     return tKeys[ _nKey ]
 end

src/main/resources/assets/computercraft/lua/rom/apis/paintutils.lua

@@ -1,3 +1,4 @@
+local expect = _G["~expect"]
 
 local function drawPixelInternal( xPos, yPos )
     term.setCursorPos( xPos, yPos )
@@ -18,9 +19,7 @@ local function parseLine( tImageArg, sLine )
 end
 
 function parseImage( sRawData )
-    if type( sRawData ) ~= "string" then
+    expect(1, sRawData, "string")
-        error( "bad argument #1 (expected string, got " .. type( sRawData ) .. ")" )
-    end
     local tImage = {}
     for sLine in ( sRawData .. "\n" ):gmatch( "(.-)\n" ) do -- read each line like original file handling did
         parseLine( tImage, sLine )
@@ -29,9 +28,7 @@ function parseImage( sRawData )
 end
 
 function loadImage( sPath )
-    if type( sPath ) ~= "string" then
+    expect(1, sPath, "string")
-        error( "bad argument #1 (expected string, got " .. type( sPath ) .. ")", 2 )
-    end
 
     if fs.exists( sPath ) then
         local file = io.open( sPath, "r" )
@@ -43,21 +40,21 @@ function loadImage( sPath )
 end
 
 function drawPixel( xPos, yPos, nColour )
-    if type( xPos ) ~= "number" then error( "bad argument #1 (expected number, got " .. type( xPos ) .. ")", 2 ) end
+    expect(1, xPos, "number")
-    if type( yPos ) ~= "number" then error( "bad argument #2 (expected number, got " .. type( yPos ) .. ")", 2 ) end
+    expect(2, yPos, "number")
-    if nColour ~= nil and type( nColour ) ~= "number" then error( "bad argument #3 (expected number, got " .. type( nColour ) .. ")", 2 ) end
+    expect(3, nColour, "number", "nil")
     if nColour then
         term.setBackgroundColor( nColour )
     end
-    drawPixelInternal( xPos, yPos )
+    return drawPixelInternal( xPos, yPos )
 end
 
 function drawLine( startX, startY, endX, endY, nColour )
-    if type( startX ) ~= "number" then error( "bad argument #1 (expected number, got " .. type( startX ) .. ")", 2 ) end
+    expect(1, startX, "number")
-    if type( startY ) ~= "number" then error( "bad argument #2 (expected number, got " .. type( startY ) .. ")", 2 ) end
+    expect(2, startY, "number")
-    if type( endX ) ~= "number" then error( "bad argument #3 (expected number, got " .. type( endX ) .. ")", 2 ) end
+    expect(3, endX, "number")
-    if type( endY ) ~= "number" then error( "bad argument #4 (expected number, got " .. type( endY ) .. ")", 2 ) end
+    expect(4, endY, "number")
-    if nColour ~= nil and type( nColour ) ~= "number" then error( "bad argument #5 (expected number, got " .. type( nColour ) .. ")", 2 ) end
+    expect(5, nColour, "number", "nil")
 
     startX = math.floor(startX)
     startY = math.floor(startY)
@@ -114,11 +111,11 @@ function drawLine( startX, startY, endX, endY, nColour )
 end
 
 function drawBox( startX, startY, endX, endY, nColour )
-    if type( startX ) ~= "number" then error( "bad argument #1 (expected number, got " .. type( startX ) .. ")", 2 ) end
+    expect(1, startX, "number")
-    if type( startY ) ~= "number" then error( "bad argument #2 (expected number, got " .. type( startY ) .. ")", 2 ) end
+    expect(2, startY, "number")
-    if type( endX ) ~= "number" then error( "bad argument #3 (expected number, got " .. type( endX ) .. ")", 2 ) end
+    expect(3, endX, "number")
-    if type( endY ) ~= "number" then error( "bad argument #4 (expected number, got " .. type( endY ) .. ")", 2 ) end
+    expect(4, endY, "number")
-    if nColour ~= nil and type( nColour ) ~= "number" then error( "bad argument #5 (expected number, got " .. type( nColour ) .. ")", 2 ) end
+    expect(5, nColour, "number", "nil")
 
     startX = math.floor(startX)
     startY = math.floor(startY)
@@ -159,11 +156,11 @@ function drawBox( startX, startY, endX, endY, nColour )
 end
 
 function drawFilledBox( startX, startY, endX, endY, nColour )
-    if type( startX ) ~= "number" then error( "bad argument #1 (expected number, got " .. type( startX ) .. ")", 2 ) end
+    expect(1, startX, "number")
-    if type( startY ) ~= "number" then error( "bad argument #2 (expected number, got " .. type( startY ) .. ")", 2 ) end
+    expect(2, startY, "number")
-    if type( endX ) ~= "number" then error( "bad argument #3 (expected number, got " .. type( endX ) .. ")", 2 ) end
+    expect(3, endX, "number")
-    if type( endY ) ~= "number" then error( "bad argument #4 (expected number, got " .. type( endY ) .. ")", 2 ) end
+    expect(4, endY, "number")
-    if nColour ~= nil and type( nColour ) ~= "number" then error( "bad argument #5 (expected number, got " .. type( nColour ) .. ")", 2 ) end
+    expect(5, nColour, "number", "nil")
 
     startX = math.floor(startX)
     startY = math.floor(startY)
@@ -198,9 +195,9 @@ function drawFilledBox( startX, startY, endX, endY, nColour )
 end
 
 function drawImage( tImage, xPos, yPos )
-    if type( tImage ) ~= "table" then error( "bad argument #1 (expected table, got " .. type( tImage ) .. ")", 2 ) end
+    expect(1, tImage, "table")
-    if type( xPos ) ~= "number" then error( "bad argument #2 (expected number, got " .. type( xPos ) .. ")", 2 ) end
+    expect(2, xPos, "number")
-    if type( yPos ) ~= "number" then error( "bad argument #3 (expected number, got " .. type( yPos ) .. ")", 2 ) end
+    expect(3, yPos, "number")
     for y=1,#tImage do
         local tLine = tImage[y]
         for x=1,#tLine do

src/main/resources/assets/computercraft/lua/rom/apis/peripheral.lua

@@ -1,3 +1,5 @@
+local expect = _G["~expect"]
+
 local native = peripheral
 
 function getNames()
@@ -17,9 +19,7 @@ function getNames()
 end
 
 function isPresent( _sSide )
-    if type( _sSide ) ~= "string" then
+    expect(1, _sSide, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sSide ) .. ")", 2 )
-    end
     if native.isPresent( _sSide ) then
         return true
     end
@@ -34,9 +34,7 @@ function isPresent( _sSide )
 end
 
 function getType( _sSide )
-    if type( _sSide ) ~= "string" then
+    expect(1, _sSide, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sSide ) .. ")", 2 )
-    end
     if native.isPresent( _sSide ) then
         return native.getType( _sSide )
     end
@@ -51,9 +49,7 @@ function getType( _sSide )
 end
 
 function getMethods( _sSide )
-    if type( _sSide ) ~= "string" then
+    expect(1, _sSide, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sSide ) .. ")", 2 )
-    end
     if native.isPresent( _sSide ) then
         return native.getMethods( _sSide )
     end
@@ -68,12 +64,8 @@ function getMethods( _sSide )
 end
 
 function call( _sSide, _sMethod, ... )
-    if type( _sSide ) ~= "string" then
+    expect(1, _sSide, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sSide ) .. ")", 2 )
+    expect(2, _sMethod, "string")
-    end
-    if type( _sSide ) ~= "string" then
-        error( "bad argument #2 (expected string, got " .. type( _sMethod ) .. ")", 2 )
-    end
     if native.isPresent( _sSide ) then
         return native.call( _sSide, _sMethod, ... )
     end
@@ -88,9 +80,7 @@ function call( _sSide, _sMethod, ... )
 end
 
 function wrap( _sSide )
-    if type( _sSide ) ~= "string" then
+    expect(1, _sSide, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sSide ) .. ")", 2 )
-    end
     if peripheral.isPresent( _sSide ) then
         local tMethods = peripheral.getMethods( _sSide )
         local tResult = {}
@@ -105,12 +95,8 @@ function wrap( _sSide )
 end
 
 function find( sType, fnFilter )
-    if type( sType ) ~= "string" then
+    expect(1, sType, "string")
-        error( "bad argument #1 (expected string, got " .. type( sType ) .. ")", 2 )
+    expect(2, fnFilter, "function", "nil")
-    end
-    if fnFilter ~= nil and type( fnFilter ) ~= "function" then
-        error( "bad argument #2 (expected function, got " .. type( fnFilter ) .. ")", 2 )
-    end
     local tResults = {}
     for n,sName in ipairs( peripheral.getNames() ) do
         if peripheral.getType( sName ) == sType then

src/main/resources/assets/computercraft/lua/rom/apis/rednet.lua

@@ -1,3 +1,4 @@
+local expect = _G["~expect"]
 
