feat: expose Crystal procs as Lua global functions

README.md

@@ -70,6 +70,33 @@ p sum.as(Lua::Function).call(3.2, 1) # => 4.2
 lua.close
 ```
 
+You can also expose Crystal procs to Lua as global functions. Argument and
+return types are taken from the proc's signature, so values flow naturally
+between the two languages:
+
+```crystal
+lua = Lua.load
+
+lua.function "add", ->(x : Float64, y : Float64) { x + y }
+lua.run "return add(3, 4)" # => 7.0
+
+# closures capture local Crystal state
+counter = 0
+lua.function "tick", -> { counter += 1; nil }
+lua.run "tick(); tick()"
+counter # => 2
+
+# method pointers work too
+def greet(name : String)
+  "Hi, #{name}"
+end
+
+lua.function "greet", ->greet(String)
+lua.run "return greet('Lua')" # => "Hi, Lua"
+
+lua.close
+```
+
 More features coming soon. Try it, that's fun :)
 
 ## Contributing

examples/crystal_callback.cr

@@ -0,0 +1,34 @@
+require "../src/lua"
+
+# crystal run examples/crystal_callback.cr
+
+lua = Lua.load
+
+# 1. inline proc literal
+lua.function "add", ->(x : Float64, y : Float64) { x + y }
+puts lua.run "return add(3, 4)" # => 7.0
+
+# 2. closure capturing local Crystal state
+counter = 0
+lua.function "tick", -> { counter += 1; nil }
+lua.run "tick(); tick(); tick()"
+puts "counter = #{counter}" # => counter = 3
+
+# 3. method pointer
+def greet(name : String)
+  "Hi, #{name}"
+end
+
+lua.function "greet", ->greet(String)
+puts lua.run "return greet('Lua')" # => Hi, Lua
+
+# 4. mixed types, used inside a larger Lua expression
+lua.function "join", ->(prefix : String, count : Int32, ok : Bool) do
+  "#{prefix}:#{count}:#{ok}"
+end
+puts lua.run %q{
+  local label = join("requests", 5, true)
+  return label .. " (" .. tostring(add(10, 20)) .. ")"
+} # => requests:5:true (30.0)
+
+lua.close

spec/lua/stack/function_support_spec.cr

@@ -0,0 +1,77 @@
+require "../../spec_helper"
+
+private def greet(name : String)
+  "Hi, #{name}"
+end
+
+module Lua::StackMixin
+  describe FunctionSupport do
+    describe "#function" do
+      it "registers a proc taking no arguments" do
+        s = Stack.new
+        s.function "answer", -> { 42.0 }
+        s.run("return answer()").should eq 42.0
+      end
+
+      it "registers a proc with typed numeric arguments" do
+        s = Stack.new
+        s.function "add", ->(x : Float64, y : Float64) { x + y }
+        s.run("return add(3, 4)").should eq 7.0
+      end
+
+      it "converts integer-typed arguments" do
+        s = Stack.new
+        s.function "mul", ->(x : Int32, y : Int32) { x * y }
+        s.run("return mul(6, 7)").should eq 42.0
+      end
+
+      it "accepts string arguments and returns a string" do
+        s = Stack.new
+        s.function "shout", ->(text : String) { text.upcase }
+        s.run("return shout('hello')").should eq "HELLO"
+      end
+
+      it "captures local Crystal state via closure" do
+        s = Stack.new
+        counter = 0
+        s.function "tick", -> { counter += 1; nil }
+        s.run "tick(); tick(); tick()"
+        counter.should eq 3
+      end
+
+      it "registers a method pointer" do
+        s = Stack.new
+        s.function "greet", ->greet(String)
+        s.run("return greet('Lua')").should eq "Hi, Lua"
+      end
+
+      it "returns no Lua value when the proc returns Nil" do
+        s = Stack.new
+        s.function "noop", -> { nil }
+        s.run("return noop()").should be_nil
+      end
+
+      it "is callable inside larger Lua expressions" do
+        s = Stack.new
+        s.function "double", ->(x : Float64) { x * 2 }
+        s.run("return 100 * double(3)").should eq 600.0
+      end
+
+      it "raises when Lua passes fewer arguments than expected" do
+        s = Stack.new
+        s.function "add", ->(x : Float64, y : Float64) { x + y }
+        expect_raises(ArgumentError, /add.*expects 2/) do
+          s.run "return add(1)"
+        end
+      end
+
+      it "supports several mixed-type arguments" do
+        s = Stack.new
+        s.function "join", ->(prefix : String, count : Int32, ok : Bool) do
+          "#{prefix}:#{count}:#{ok}"
+        end
+        s.run("return join('n', 5, true)").should eq "n:5:true"
+      end
+    end
+  end
+end

src/lua/stack.cr

@@ -11,6 +11,7 @@ module Lua
     include StackMixin::CoroutineSupport
     include StackMixin::StandardLibraries
     include StackMixin::ClassSupport
+    include StackMixin::FunctionSupport
 
     getter state
     getter libs = Set(Symbol).new

src/lua/stack/function_support.cr

@@ -0,0 +1,58 @@
+module Lua
+  module StackMixin::FunctionSupport
+    # Registers a Crystal `Proc` as a global Lua function under the given name.
+    #
+    # Argument types come from the proc's signature: each Lua argument is
+    # converted to the matching Crystal type before the proc is called, and
+    # the proc's return value is pushed back onto the Lua stack. A proc
+    # returning `Nil` produces no Lua return value.
+    #
+    # ```
+    # lua = Lua.load
+    #
+    # # inline proc literal
+    # lua.function "add", ->(x : Float64, y : Float64) { x + y }
+    # lua.run "return add(3, 4)" # => 7.0
+    #
+    # # closure over local state
+    # counter = 0
+    # lua.function "tick", -> { counter += 1; nil }
+    # lua.run "tick(); tick()"
+    # counter # => 2
+    #
+    # # method pointer
+    # def greet(name : String)
+    #   "Hi, #{name}"
+    # end
+    #
+    # lua.function "greet", ->greet(String)
+    # lua.run "return greet('Lua')" # => "Hi, Lua"
+    # ```
+    def function(name : String, proc : Proc(*Args, R)) forall Args, R
+      libs_snapshot = libs.to_a
+      arity = {{ Args.type_vars.size }}
+      wrapper = ->(state : LibLua::State) do
+        if LibLua.gettop(state) < arity
+          raise ArgumentError.new("'#{name}' expects #{arity} argument(s)")
+        end
+        stack = Lua::Stack.new(state, libs_snapshot)
+        {% if Args.type_vars.empty? %}
+          result = proc.call
+        {% else %}
+          result = proc.call(
+            {% for type, i in Args.type_vars %}
+              ::LuaCallable::LuaConvert({{type}}).convert(stack[{{i + 1}}]),
+            {% end %}
+          )
+        {% end %}
+        {% if R.resolve == Nil %}
+          0
+        {% else %}
+          stack << result
+          1
+        {% end %}
+      end
+      set_global(name, wrapper)
+    end
+  end
+end