Allow returning lua functions

Not sure how this will play with persistence when it happens (badly,
most likely), but it's not a bad idea to support it.

Closes #466

src/main/java/dan200/computercraft/api/lua/ILuaFunction.java

@@ -0,0 +1,29 @@
+/*
+ * This file is part of the public ComputerCraft API - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2020. This API may be redistributed unmodified and in full only.
+ * For help using the API, and posting your mods, visit the forums at computercraft.info.
+ */
+package dan200.computercraft.api.lua;
+
+import javax.annotation.Nonnull;
+
+/**
+ * A function, which can be called from Lua. If you need to return a table of functions, it is recommended to use
+ * an object with {@link LuaFunction} methods, or implement {@link IDynamicLuaObject}.
+ *
+ * @see MethodResult#of(Object)
+ */
+@FunctionalInterface
+public interface ILuaFunction
+{
+    /**
+     * Call this function with a series of arguments. Note, this will <em>always</em> be called on the computer thread,
+     * and so its implementation must be thread-safe.
+     *
+     * @param arguments The arguments for this function
+     * @return The result of calling this function.
+     * @throws LuaException Upon Lua errors.
+     */
+    @Nonnull
+    MethodResult call( @Nonnull IArguments arguments ) throws LuaException;
+}

src/main/java/dan200/computercraft/api/lua/MethodResult.java

@@ -58,7 +58,7 @@ public final class MethodResult
      *
      * Integers, doubles, floats, strings, booleans, {@link Map}, {@link Collection}s, arrays and {@code null} will be
      * converted to their corresponding Lua type. {@code byte[]} and {@link ByteBuffer} will be treated as binary
-     * strings.
+     * strings. {@link ILuaFunction} will be treated as a function.
      *
      * In order to provide a custom object with methods, one may return a {@link IDynamicLuaObject}, or an arbitrary
      * class with {@link LuaFunction} annotations. Anything else will be converted to {@code nil}.

src/main/java/dan200/computercraft/core/lua/CobaltLuaMachine.java

@@ -48,6 +48,8 @@ public class CobaltLuaMachine implements ILuaMachine
         ThreadUtils.factory( "Coroutine" )
     );
 
+    private static final LuaMethod FUNCTION_METHOD = ( target, context, args ) -> ((ILuaFunction) target).call( args );
+
     private final Computer m_computer;
     private final TimeoutState timeout;
     private final TimeoutDebugHandler debug;
@@ -275,6 +277,11 @@ public class CobaltLuaMachine implements ILuaMachine
         LuaValue result = values.get( object );
         if( result != null ) return result;
 
+        if( object instanceof ILuaFunction )
+        {
+            return new ResultInterpreterFunction( this, FUNCTION_METHOD, object, context, object.toString() );
+        }
+
         if( object instanceof IDynamicLuaObject )
         {
             LuaValue wrapped = wrapLuaObject( object );

src/test/java/dan200/computercraft/core/asm/MethodTest.java

@@ -87,6 +87,14 @@ public class MethodTest
             x -> x.addApi( new ManyMethods() ), 50 );
     }
 
+    @Test
+    public void testFunction()
+    {
+        ComputerBootstrap.run(
+            "assert(func.call()(123) == 123)",
+            x -> x.addApi( new ReturnFunction() ), 50 );
+    }
+
     public static class MainThread implements ILuaAPI, IPeripheral
     {
         public final String thread = Thread.currentThread().getName();
@@ -239,4 +247,19 @@ public class MethodTest
             return new String[] { "many" };
         }
     }
+
+    public static class ReturnFunction implements ILuaAPI
+    {
+        @LuaFunction
+        public final ILuaFunction call()
+        {
+            return args -> MethodResult.of( args.getAll() );
+        }
+
+        @Override
+        public String[] getNames()
+        {
+            return new String[] { "func" };
+        }
+    }
 }