 CHANNEL_BROADCAST = 65535
 CHANNEL_REPEAT = 65533
@@ -7,9 +8,7 @@ local tReceivedMessageTimeouts = {}
 local tHostnames = {}
 
 function open( sModem )
-    if type( sModem ) ~= "string" then
+    expect(1, sModem, "string")
-        error( "bad argument #1 (expected string, got " .. type( sModem ) .. ")", 2 )
-    end
     if peripheral.getType( sModem ) ~= "modem" then
         error( "No such modem: "..sModem, 2 )
     end
@@ -18,11 +17,9 @@ function open( sModem )
 end
 
 function close( sModem )
+    expect(1, sModem, "string", "nil")
     if sModem then
         -- Close a specific modem
-        if type( sModem ) ~= "string" then
-            error( "bad argument #1 (expected string, got " .. type( sModem ) .. ")", 2 )
-        end
         if peripheral.getType( sModem ) ~= "modem" then
             error( "No such modem: "..sModem, 2 )
         end
@@ -39,11 +36,9 @@ function close( sModem )
 end
 
 function isOpen( sModem )
+    expect(1, sModem, "string", "nil")
     if sModem then
         -- Check if a specific modem is open
-        if type( sModem ) ~= "string" then
-            error( "bad argument #1 (expected string, got " .. type( sModem ) .. ")", 2 )
-        end
         if peripheral.getType( sModem ) == "modem" then
             return peripheral.call( sModem, "isOpen", os.getComputerID() ) and peripheral.call( sModem, "isOpen", CHANNEL_BROADCAST )
         end
@@ -59,12 +54,8 @@ function isOpen( sModem )
 end
 
 function send( nRecipient, message, sProtocol )
-    if type( nRecipient ) ~= "number" then
+    expect(1, nRecipient, "number")
-        error( "bad argument #1 (expected number, got " .. type( nRecipient ) .. ")", 2 )
+    expect(3, sProtocol, "string", "nil")
-    end
-    if sProtocol ~= nil and type( sProtocol ) ~= "string" then
-        error( "bad argument #3 (expected string, got " .. type( sProtocol ) .. ")", 2 )
-    end
     -- Generate a (probably) unique message ID
     -- We could do other things to guarantee uniqueness, but we really don't need to
     -- Store it to ensure we don't get our own messages back
@@ -101,9 +92,7 @@ function send( nRecipient, message, sProtocol )
 end
 
 function broadcast( message, sProtocol )
-    if sProtocol ~= nil and type( sProtocol ) ~= "string" then
+    expect(2, sProtocol, "string", "nil")
-        error( "bad argument #2 (expected string, got " .. type( sProtocol ) .. ")", 2 )
-    end
     send( CHANNEL_BROADCAST, message, sProtocol )
 end
 
@@ -112,12 +101,8 @@ function receive( sProtocolFilter, nTimeout )
     if type(sProtocolFilter) == "number" and nTimeout == nil then
         sProtocolFilter, nTimeout = nil, sProtocolFilter
     end
-    if sProtocolFilter ~= nil and type( sProtocolFilter ) ~= "string" then
+    expect(1, sProtocolFilter, "string", "nil")
-        error( "bad argument #1 (expected string, got " .. type( sProtocolFilter ) .. ")", 2 )
+    expect(2, nTimeout, "number", "nil")
-    end
-    if nTimeout ~= nil and type( nTimeout ) ~= "number" then
-        error( "bad argument #2 (expected number, got " .. type( nTimeout ) .. ")", 2 )
-    end
 
     -- Start the timer
     local timer = nil
@@ -148,12 +133,8 @@ function receive( sProtocolFilter, nTimeout )
 end
 
 function host( sProtocol, sHostname )
-    if type( sProtocol ) ~= "string" then
+    expect(1, sProtocol, "string")
-        error( "bad argument #1 (expected string, got " .. type( sProtocol ) .. ")", 2 )
+    expect(2, sHostname, "string")
-    end
-    if type( sHostname ) ~= "string" then
-        error( "bad argument #2 (expected string, got " .. type( sHostname ) .. ")", 2 )
-    end
     if sHostname == "localhost" then
         error( "Reserved hostname", 2 )
     end
@@ -166,19 +147,13 @@ function host( sProtocol, sHostname )
 end
 
 function unhost( sProtocol )
-    if type( sProtocol ) ~= "string" then
+    expect(1, sProtocol, "string")
-        error( "bad argument #1 (expected string, got " .. type( sProtocol ) .. ")", 2 )
-    end
     tHostnames[ sProtocol ] = nil
 end
 
 function lookup( sProtocol, sHostname )
-    if type( sProtocol ) ~= "string" then
+    expect(1, sProtocol, "string")
-        error( "bad argument #1 (expected string, got " .. type( sProtocol ) .. ")", 2 )
+    expect(2, sHostname, "string")
-    end
-    if sHostname ~= nil and type( sHostname ) ~= "string" then
-        error( "bad argument #2 (expected string, got " .. type( sHostname ) .. ")", 2 )
-    end
 
     -- Build list of host IDs
     local tResults = nil

src/main/resources/assets/computercraft/lua/rom/apis/settings.lua

@@ -1,14 +1,12 @@
+local expect = _G["~expect"]
 
 local tSettings = {}
 
 function set( sName, value )
-    if type( sName ) ~= "string" then error( "bad argument #1 (expected string, got " .. type( sName ) .. ")", 2 ) end
+    expect(1, sName, "string")
+    expect(2, value, "number", "string", "boolean", "table")
 
-    local sValueTy = type(value)
+    if type(value) == "table" then
-    if sValueTy ~= "number" and sValueTy ~= "string" and sValueTy ~= "boolean" and sValueTy ~= "table" then
-        error( "bad argument #2 (expected value, got " .. sValueTy .. ")", 2 )
-    end
-    if sValueTy == "table" then
         -- Ensure value is serializeable
         value = textutils.unserialize( textutils.serialize(value) )
     end
@@ -29,9 +27,7 @@ function copy( value )
 end
 
 function get( sName, default )
-    if type(sName) ~= "string" then
+    expect(1, sName, "string")
-        error( "bad argument #1 (expected string, got " .. type( sName ) .. ")", 2 )
-    end
     local result = tSettings[ sName ]
     if result ~= nil then
         return copy(result)
@@ -41,9 +37,7 @@ function get( sName, default )
 end
 
 function unset( sName )
-    if type(sName) ~= "string" then
+    expect(1, sName, "string")
-        error( "bad argument #1 (expected string, got " .. type( sName ) .. ")", 2 )
-    end
     tSettings[ sName ] = nil
 end
 
@@ -61,9 +55,7 @@ function getNames()
 end
 
 function load( sPath )
-    if type(sPath) ~= "string" then
+    expect(1, sPath, "string")
-        error( "bad argument #1 (expected string, got " .. type( sPath ) .. ")", 2 )
-    end
     local file = fs.open( sPath, "r" )
     if not file then
         return false
@@ -88,9 +80,7 @@ function load( sPath )
 end
 
 function save( sPath )
-    if type(sPath) ~= "string" then
+    expect(1, sPath, "string")
-        error( "bad argument #1 (expected string, got " .. type( sPath ) .. ")", 2 )
-    end
     local file = fs.open( sPath, "w" )
     if not file then
         return false

src/main/resources/assets/computercraft/lua/rom/apis/term.lua

@@ -1,3 +1,4 @@
+local expect = _G["~expect"]
 
 local native = (term.native and term.native()) or term
 local redirectTarget = native
@@ -11,10 +12,8 @@ end
 local term = {}
 
 term.redirect = function( target )
-    if type( target ) ~= "table" then
+    expect(1, target, "table")
-        error( "bad argument #1 (expected table, got " .. type( target ) .. ")", 2 )
+    if target == term or target == _G.term then
-    end
-    if target == term then
         error( "term is not a recommended redirect target, try term.current() instead", 2 )
     end
     for k,v in pairs( native ) do

src/main/resources/assets/computercraft/lua/rom/apis/textutils.lua

@@ -1,8 +1,7 @@
+local expect = _G["~expect"]
 
 function slowWrite( sText, nRate )
-    if nRate ~= nil and type( nRate ) ~= "number" then
+    expect(2, nRate, "number", "nil")
-        error( "bad argument #2 (expected number, got " .. type( nRate ) .. ")", 2 )
-    end
     nRate = nRate or 20
     if nRate < 0 then
         error( "Rate must be positive", 2 )
@@ -28,12 +27,8 @@ function slowPrint( sText, nRate )
 end
 
 function formatTime( nTime, bTwentyFourHour )
-    if type( nTime ) ~= "number" then
+    expect(1, nTime, "number")
-        error( "bad argument #1 (expected number, got " .. type( nTime ) .. ")", 2 )
+    expect(2, bTwentyFourHour, "boolean", "nil")
-    end
-    if bTwentyFourHour ~= nil and type( bTwentyFourHour ) ~= "boolean" then
-        error( "bad argument #2 (expected boolean, got " .. type( bTwentyFourHour ) .. ")", 2 )
-    end
     local sTOD = nil
     if not bTwentyFourHour then
         if nTime >= 12 then
@@ -77,9 +72,7 @@ local function makePagedScroll( _term, _nFreeLines )
 end
 
