Add utility to check table keys

expect.field acts very similarly to expect.expect, though checks a
specific table key rather than a function argument.

src/main/resources/assets/computercraft/lua/rom/modules/main/cc/expect.lua

@@ -5,28 +5,29 @@
 
 local native_select, native_type = select, type
 
---- Expect an argument to have a specific type.
---
--- @tparam number 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 function get_type_names(...)
     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(...)
+        return tostring(...)
     else
-        type_names = table.concat(types, ", ", 1, #types - 1) .. " or " .. types[#types]
+        return table.concat(types, ", ", 1, #types - 1) .. " or " .. types[#types]
+    end
+end
+--- Expect an argument to have a specific type.
+--
+-- @tparam number index The 1-based argument index.
+-- @param value The argument's value.
+-- @tparam string ... The allowed types of the argument.
+-- @return The given `value`.
+-- @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 value end
     end
 
     -- If we can determine the function name with a high level of confidence, try to include it.
@@ -36,6 +37,7 @@ local function expect(index, value, ...)
         if ok and info.name and #info.name ~= "" and info.what ~= "C" then name = info.name end
     end
 
+    local type_names = get_type_names(...)
     if name then
         error(("bad argument #%d to '%s' (expected %s, got %s)"):format(index, name, type_names, t), 3)
     else
@@ -43,4 +45,31 @@ local function expect(index, value, ...)
     end
 end
 
-return { expect = expect }
+--- Expect an field to have a specific type.
+--
+-- @tparam table tbl The table to index.
+-- @tparam string index The field name to check.
+-- @tparam string ... The allowed types of the argument.
+-- @return The contents of the given field.
+-- @throws If the field is not one of the allowed types.
+local function field(tbl, index, ...)
+    expect(1, tbl, "table")
+    expect(2, index, "string")
+
+    local value = tbl[index]
+    local t = native_type(value)
+    for i = 1, native_select("#", ...) do
+        if t == native_select(i, ...) then return value end
+    end
+
+    if value == nil then
+        error(("field '%s' missing from table"):format(index), 3)
+    else
+        error(("bad field '%s' (expected %s, got %s)"):format(index, get_type_names(...), t), 3)
+    end
+end
+
+return {
+    expect = expect,
+    field = field,
+}

src/test/resources/test-rom/spec/modules/cc/expect_spec.lua

@@ -1,17 +1,18 @@
 describe("cc.expect", function()
     local e = require("cc.expect")
 
+    describe("expect", function()
         it("checks a single type", function()
-        expect(e.expect(1, "test", "string")):eq(true)
-        expect(e.expect(1, 2, "number")):eq(true)
+            expect(e.expect(1, "test", "string")):eq("test")
+            expect(e.expect(1, 2, "number")):eq(2)
 
             expect.error(e.expect, 1, nil, "string"):eq("bad argument #1 (expected string, got nil)")
             expect.error(e.expect, 2, 1, "nil"):eq("bad argument #2 (expected nil, got number)")
         end)
 
         it("checks multiple types", function()
-        expect(e.expect(1, "test", "string", "number")):eq(true)
-        expect(e.expect(1, 2, "string", "number")):eq(true)
+            expect(e.expect(1, "test", "string", "number")):eq("test")
+            expect(e.expect(1, 2, "string", "number")):eq(2)
 
             expect.error(e.expect, 1, nil, "string", "number"):eq("bad argument #1 (expected string or number, got nil)")
             expect.error(e.expect, 2, false, "string", "table", "number", "nil")
@@ -26,6 +27,27 @@ describe("cc.expect", function()
                 worker()
             end
 
-        expect.error(trampoline):eq("expect_spec.lua:26: bad argument #1 to 'worker' (expected string, got nil)")
+            expect.error(trampoline):eq("expect_spec.lua:27: bad argument #1 to 'worker' (expected string, got nil)")
+        end)
+    end)
+
+    describe("field", function()
+        it("checks a single type", function()
+            expect(e.field({ k = "test" }, "k", "string")):eq("test")
+            expect(e.field({ k = 2 }, "k", "number")):eq(2)
+
+            expect.error(e.field, { k = nil }, "k", "string"):eq("field 'k' missing from table")
+            expect.error(e.field, { l = 1 }, "l", "nil"):eq("bad field 'l' (expected nil, got number)")
+        end)
+
+        it("checks multiple types", function()
+            expect(e.field({ k = "test" }, "k", "string", "number")):eq("test")
+            expect(e.field({ k = 2 }, "k", "string", "number")):eq(2)
+
+            expect.error(e.field, { k = nil }, "k", "string", "number")
+                :eq("field 'k' missing from table")
+            expect.error(e.field, { l = false }, "l", "string", "table", "number", "nil")
+                :eq("bad field 'l' (expected string, table or number, got boolean)")
+        end)
     end)
 end)