 function pagedPrint( _sText, _nFreeLines )
-    if _nFreeLines ~= nil and type( _nFreeLines ) ~= "number" then
+    expect(2, _nFreeLines, "number", "nil")
-        error( "bad argument #2 (expected number, got " .. type( _nFreeLines ) .. ")", 2 )
-    end
     -- Setup a redirector
     local oldTerm = term.current()
     local newTerm = {}
@@ -321,9 +314,7 @@ function serialize( t )
 end
 
 function unserialize( s )
-    if type( s ) ~= "string" then
+    expect(1, s, "string")
-        error( "bad argument #1 (expected string, got " .. type( s ) .. ")", 2 )
-    end
     local func = load( "return "..s, "unserialize", "t", {} )
     if func then
         local ok, result = pcall( func )
@@ -335,20 +326,14 @@ function unserialize( s )
 end
 
 function serializeJSON( t, bNBTStyle )
-    if type( t ) ~= "table" and type( t ) ~= "string" and type( t ) ~= "number" and type( t ) ~= "boolean" then
+    expect(1, t, "table", "string", "number", "boolean")
-        error( "bad argument #1 (expected table/string/number/boolean, got " .. type( t ) .. ")", 2 )
+    expect(2, bNBTStyle, "boolean", "nil")
-    end
-    if bNBTStyle ~= nil and type( bNBTStyle ) ~= "boolean" then
-        error( "bad argument #2 (expected boolean, got " .. type( bNBTStyle ) .. ")", 2 )
-    end
     local tTracking = {}
     return serializeJSONImpl( t, tTracking, bNBTStyle or false )
 end
 
 function urlEncode( str )
-    if type( str ) ~= "string" then
+    expect(1, str, "string")
-        error( "bad argument #1 (expected string, got " .. type( str ) .. ")", 2 )
-    end
     if str then
         str = string.gsub(str, "\n", "\r\n")
         str = string.gsub(str, "([^A-Za-z0-9 %-%_%.])", function(c)
@@ -370,12 +355,8 @@ end
 
 local tEmpty = {}
 function complete( sSearchText, tSearchTable )
-    if type( sSearchText ) ~= "string" then
+    expect(1, sSearchText, "string")
-        error( "bad argument #1 (expected string, got " .. type( sSearchText ) .. ")", 2 )
+    expect(2, tSearchTable, "table", "nil")
-    end
-    if tSearchTable ~= nil and type( tSearchTable ) ~= "table" then
-        error( "bad argument #2 (expected table, got " .. type( tSearchTable ) .. ")", 2 )
-    end
 
     if g_tLuaKeywords[sSearchText] then return tEmpty end
     local nStart = 1

src/main/resources/assets/computercraft/lua/rom/apis/window.lua

@@ -1,3 +1,4 @@
+local expect = _G["~expect"]
 
 local tHex = {
     [ colors.white ] = "0",
@@ -21,15 +22,14 @@ local tHex = {
 local type = type
 local string_rep = string.rep
 local string_sub = string.sub
-local table_unpack = table.unpack
 
 function create( parent, nX, nY, nWidth, nHeight, bStartVisible )
-    if type( parent ) ~= "table" then error( "bad argument #1 (expected table, got " .. type( parent ) .. ")", 2 ) end
+    expect(1, parent, "table")
-    if type( nX ) ~= "number" then error( "bad argument #2 (expected number, got " .. type( nX ) .. ")", 2 ) end
+    expect(2, nX, "number")
-    if type( nY ) ~= "number" then error( "bad argument #3 (expected number, got " .. type( nY ) .. ")", 2 ) end
+    expect(3, nY, "number")
-    if type( nWidth ) ~= "number" then error( "bad argument #4 (expected number, got " .. type( nWidth ) .. ")", 2 ) end
+    expect(4, nWidth, "number")
-    if type( nHeight ) ~= "number" then error( "bad argument #5 (expected number, got " .. type( nHeight ) .. ")", 2 ) end
+    expect(5, nHeight, "number")
-    if bStartVisible ~= nil and type( bStartVisible ) ~= "boolean" then error( "bad argument #6 (expected boolean, got " .. type( bStartVisible ) .. ")", 2 ) end
+    expect(6, bStartVisible, "boolean", "nil")
 
     if parent == term then
         error( "term is not a recommended window parent, try term.current() instead", 2 )
@@ -191,9 +191,9 @@ function create( parent, nX, nY, nWidth, nHeight, bStartVisible )
     end
 
     function window.blit( sText, sTextColor, sBackgroundColor )
-        if type( sText ) ~= "string" then error( "bad argument #1 (expected string, got " .. type( sText ) .. ")", 2 ) end
+        if type(sText) ~= "string" then expect(1, sText, "string") end
-        if type( sTextColor ) ~= "string" then error( "bad argument #2 (expected string, got " .. type( sTextColor ) .. ")", 2 ) end
+        if type(sTextColor) ~= "string" then expect(2, sTextColor, "string") end
-        if type( sBackgroundColor ) ~= "string" then error( "bad argument #3 (expected string, got " .. type( sBackgroundColor ) .. ")", 2 ) end
+        if type(sBackgroundColor) ~= "string" then expect(3, sBackgroundColor, "string") end
         if #sTextColor ~= #sText or #sBackgroundColor ~= #sText then
             error( "Arguments must be the same length", 2 )
         end
@@ -241,8 +241,8 @@ function create( parent, nX, nY, nWidth, nHeight, bStartVisible )
     end
 
     function window.setCursorPos( x, y )
-        if type( x ) ~= "number" then error( "bad argument #1 (expected number, got " .. type( x ) .. ")", 2 ) end
+        if type(x) ~= "number" then expect(1, x, "number") end
-        if type( y ) ~= "number" then error( "bad argument #2 (expected number, got " .. type( y ) .. ")", 2 ) end
+        if type(y) ~= "number" then expect(2, y, "number") end
         nCursorX = math.floor( x )
         nCursorY = math.floor( y )
         if bVisible then
@@ -251,7 +251,7 @@ function create( parent, nX, nY, nWidth, nHeight, bStartVisible )
     end
 
     function window.setCursorBlink( blink )
-        if type( blink ) ~= "boolean" then error( "bad argument #1 (expected boolean, got " .. type( blink ) .. ")", 2 ) end
+        if type(blink) ~= "boolean" then expect(1, blink, "boolean") end
         bCursorBlink = blink
         if bVisible then
             updateCursorBlink()
@@ -275,11 +275,11 @@ function create( parent, nX, nY, nWidth, nHeight, bStartVisible )
     end
 
     local function setTextColor( color )
-        if type( color ) ~= "number" then
+        if type(color) ~= "number" then expect(1, color, "number") end
-            error( "bad argument #1 (expected number, got " .. type( color ) .. ")", 2 )
+        if tHex[color] == nil then
-        elseif tHex[color] == nil then
             error( "Invalid color (got " .. color .. ")" , 2 )
         end
+
         nTextColor = color
         if bVisible then
             updateCursorColor()
@@ -290,7 +290,7 @@ function create( parent, nX, nY, nWidth, nHeight, bStartVisible )
     window.setTextColour = setTextColor
 
     function window.setPaletteColour( colour, r, g, b )
-        if type( colour ) ~= "number" then error( "bad argument #1 (expected number, got " .. type( colour ) .. ")", 2 ) end
+        if type(colour) ~= "number" then expect(1, colour, "number") end
 
         if tHex[colour] == nil then
             error( "Invalid color (got " .. colour .. ")" , 2 )
@@ -301,9 +301,9 @@ function create( parent, nX, nY, nWidth, nHeight, bStartVisible )
             tCol = { colours.unpackRGB( r ) }
             tPalette[ colour ] = tCol
         else
-            if type( r ) ~= "number" then error( "bad argument #2 (expected number, got " .. type( r ) .. ")", 2 ) end
+            if type(r) ~= "number" then expect(2, r, "number") end
-            if type( g ) ~= "number" then error( "bad argument #3 (expected number, got " .. type( g ) .. ")", 2 ) end
+            if type(g) ~= "number" then expect(3, g, "number") end
-            if type( b ) ~= "number" then error( "bad argument #4 (expected number, got " .. type( b ) .. ")", 2 ) end
+            if type(b) ~= "number" then expect(4, b, "number") end
 
             tCol = tPalette[ colour ]
             tCol[1] = r
@@ -319,7 +319,7 @@ function create( parent, nX, nY, nWidth, nHeight, bStartVisible )
     window.setPaletteColor = window.setPaletteColour
 
     function window.getPaletteColour( colour )
-        if type( colour ) ~= "number" then error( "bad argument #1 (expected number, got " .. type( colour ) .. ")", 2 ) end
+        if type(colour) ~= "number" then expect(1, colour, "number") end
         if tHex[colour] == nil then
             error( "Invalid color (got " .. colour .. ")" , 2 )
         end
@@ -330,9 +330,8 @@ function create( parent, nX, nY, nWidth, nHeight, bStartVisible )
     window.getPaletteColor = window.getPaletteColour
 
     local function setBackgroundColor( color )
-        if type( color ) ~= "number" then
+        if type(color) ~= "number" then expect(1, color, "number") end
-            error( "bad argument #1 (expected number, got " .. type( color ) .. ")", 2 )
+        if tHex[color] == nil then
-        elseif tHex[color] == nil then
             error( "Invalid color (got " .. color .. ")", 2 )
         end
         nBackgroundColor = color
@@ -346,7 +345,7 @@ function create( parent, nX, nY, nWidth, nHeight, bStartVisible )
     end
 
     function window.scroll( n )
-        if type( n ) ~= "number" then error( "bad argument #1 (expected number, got " .. type( n ) .. ")", 2 ) end
+        if type(n) ~= "number" then expect(1, n, "number") end
         if n ~= 0 then
             local tNewLines = {}
             local sEmptyText = sEmptySpaceLine
@@ -391,7 +390,7 @@ function create( parent, nX, nY, nWidth, nHeight, bStartVisible )
 
     -- Other functions
     function window.setVisible( bVis )
-        if type( bVis ) ~= "boolean" then error( "bad argument #1 (expected boolean, got " .. type( bVis ) .. ")", 2 ) end
+        if type(bVis) ~= "boolean" then expect(1, bVis, "boolean") end
         if bVisible ~= bVis then
             bVisible = bVis
             if bVisible then
@@ -423,11 +422,11 @@ function create( parent, nX, nY, nWidth, nHeight, bStartVisible )
     end
 
     function window.reposition( nNewX, nNewY, nNewWidth, nNewHeight )
-        if type( nNewX ) ~= "number" then error( "bad argument #1 (expected number, got " .. type( nNewX ) .. ")", 2 ) end
+        if type(nNewX) ~= "number" then expect(1, nNewX, "number") end
-        if type( nNewY ) ~= "number" then error( "bad argument #2 (expected number, got " .. type( nNewY ) .. ")", 2 ) end
+        if type(nNewY) ~= "number" then expect(2, nNewY, "number") end
         if nNewWidth ~= nil or nNewHeight ~= nil then
-            if type( nNewWidth ) ~= "number" then error( "bad argument #3 (expected number, got " .. type( nNewWidth ) .. ")", 2 ) end
+            expect(3, nNewWidth, "number")
-            if type( nNewHeight ) ~= "number" then error( "bad argument #4 (expected number, got " .. type( nNewHeight ) .. ")", 2 ) end
+            expect(4, nNewHeight, "number")
         end
 
         nX = nNewX

src/main/resources/assets/computercraft/lua/rom/programs/advanced/multishell.lua

@@ -1,3 +1,4 @@
+local expect = _G["~expect"]
 
 -- Setup process switching
 local parentTerm = term.current()
@@ -197,9 +198,7 @@ function multishell.getFocus()
 end
 
 function multishell.setFocus( n )
-    if type( n ) ~= "number" then
+    expect(1, n, "number")
-        error( "bad argument #1 (expected number, got " .. type( n ) .. ")", 2 )
-    end
     if n >= 1 and n <= #tProcesses then
         selectProcess( n )
         redrawMenu()
@@ -209,9 +208,7 @@ function multishell.setFocus( n )
 end
 
 function multishell.getTitle( n )
-    if type( n ) ~= "number" then
+    expect(1, n, "number")
-        error( "bad argument #1 (expected number, got " .. type( n ) .. ")", 2 )
-    end
     if n >= 1 and n <= #tProcesses then
         return tProcesses[n].sTitle
     end
@@ -219,12 +216,8 @@ function multishell.getTitle( n )
 end
 
 function multishell.setTitle( n, sTitle )
-    if type( n ) ~= "number" then
+    expect(1, n, "number")
-        error( "bad argument #1 (expected number, got " .. type( n ) .. ")", 2 )
+    expect(2, sTitle, "string")
-    end
-    if type( sTitle ) ~= "string" then
-        error( "bad argument #2 (expected string, got " .. type( sTitle ) .. ")", 2 )
-    end
     if n >= 1 and n <= #tProcesses then
         setProcessTitle( n, sTitle )
         redrawMenu()
@@ -236,12 +229,8 @@ function multishell.getCurrent()
 end
 
 function multishell.launch( tProgramEnv, sProgramPath, ... )
-    if type( tProgramEnv ) ~= "table" then
+    expect(1, tProgramEnv, "table")
-        error( "bad argument #1 (expected table, got " .. type( tProgramEnv ) .. ")", 2 )
+    expect(2, sProgramPath, "string")
-    end
-    if type( sProgramPath ) ~= "string" then
-        error( "bad argument #2 (expected string, got " .. type( sProgramPath ) .. ")", 2 )
-    end
     local previousTerm = term.current()
     setMenuVisible( (#tProcesses + 1) >= 2 )
     local nResult = launchProcess( false, tProgramEnv, sProgramPath, ... )

src/main/resources/assets/computercraft/lua/rom/programs/shell.lua

@@ -1,3 +1,4 @@
+local expect = _G["~expect"]
 
 local multishell = multishell
 local parentShell = shell
@@ -74,9 +75,7 @@ local function createShellEnv( sDir )
 
     local sentinel = {}
     local function require( name )
-        if type( name ) ~= "string" then
+        expect(1, name, "string")
-            error( "bad argument #1 (expected string, got " .. type( name ) .. ")", 2 )
-        end
         if package.loaded[name] == sentinel then
             error("loop or previous error loading module '" .. name .. "'", 0)
         end
@@ -191,9 +190,7 @@ function shell.dir()
 end
 
 function shell.setDir( _sDir )
-    if type( _sDir ) ~= "string" then
+    expect(1, _sDir, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sDir ) .. ")", 2 )
-    end
     if not fs.isDir( _sDir ) then
         error( "Not a directory", 2 )
     end
@@ -205,16 +202,12 @@ function shell.path()
 end
 
 function shell.setPath( _sPath )
-    if type( _sPath ) ~= "string" then
+    expect(1, _sPath, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sPath ) .. ")", 2 )
-    end
     sPath = _sPath
 end
 
 function shell.resolve( _sPath )
-    if type( _sPath ) ~= "string" then
+    expect(1, _sPath, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sPath ) .. ")", 2 )
-    end
     local sStartChar = string.sub( _sPath, 1, 1 )
     if sStartChar == "/" or sStartChar == "\\" then
         return fs.combine( "", _sPath )
@@ -234,9 +227,7 @@ local function pathWithExtension( _sPath, _sExt )
 end
 
 function shell.resolveProgram( _sCommand )
-    if type( _sCommand ) ~= "string" then
+    expect(1, _sCommand, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sCommand ) .. ")", 2 )
-    end
     -- Substitute aliases firsts
     if tAliases[ _sCommand ] ~= nil then
         _sCommand = tAliases[ _sCommand ]
@@ -361,9 +352,7 @@ local function completeProgramArgument( sProgram, nArgument, sPart, tPreviousPar
 end
 
 function shell.complete( sLine )
-    if type( sLine ) ~= "string" then
+    expect(1, sLine, "string")
-        error( "bad argument #1 (expected string, got " .. type( sLine ) .. ")", 2 )
-    end
     if #sLine > 0 then
         local tWords = tokenise( sLine )
         local nIndex = #tWords
@@ -400,19 +389,13 @@ function shell.complete( sLine )
 end
 
 function shell.completeProgram( sProgram )
-    if type( sProgram ) ~= "string" then
+    expect(1, sProgram, "string")
-        error( "bad argument #1 (expected string, got " .. type( sProgram ) .. ")", 2 )
-    end
     return completeProgram( sProgram )
 end
 
 function shell.setCompletionFunction( sProgram, fnComplete )
-    if type( sProgram ) ~= "string" then
+    expect(1, sProgram, "string")
-        error( "bad argument #1 (expected string, got " .. type( sProgram ) .. ")", 2 )
+    expect(2, fnComplete, "function")
-    end
-    if type( fnComplete ) ~= "function" then
-        error( "bad argument #2 (expected function, got " .. type( fnComplete ) .. ")", 2 )
-    end
     tCompletionInfo[ sProgram ] = {
         fnComplete = fnComplete
     }
@@ -430,19 +413,13 @@ function shell.getRunningProgram()
 end
 
 function shell.setAlias( _sCommand, _sProgram )
-    if type( _sCommand ) ~= "string" then
+    expect(1, _sCommand, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sCommand ) .. ")", 2 )
+    expect(2, _sProgram, "string")
-    end
-    if type( _sProgram ) ~= "string" then
-        error( "bad argument #2 (expected string, got " .. type( _sProgram ) .. ")", 2 )
-    end
     tAliases[ _sCommand ] = _sProgram
 end
 
 function shell.clearAlias( _sCommand )
-    if type( _sCommand ) ~= "string" then
+    expect(1, _sCommand, "string")
-        error( "bad argument #1 (expected string, got " .. type( _sCommand ) .. ")", 2 )
-    end
     tAliases[ _sCommand ] = nil
 end
 
@@ -472,9 +449,7 @@ if multishell then
     end
 
     function shell.switchTab( nID )
-        if type( nID ) ~= "number" then
+        expect(1, nID, "number")
-            error( "bad argument #1 (expected number, got " .. type( nID ) .. ")", 2 )
-        end
         multishell.setFocus( nID )
     end
 end

src/test/java/dan200/computercraft/core/computer/ComputerBootstrap.java

@@ -20,6 +20,7 @@ import org.junit.jupiter.api.Assertions;
 
 import javax.annotation.Nonnull;
 import javax.annotation.Nullable;
+import java.util.Arrays;
 import java.util.function.Consumer;
 
 /**
@@ -122,7 +123,7 @@ public class ComputerBootstrap
         @Override
         public String[] getMethodNames()
         {
-            return new String[] { "assert" };
+            return new String[] { "assert", "log" };
         }
 
         @Nullable
@@ -144,6 +145,9 @@ public class ComputerBootstrap
 
                     return arguments;
                 }
+                case 1:
+                    ComputerCraft.log.info( "[Computer] {}", Arrays.toString( arguments ) );
+                    return null;
 
                 default:
                     return null;

src/test/resources/test-rom/mcfly.lua

@@ -27,6 +27,43 @@ local function check(func, arg, ty, val)
     end
 end
 
+local active_stubs = {}
+
+--- Stub a global variable with a specific value
+--
+-- @tparam string var The variable to stub
+-- @param value The value to stub it with
+local function stub(var, value)
+    if not active_stubs[var] then
+        active_stubs[var] = { value = _G[var] }
+    end
+
+    _G[var] = value
+end
+
+
+--- Capture the current global state of the computer
+local function push_state()
+    local stubs = active_stubs
+    active_stubs = {}
+    return {
+        term = term.current(),
+        input = io.input(),
+        output = io.output(),
+        stubs = stubs,
+    }
+end
+
+--- Restore the global state of the computer to a previous version
+local function pop_state(state)
+    for k, v in pairs(active_stubs) do _G[k] = v.value end
+    active_stubs = state.stubs
+
+    term.redirect(state.term)
+    io.input(state.input)
+    io.output(state.output)
+end
+
 local error_mt = { __tostring = function(self) return self.message end }
 
 --- Attempt to execute the provided function, gathering a stack trace when it
@@ -50,10 +87,6 @@ local function try(fn)
         return { message = err, trace = debug.traceback(nil, 2) }
     end)
 
-    -- Restore a whole bunch of state
-    io.input(io.stdin)
-    io.output(io.stdout)
-
     -- If we succeeded, propagate it
     if ok then return ok, err end
 
@@ -196,13 +229,32 @@ function expect_mt:matches(value)
     return self
 end
 
+local expect = setmetatable( {
+    --- Construct an expectation on the error message calling this function
+    -- produces
+    --
+    -- @tparam fun The function to call
+    -- @param ... The function arguments
+    -- @return The new expectation
+    error = function(fun, ...)
+        local ok, res = pcall(fun, ...) local _, line = pcall(error, "", 2)
+        if ok then fail("expected function to error") end
+        if res:sub(1, #line) == line then
+            res = res:sub(#line + 1)
+        elseif res:sub(1, 7) == "pcall: " then
+            res = res:sub(8)
+        end
+        return setmetatable({ value = res }, expect_mt)
+    end,
+}, {
     --- Construct a new expectation from the provided value
     --
     -- @param value The value to apply assertions to
     -- @return The new expectation
-local function expect(value)
+    __call = function(_, value)
         return setmetatable({ value = value }, expect_mt)
     end
+})
 
 --- The stack of "describe"s.
 local test_stack = { n = 0 }
@@ -302,10 +354,15 @@ end
 
 do
     -- Load in the tests from all our files
-    local env = setmetatable({
+    local env = setmetatable({}, { __index = _ENV })
-        expect = expect, fail = fail,
+
-        describe = describe, it = it, pending = pending
+    local function set_env(tbl)
-    }, { __index = _ENV })
+        for k in pairs(env) do env[k] = nil end
+        for k, v in pairs(tbl) do env[k] = v end
+    end
+
+    -- When declaring tests, you shouldn't be able to use test methods
+    set_env { describe = describe, it = it, pending = pending }
 
     local suffix = "_spec.lua"
     local function run_in(sub_dir)
@@ -326,6 +383,9 @@ do
     end
 
     run_in(root_dir)
+
+    -- When running tests, you shouldn't be able to declare new ones.
+    set_env { expect = expect, fail = fail, stub = stub }
 end
 
 -- Error if we've found no tests
@@ -359,9 +419,13 @@ local function do_run(test)
         err = test.error
         status = "error"
     elseif test.action then
+        local state = push_state()
+
         local ok
         ok, err = try(test.action)
         status = ok and "pass" or (err.fail and "fail" or "error")
+
+        pop_state(state)
     end
 
     -- If we've a boolean status, then convert it into a string

src/test/resources/test-rom/spec/apis/colors_spec.lua

@@ -1,9 +1,24 @@
 describe("The colors library", function()
-    it("colors.combine", function()
+    describe("colors.combine", function()
-        expect(colors.combine(colors.red, colors.brown, colors.green)):equals(0x7000)
+        it("validates arguments", function()
+            -- FIXME: Error when last argument is nil - use table.pack instead.
+            expect.error(colors.combine, 1, false):eq("bad argument #2 (expected number, got boolean)")
+            expect.error(colors.combine, 1, 1, false):eq("bad argument #3 (expected number, got boolean)")
+        end)
+
+        it("combines colours", function()
+            expect(colors.combine()):eq(0)
+            expect(colors.combine(colors.red, colors.brown, colors.green)):eq(0x7000)
+        end)
     end)
 
     describe("colors.subtract", function()
+        it("validates arguments", function()
+            expect.error(colors.subtract, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(colors.subtract, 1, false):eq("bad argument #2 (expected number, got boolean)")
+            expect.error(colors.subtract, 1, 1, false):eq("bad argument #3 (expected number, got boolean)")
+        end)
+
         it("subtracts colours", function()
             expect(colors.subtract(0x7000, colors.green)):equals(0x5000)
             expect(colors.subtract(0x5000, colors.red)):equals(0x1000)
@@ -17,6 +32,11 @@ describe("The colors library", function()
     end)
 
     describe("colors.test", function()
+        it("validates arguments", function()
+            expect.error(colors.test, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(colors.test, 1, nil):eq("bad argument #2 (expected number, got nil)")
+        end)
+
         it("returns true when present", function()
             expect(colors.test(0x7000, colors.green)):equals(true)
         end)
@@ -28,13 +48,27 @@ describe("The colors library", function()
         end)
     end)
 
-    it("colors.packRGB", function()
+    describe("colors.packRGB", function()
-        expect(colors.packRGB(0.3, 0.5, 0.6)):equals(0x4c7f99)
+        it("validates arguments", function()
+            expect.error(colors.packRGB, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(colors.packRGB, 1, nil):eq("bad argument #2 (expected number, got nil)")
+            expect.error(colors.packRGB, 1, 1, nil):eq("bad argument #3 (expected number, got nil)")
         end)
 
-    it("colors.unpackRGB", function()
+        it("packs colours", function()
+            expect(colors.packRGB(0.3, 0.5, 0.6)):equals(0x4c7f99)
+        end)
+    end)
+
+    describe("colors.unpackRGB", function()
+        it("validates arguments", function()
+            expect.error(colors.unpackRGB, nil):eq("bad argument #1 (expected number, got nil)")
+        end)
+
+        it("unpacks colours", function()
             expect({ colors.unpackRGB(0x4c7f99) }):same { 0x4c / 0xFF, 0x7f / 0xFF, 0.6 }
         end)
+    end)
 
     it("colors.rgb8", function()
         expect(colors.rgb8(0.3, 0.5, 0.6)):equals(0x4c7f99)

src/test/resources/test-rom/spec/apis/fs_spec.lua

@@ -0,0 +1,14 @@
+describe("The fs library", function()
+    describe("fs.complete", function()
+        it("validates arguments", function()
+            fs.complete("", "")
+            fs.complete("", "", true)
+            fs.complete("", "", nil, true)
+
+            expect.error(fs.complete, nil):eq("bad argument #1 (expected string, got nil)")
+            expect.error(fs.complete, "", nil):eq("bad argument #2 (expected string, got nil)")
+            expect.error(fs.complete, "", "", 1):eq("bad argument #3 (expected boolean, got number)")
+            expect.error(fs.complete, "", "", true, 1):eq("bad argument #4 (expected boolean, got number)")
+        end)
+    end)
+end)

src/test/resources/test-rom/spec/apis/gps_spec.lua

@@ -0,0 +1,15 @@
+describe("The gps library", function()
+    describe("gps.locate", function()
+        it("validates arguments", function()
+            stub("commands", { getBlockPosition = function()
+            end })
+
+            gps.locate()
+            gps.locate(1)
+            gps.locate(1, true)
+
+            expect.error(gps.locate, ""):eq("bad argument #1 (expected number, got string)")
+            expect.error(gps.locate, 1, ""):eq("bad argument #2 (expected boolean, got string)")
+        end)
+    end)
+end)

src/test/resources/test-rom/spec/apis/help_spec.lua

@@ -0,0 +1,22 @@
+describe("The help library", function()
+    describe("help.setPath", function()
+        it("validates arguments", function()
+            help.setPath(help.path())
+            expect.error(help.setPath, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("help.lookup", function()
+        it("validates arguments", function()
+            help.lookup("")
+            expect.error(help.lookup, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("help.completeTopic", function()
+        it("validates arguments", function()
+            help.completeTopic("")
+            expect.error(help.completeTopic, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+end)

src/test/resources/test-rom/spec/apis/io_spec.lua

@@ -18,13 +18,32 @@ describe("The io library", function()
             expect(io.type(handle)):equals("file")
         end)
 
-        it("returns nil on values", function() expect(io.type(8)):equals(nil) end)
+        it("returns nil on values", function()
+            expect(io.type(8)):equals(nil)
+        end)
+
         it("returns nil on tables", function()
             expect(io.type(setmetatable({}, {}))):equals(nil)
         end)
     end)
 
+    describe("io.lines", function()
+        it("validates arguments", function()
+            io.lines(nil)
+            expect.error(io.lines, ""):eq("/: No such file")
+            expect.error(io.lines, false):eq("bad argument #1 (expected string, got boolean)")
+        end)
+    end)
+
     describe("io.open", function()
+        it("validates arguments", function()
+            io.open("")
+            io.open("", "r")
+
+            expect.error(io.open, nil):eq("bad argument #1 (expected string, got nil)")
+            expect.error(io.open, "", false):eq("bad argument #2 (expected string, got boolean)")
+        end)
+
         it("returns an error message on non-existent files", function()
             local a, b = io.open('xuxu_nao_existe')
             expect(a):equals(nil)

src/test/resources/test-rom/spec/apis/keys_spec.lua

@@ -0,0 +1,8 @@
+describe("The keys library", function()
+    describe("keys.getName", function()
+        it("validates arguments", function()
+            keys.getName(1)
+            expect.error(keys.getName, nil):eq("bad argument #1 (expected number, got nil)")
+        end)
+    end)
+end)

src/test/resources/test-rom/spec/apis/os_spec.lua

@@ -1,4 +1,4 @@
-describe("The os api", function()
+describe("The os library", function()
     describe("os.date and os.time", function()
         it("round trips correctly", function()
             local t = math.floor(os.epoch("local") / 1000)
@@ -118,4 +118,16 @@ describe("The os api", function()
             expect(t1 - t2):eq(60 * 2 - 2)
         end)
     end)
+
+    describe("os.loadAPI", function()
+        it("validates arguments", function()
+            expect.error(os.loadAPI, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("os.unloadAPI", function()
+        it("validates arguments", function()
+            expect.error(os.loadAPI, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
 end)

src/test/resources/test-rom/spec/apis/paintutils_spec.lua

@@ -0,0 +1,60 @@
+describe("The paintutils library", function()
+    describe("paintutils.parseImage", function()
+        it("validates arguments", function()
+            paintutils.parseImage("")
+            expect.error(paintutils.parseImage, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("paintutils.loadImage", function()
+        it("validates arguments", function()
+            expect.error(paintutils.loadImage, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("paintutils.drawPixel", function()
+        it("validates arguments", function()
+            expect.error(paintutils.drawPixel, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(paintutils.drawPixel, 1, nil):eq("bad argument #2 (expected number, got nil)")
+            expect.error(paintutils.drawPixel, 1, 1, false):eq("bad argument #3 (expected number, got boolean)")
+        end)
+    end)
+
+    describe("paintutils.drawLine", function()
+        it("validates arguments", function()
+            expect.error(paintutils.drawLine, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(paintutils.drawLine, 1, nil):eq("bad argument #2 (expected number, got nil)")
+            expect.error(paintutils.drawLine, 1, 1, nil):eq("bad argument #3 (expected number, got nil)")
+            expect.error(paintutils.drawLine, 1, 1, 1, nil):eq("bad argument #4 (expected number, got nil)")
+            expect.error(paintutils.drawLine, 1, 1, 1, 1, false):eq("bad argument #5 (expected number, got boolean)")
+        end)
+    end)
+
+    describe("paintutils.drawBox", function()
+        it("validates arguments", function()
+            expect.error(paintutils.drawBox, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(paintutils.drawBox, 1, nil):eq("bad argument #2 (expected number, got nil)")
+            expect.error(paintutils.drawBox, 1, 1, nil):eq("bad argument #3 (expected number, got nil)")
+            expect.error(paintutils.drawBox, 1, 1, 1, nil):eq("bad argument #4 (expected number, got nil)")
+            expect.error(paintutils.drawBox, 1, 1, 1, 1, false):eq("bad argument #5 (expected number, got boolean)")
+        end)
+    end)
+
+    describe("paintutils.drawFilledBox", function()
+        it("validates arguments", function()
+            expect.error(paintutils.drawFilledBox, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(paintutils.drawFilledBox, 1, nil):eq("bad argument #2 (expected number, got nil)")
+            expect.error(paintutils.drawFilledBox, 1, 1, nil):eq("bad argument #3 (expected number, got nil)")
+            expect.error(paintutils.drawFilledBox, 1, 1, 1, nil):eq("bad argument #4 (expected number, got nil)")
+            expect.error(paintutils.drawFilledBox, 1, 1, 1, 1, false):eq("bad argument #5 (expected number, got boolean)")
+        end)
+    end)
+
+    describe("paintutils.drawImage", function()
+        it("validates arguments", function()
+            expect.error(paintutils.drawImage, nil):eq("bad argument #1 (expected table, got nil)")
+            expect.error(paintutils.drawImage, {}, nil):eq("bad argument #2 (expected number, got nil)")
+            expect.error(paintutils.drawImage, {}, 1, nil):eq("bad argument #3 (expected number, got nil)")
+        end)
+    end)
+end)

src/test/resources/test-rom/spec/apis/peripheral_spec.lua

@@ -0,0 +1,47 @@
+describe("The peripheral library", function()
+    describe("peripheral.isPresent", function()
+        it("validates arguments", function()
+            peripheral.isPresent("")
+            expect.error(peripheral.isPresent, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("peripheral.getType", function()
+        it("validates arguments", function()
+            peripheral.getType("")
+            expect.error(peripheral.getType, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("peripheral.getMethods", function()
+        it("validates arguments", function()
+            peripheral.getMethods("")
+            expect.error(peripheral.getMethods, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("peripheral.call", function()
+        it("validates arguments", function()
+            peripheral.call("", "")
+            expect.error(peripheral.call, nil):eq("bad argument #1 (expected string, got nil)")
+            expect.error(peripheral.call, "", nil):eq("bad argument #2 (expected string, got nil)")
+        end)
+    end)
+
+    describe("peripheral.wrap", function()
+        it("validates arguments", function()
+            peripheral.wrap("")
+            expect.error(peripheral.wrap, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("peripheral.find", function()
+        it("validates arguments", function()
+            peripheral.find("")
+            peripheral.find("", function()
+            end)
+            expect.error(peripheral.find, nil):eq("bad argument #1 (expected string, got nil)")
+            expect.error(peripheral.find, "", false):eq("bad argument #2 (expected function, got boolean)")
+        end)
+    end)
+end)

src/test/resources/test-rom/spec/apis/rednet_spec.lua

@@ -0,0 +1,78 @@
+describe("The rednet library", function()
+    describe("rednet.open", function()
+        it("validates arguments", function()
+            expect.error(rednet.open, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+
+        it("requires a modem to be present", function()
+            expect.error(rednet.open, "not_there"):eq("No such modem: not_there")
+        end)
+    end)
+
+    describe("rednet.close", function()
+        it("validates arguments", function()
+            rednet.close()
+            expect.error(rednet.close, 1):eq("bad argument #1 (expected string, got number)")
+            expect.error(rednet.close, false):eq("bad argument #1 (expected string, got boolean)")
+        end)
+
+        it("requires a modem to be present", function()
+            expect.error(rednet.close, "not_there"):eq("No such modem: not_there")
+        end)
+    end)
+
+    describe("rednet.isOpen", function()
+        it("validates arguments", function()
+            rednet.isOpen()
+            rednet.isOpen("")
+            expect.error(rednet.isOpen, 1):eq("bad argument #1 (expected string, got number)")
+            expect.error(rednet.isOpen, false):eq("bad argument #1 (expected string, got boolean)")
+        end)
+    end)
+
+    describe("rednet.send", function()
+        it("validates arguments", function()
+            rednet.send(1)
+            rednet.send(1, nil, "")
+            expect.error(rednet.send, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(rednet.send, 1, nil, false):eq("bad argument #3 (expected string, got boolean)")
+        end)
+    end)
+
+    describe("rednet.broadcast", function()
+        it("validates arguments", function()
+            rednet.broadcast(nil)
+            rednet.broadcast(nil, "")
+            expect.error(rednet.broadcast, nil, false):eq("bad argument #2 (expected string, got boolean)")
+        end)
+    end)
+
+    describe("rednet.receive", function()
+        it("validates arguments", function()
+            expect.error(rednet.receive, false):eq("bad argument #1 (expected string, got boolean)")
+            expect.error(rednet.receive, "", false):eq("bad argument #2 (expected number, got boolean)")
+        end)
+    end)
+
+    describe("rednet.host", function()
+        it("validates arguments", function()
+            expect.error(rednet.host, "", "localhost"):eq("Reserved hostname")
+            expect.error(rednet.host, nil):eq("bad argument #1 (expected string, got nil)")
+            expect.error(rednet.host, "", nil):eq("bad argument #2 (expected string, got nil)")
+        end)
+    end)
+
+    describe("rednet.unhost", function()
+        it("validates arguments", function()
+            rednet.unhost("")
+            expect.error(rednet.unhost, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("rednet.lookup", function()
+        it("validates arguments", function()
+            expect.error(rednet.lookup, nil):eq("bad argument #1 (expected string, got nil)")
+            expect.error(rednet.lookup, "", false):eq("bad argument #2 (expected string, got boolean)")
+        end)
+    end)
+end)

src/test/resources/test-rom/spec/apis/settings_spec.lua

@@ -0,0 +1,43 @@
+describe("The settings library", function()
+    describe("settings.set", function()
+        it("validates arguments", function()
+            settings.set("test", 1)
+            settings.set("test", "")
+            settings.set("test", {})
+            settings.set("test", false)
+
+            expect.error(settings.set, nil):eq("bad argument #1 (expected string, got nil)")
+            expect.error(settings.set, "", nil):eq("bad argument #2 (expected number, string, boolean or table, got nil)")
+        end)
+
+        it("prevents storing unserialisable types", function()
+            expect.error(settings.set, "", { print }):eq("Cannot serialize type function")
+        end)
+    end)
+
+    describe("settings.get", function()
+        it("validates arguments", function()
+            settings.get("test")
+            expect.error(settings.get, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("settings.unset", function()
+        it("validates arguments", function()
+            settings.unset("test")
+            expect.error(settings.unset, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("settings.load", function()
+        it("validates arguments", function()
+            expect.error(settings.load, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("settings.save", function()
+        it("validates arguments", function()
+            expect.error(settings.save, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+end)

src/test/resources/test-rom/spec/apis/term_spec.lua

@@ -0,0 +1,12 @@
+describe("The term library", function()
+    describe("term.redirect", function()
+        it("validates arguments", function()
+            expect.error(term.redirect, nil):eq("bad argument #1 (expected table, got nil)")
+        end)
+
+        it("prevents redirecting to term", function()
+            expect.error(term.redirect, term)
+                  :eq("term is not a recommended redirect target, try term.current() instead")
+        end)
+    end)
+end)

src/test/resources/test-rom/spec/apis/textutils_spec.lua

@@ -0,0 +1,57 @@
+describe("The textutils library", function()
+    describe("textutils.slowWrite", function()
+        it("validates arguments", function()
+            expect.error(textutils.slowWrite, nil, false):eq("bad argument #2 (expected number, got boolean)")
+        end)
+    end)
+
+    describe("textutils.formatTime", function()
+        it("validates arguments", function()
+            textutils.formatTime(0)
+            textutils.formatTime(0, false)
+            expect.error(textutils.formatTime, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(textutils.formatTime, 1, 1):eq("bad argument #2 (expected boolean, got number)")
+        end)
+    end)
+
+    describe("textutils.pagedPrint", function()
+        it("validates arguments", function()
+            expect.error(textutils.pagedPrint, nil, false):eq("bad argument #2 (expected number, got boolean)")
+        end)
+    end)
+
+    describe("textutils.unserialise", function()
+        it("validates arguments", function()
+            textutils.unserialise("")
+            expect.error(textutils.unserialise, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("textutils.serialiseJSON", function()
+        it("validates arguments", function()
+            textutils.serialiseJSON("")
+            textutils.serialiseJSON(1)
+            textutils.serialiseJSON({})
+            textutils.serialiseJSON(false)
+            textutils.serialiseJSON("", true)
+            expect.error(textutils.serialiseJSON, nil):eq("bad argument #1 (expected table, string, number or boolean, got nil)")
+            expect.error(textutils.serialiseJSON, "", 1):eq("bad argument #2 (expected boolean, got number)")
+        end)
+    end)
+
+    describe("textutils.urlEncode", function()
+        it("validates arguments", function()
+            textutils.urlEncode("")
+            expect.error(textutils.urlEncode, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("textutils.complete", function()
+        it("validates arguments", function()
+            textutils.complete("pri")
+            textutils.complete("pri", _G)
+            expect.error(textutils.complete, nil):eq("bad argument #1 (expected string, got nil)")
+            expect.error(textutils.complete, "", false):eq("bad argument #2 (expected table, got boolean)")
+        end)
+    end)
+end)

src/test/resources/test-rom/spec/apis/window_spec.lua

@@ -0,0 +1,123 @@
+describe("The window library", function()
+    local function mk()
+        return window.create(term.current(), 1, 1, 5, 5, false)
+    end
+
+    describe("window.create", function()
+        it("validates arguments", function()
+            local r = mk()
+            window.create(r, 1, 1, 5, 5)
+            window.create(r, 1, 1, 5, 5, false)
+
+            expect.error(window.create, nil):eq("bad argument #1 (expected table, got nil)")
+            expect.error(window.create, r, nil):eq("bad argument #2 (expected number, got nil)")
+            expect.error(window.create, r, 1, nil):eq("bad argument #3 (expected number, got nil)")
+            expect.error(window.create, r, 1, 1, nil):eq("bad argument #4 (expected number, got nil)")
+            expect.error(window.create, r, 1, 1, 1, nil):eq("bad argument #5 (expected number, got nil)")
+            expect.error(window.create, r, 1, 1, 1, 1, ""):eq("bad argument #6 (expected boolean, got string)")
+        end)
+    end)
+
+    describe("Window.blit", function()
+        it("validates arguments", function()
+            local w = mk()
+            w.blit("a", "a", "a")
+
+            expect.error(w.blit, nil):eq("bad argument #1 (expected string, got nil)")
+            expect.error(w.blit, "", nil):eq("bad argument #2 (expected string, got nil)")
+            expect.error(w.blit, "", "", nil):eq("bad argument #3 (expected string, got nil)")
+            expect.error(w.blit, "", "", "a"):eq("Arguments must be the same length")
+        end)
+    end)
+
+    describe("Window.setCursorPos", function()
+        it("validates arguments", function()
+            local w = mk()
+            w.setCursorPos(1, 1)
+
+            expect.error(w.setCursorPos, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(w.setCursorPos, 1, nil):eq("bad argument #2 (expected number, got nil)")
+        end)
+    end)
+
+    describe("Window.setCursorBlink", function()
+        it("validates arguments", function()
+            local w = mk()
+            w.setCursorBlink(false)
+            expect.error(w.setCursorBlink, nil):eq("bad argument #1 (expected boolean, got nil)")
+        end)
+    end)
+
+    describe("Window.setTextColour", function()
+        it("validates arguments", function()
+            local w = mk()
+            w.setTextColour(colors.white)
+
+            expect.error(w.setTextColour, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(w.setTextColour, -5):eq("Invalid color (got -5)")
+        end)
+    end)
+
+    describe("Window.setPaletteColour", function()
+        it("validates arguments", function()
+            local w = mk()
+            w.setPaletteColour(colors.white, 0, 0, 0)
+            w.setPaletteColour(colors.white, 0x000000)
+
+            expect.error(w.setPaletteColour, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(w.setPaletteColour, -5):eq("Invalid color (got -5)")
+            expect.error(w.setPaletteColour, colors.white):eq("bad argument #2 (expected number, got nil)")
+            expect.error(w.setPaletteColour, colors.white, 1, false):eq("bad argument #3 (expected number, got boolean)")
+            expect.error(w.setPaletteColour, colors.white, 1, nil, 1):eq("bad argument #3 (expected number, got nil)")
+            expect.error(w.setPaletteColour, colors.white, 1, 1, nil):eq("bad argument #4 (expected number, got nil)")
+        end)
+    end)
+
+    describe("Window.getPaletteColour", function()
+        it("validates arguments", function()
+            local w = mk()
+            w.getPaletteColour(colors.white)
+            expect.error(w.getPaletteColour, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(w.getPaletteColour, -5):eq("Invalid color (got -5)")
+        end)
+    end)
+
+    describe("Window.setBackgroundColour", function()
+        it("validates arguments", function()
+            local w = mk()
+            w.setBackgroundColour(colors.white)
+
+            expect.error(w.setBackgroundColour, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(w.setBackgroundColour, -5):eq("Invalid color (got -5)")
+        end)
+    end)
+
+    describe("Window.scroll", function()
+        it("validates arguments", function()
+            local w = mk()
+            w.scroll(0)
+            expect.error(w.scroll, nil):eq("bad argument #1 (expected number, got nil)")
+        end)
+    end)
+
+    describe("Window.setVisible", function()
+        it("validates arguments", function()
+            local w = mk()
+            w.setVisible(false)
+            expect.error(w.setVisible, nil):eq("bad argument #1 (expected boolean, got nil)")
+        end)
+    end)
+
+    describe("Window.reposition", function()
+        it("validates arguments", function()
+            local w = mk()
+            w.reposition(1, 1)
+            w.reposition(1, 1, 5, 5)
+            expect.error(w.reposition, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(w.reposition, 1, nil):eq("bad argument #2 (expected number, got nil)")
+            expect.error(w.reposition, 1, 1, false, 1):eq("bad argument #3 (expected number, got boolean)")
+            expect.error(w.reposition, 1, 1, nil, 1):eq("bad argument #3 (expected number, got nil)")
+            expect.error(w.reposition, 1, 1, 1, nil):eq("bad argument #4 (expected number, got nil)")
+        end)
+    end)
+end)

src/test/resources/test-rom/spec/base_spec.lua

@@ -1,11 +1,74 @@
 describe("The Lua base library", function()
+    describe("expect", function()
+        local e = _G["~expect"]
+
+        it("checks a single type", function()
+            expect(e(1, "test", "string")):eq(true)
+            expect(e(1, 2, "number")):eq(true)
+
+            expect.error(e, 1, nil, "string"):eq("bad argument #1 (expected string, got nil)")
+            expect.error(e, 2, 1, "nil"):eq("bad argument #2 (expected nil, got number)")
+        end)
+
+        it("checks multiple types", function()
+            expect(e(1, "test", "string", "number")):eq(true)
+            expect(e(1, 2, "string", "number")):eq(true)
+
+            expect.error(e, 1, nil, "string", "number"):eq("bad argument #1 (expected string or number, got nil)")
+            expect.error(e, 2, false, "string", "table", "number", "nil")
+                  :eq("bad argument #2 (expected string, table or number, got boolean)")
+        end)
+
+        it("includes the function name", function()
+            local function worker()
+                expect(e(1, nil, "string")):eq(true)
+            end
+            local function trampoline()
+                worker()
+            end
+
+            expect.error(trampoline):eq("base_spec.lua:27: bad argument #1 to 'worker' (expected string, got nil)")
+        end)
+    end)
+
+    describe("sleep", function()
+        it("validates arguments", function()
+            sleep(0)
+            sleep(nil)
+
+            expect.error(sleep, false):eq("bad argument #1 (expected number, got boolean)")
+        end)
+    end)
+
+    describe("write", function()
+        it("validates arguments", function()
+            write("")
+            expect.error(write, nil):eq("bad argument #1 (expected string or number, got nil)")
+        end)
+    end)
+
     describe("loadfile", function()
+        it("validates arguments", function()
+            loadfile("")
+            loadfile("", {})
+
+            expect.error(loadfile, nil):eq("bad argument #1 (expected string, got nil)")
+            expect.error(loadfile, "", false):eq("bad argument #2 (expected table, got boolean)")
+        end)
+
         it("prefixes the filename with @", function()
             local info = debug.getinfo(loadfile("/rom/startup.lua"), "S")
             expect(info):matches { short_src = "startup.lua", source = "@startup.lua" }
         end)
     end)
 
+    describe("dofile", function()
+        it("validates arguments", function()
+            expect.error(dofile, ""):eq("File not found")
+            expect.error(dofile, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
     describe("loadstring", function()
         it("prefixes the chunk name with '='", function()
             local info = debug.getinfo(loadstring("return 1", "name"), "S")
@@ -27,9 +90,25 @@ describe("The Lua base library", function()
     end)
 
     describe("load", function()
+        it("validates arguments", function()
+            load("")
+            load(function()
+            end)
+            load("", "")
+            load("", "", "")
+            load("", "", "", _ENV)
+
+            expect.error(load, nil):eq("bad argument #1 (expected function or string, got nil)")
+            expect.error(load, "", false):eq("bad argument #2 (expected string, got boolean)")
+            expect.error(load, "", "", false):eq("bad argument #3 (expected string, got boolean)")
+            expect.error(load, "", "", "", false):eq("bad argument #4 (expected table, got boolean)")
+        end)
+
         local function generator(parts)
             return coroutine.wrap(function()
-                for i = 1, #parts do coroutine.yield(parts[i]) end
+                for i = 1, #parts do
+                    coroutine.yield(parts[i])
+                end
             end)
         end
 

src/test/resources/test-rom/spec/programs/advanced/multishell_spec.lua

@@ -0,0 +1,30 @@
+describe("The multishell program", function()
+    describe("multishell.setFocus", function()
+        it("validates arguments", function()
+            multishell.setFocus(multishell.getFocus())
+            expect.error(multishell.setFocus, nil):eq("bad argument #1 (expected number, got nil)")
+        end)
+    end)
+
+    describe("multishell.getTitle", function()
+        it("validates arguments", function()
+            multishell.getTitle(1)
+            expect.error(multishell.getTitle, nil):eq("bad argument #1 (expected number, got nil)")
+        end)
+    end)
+
+    describe("multishell.setTitle", function()
+        it("validates arguments", function()
+            multishell.setTitle(1, multishell.getTitle(1))
+            expect.error(multishell.setTitle, nil):eq("bad argument #1 (expected number, got nil)")
+            expect.error(multishell.setTitle, 1, nil):eq("bad argument #2 (expected string, got nil)")
+        end)
+    end)
+
+    describe("multishell.launch", function()
+        it("validates arguments", function()
+            expect.error(multishell.launch, nil):eq("bad argument #1 (expected table, got nil)")
+            expect.error(multishell.launch, _ENV, nil):eq("bad argument #2 (expected string, got nil)")
+        end)
+    end)
+end)

src/test/resources/test-rom/spec/programs/shell_spec.lua

@@ -1,4 +1,11 @@
 describe("The shell", function()
+    describe("require", function()
+        it("validates arguments", function()
+            require("math")
+            expect.error(require, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
     describe("shell.run", function()
         it("sets the arguments", function()
             local handle = fs.open("test-files/out.txt", "w")
@@ -14,4 +21,74 @@ describe("The shell", function()
             expect(args):same { [0] = "/test-files/out.txt", "arg1", "arg2" }
         end)
     end)
+
+    describe("shell.setDir", function()
+        it("validates arguments", function()
+            shell.setDir(shell.dir())
+            expect.error(shell.setDir, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("shell.setPath", function()
+        it("validates arguments", function()
+            shell.setPath(shell.path())
+            expect.error(shell.setPath, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("shell.resolve", function()
+        it("validates arguments", function()
+            shell.resolve("")
+            expect.error(shell.resolve, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("shell.resolveProgram", function()
+        it("validates arguments", function()
+            shell.resolveProgram("ls")
+            expect.error(shell.resolveProgram, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("shell.complete", function()
+        it("validates arguments", function()
+            shell.complete("ls")
+            expect.error(shell.complete, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("shell.setCompletionFunction", function()
+        it("validates arguments", function()
+            expect.error(shell.setCompletionFunction, nil):eq("bad argument #1 (expected string, got nil)")
+            expect.error(shell.setCompletionFunction, "", nil):eq("bad argument #2 (expected function, got nil)")
+        end)
+    end)
+
+    describe("shell.setCompletionFunction", function()
+        it("validates arguments", function()
+            expect.error(shell.setCompletionFunction, nil):eq("bad argument #1 (expected string, got nil)")
+            expect.error(shell.setCompletionFunction, "", nil):eq("bad argument #2 (expected function, got nil)")
+        end)
+    end)
+
+    describe("shell.setAlias", function()
+        it("validates arguments", function()
+            shell.setAlias("sl", "ls")
+            expect.error(shell.setAlias, nil):eq("bad argument #1 (expected string, got nil)")
+            expect.error(shell.setAlias, "", nil):eq("bad argument #2 (expected string, got nil)")
+        end)
+    end)
+
+    describe("shell.clearAlias", function()
+        it("validates arguments", function()
+            shell.clearAlias("sl")
+            expect.error(shell.clearAlias, nil):eq("bad argument #1 (expected string, got nil)")
+        end)
+    end)
+
+    describe("shell.switchTab", function()
+        it("validates arguments", function()
+            expect.error(shell.switchTab, nil):eq("bad argument #1 (expected number, got nil)")
+        end)
+    end)
 end)