Support arrow functions (=>)

spec/giavascript_spec.cr

@@ -428,6 +428,64 @@ describe GiavaScript do
     interpreter.eval("fact;").should eq(["Error: variable 'fact' does not exist"])
   end
 
+  it "supports no-parameter arrow with expression body" do
+    interpreter = GiavaScript::Interpreter.new
+    interpreter.eval("(() => 42)();").should eq(["42"])
+  end
+
+  it "supports single-parameter arrow without parens with expression body" do
+    interpreter = GiavaScript::Interpreter.new
+    interpreter.eval("var double = x => x * 2;").should eq([] of String)
+    interpreter.eval("double(5);").should eq(["10"])
+  end
+
+  it "supports multi-parameter arrow with expression body" do
+    interpreter = GiavaScript::Interpreter.new
+    interpreter.eval("var sum = (a, b) => a + b;").should eq([] of String)
+    interpreter.eval("sum(2, 3);").should eq(["5"])
+  end
+
+  it "supports no-parameter arrow with block body" do
+    interpreter = GiavaScript::Interpreter.new
+    interpreter.eval("var fn = () => { return 42; };").should eq([] of String)
+    interpreter.eval("fn();").should eq(["42"])
+  end
+
+  it "supports single-parameter arrow with block body" do
+    interpreter = GiavaScript::Interpreter.new
+    interpreter.eval("var triple = x => { return x * 3; };").should eq([] of String)
+    interpreter.eval("triple(4);").should eq(["12"])
+  end
+
+  it "supports multi-parameter arrow with block body" do
+    interpreter = GiavaScript::Interpreter.new
+    interpreter.eval("var multiply = (a, b) => { return a * b; };").should eq([] of String)
+    interpreter.eval("multiply(4, 5);").should eq(["20"])
+  end
+
+  it "supports immediately-invoked arrow function" do
+    interpreter = GiavaScript::Interpreter.new
+    interpreter.eval("((a, b) => a + b)(4, 6);").should eq(["10"])
+  end
+
+  it "returns function for typeof arrow function" do
+    interpreter = GiavaScript::Interpreter.new
+    interpreter.eval("var fn = () => 1;").should eq([] of String)
+    interpreter.eval("typeof fn;").should eq(["\"function\""])
+  end
+
+  it "supports arrow with implicit return of empty block" do
+    interpreter = GiavaScript::Interpreter.new
+    interpreter.eval("var fn = () => {};").should eq([] of String)
+    interpreter.eval("fn();").should eq(["undefined"])
+  end
+
+  it "supports arrow functions returning expressions with operators" do
+    interpreter = GiavaScript::Interpreter.new
+    interpreter.eval("var fn = (a, b) => a * b + 1;").should eq([] of String)
+    interpreter.eval("fn(3, 4);").should eq(["13"])
+  end
+
   it "prints error when print is not defined" do
     interpreter = GiavaScript::Interpreter.new
     interpreter.eval("print(\"hello world\");").should eq(["Error: function 'print' does not exist"])

src/giavascript/ast.cr

@@ -66,6 +66,14 @@ module GiavaScript
     end
   end
 
+  class ArrowFunctionExpr < Expr
+    getter parameters : Array(String)
+    getter body_source : String
+
+    def initialize(@parameters : Array(String), @body_source : String)
+    end
+  end
+
   class ArrayLiteral < Expr
     getter elements : Array(Expr)
 

src/giavascript/expression_evaluator.cr

@@ -38,6 +38,8 @@ module GiavaScript
         evaluate_new_expression(expr)
       when FunctionExpr
         UserFunction.new(expr.name, expr.parameters, expr.body_source, @env)
+      when ArrowFunctionExpr
+        UserFunction.new(nil, expr.parameters, expr.body_source, @env)
       when ArrayLiteral
         values = Array(Value).new(expr.elements.size)
         expr.elements.each do |element|

src/giavascript/expression_parser.cr

@@ -196,6 +196,9 @@ module GiavaScript
     private def parse_primary : Expr
       case @current.kind
       when Tokenizer::TokenKind::LParen
+        parsed_arrow = try_parse_paren_arrow_function
+        return parsed_arrow if parsed_arrow
+
         advance_token
         value = parse_expression
         raise invalid_rhs_error unless @current.kind == Tokenizer::TokenKind::RParen
@@ -226,6 +229,8 @@ module GiavaScript
         advance_token
         LiteralExpr.new(parse_number_value(number_lexeme))
       when Tokenizer::TokenKind::Identifier
+        parsed_arrow = try_parse_identifier_arrow_function
+        return parsed_arrow if parsed_arrow
         parse_identifier_expression
       else
         raise invalid_rhs_error
@@ -348,6 +353,96 @@ module GiavaScript
       FunctionExpr.new(function_name, parameters, body_source)
     end
 
+    private def try_parse_paren_arrow_function : ArrowFunctionExpr?
+      return nil unless @current.kind == Tokenizer::TokenKind::LParen
+
+      saved_cursor = @tokenizer.cursor
+      saved_token = @current
+
+      advance_token
+
+      parameters = [] of String
+
+      if @current.kind == Tokenizer::TokenKind::RParen
+        advance_token
+        if @current.kind == Tokenizer::TokenKind::Arrow
+          advance_token
+          return parse_arrow_body(parameters)
+        end
+      elsif @current.kind == Tokenizer::TokenKind::Identifier
+        loop do
+          param = @current.lexeme
+          return restore_and_nil(saved_cursor, saved_token) if parameters.includes?(param)
+          parameters << param
+          advance_token
+
+          if @current.kind == Tokenizer::TokenKind::Comma
+            advance_token
+            return restore_and_nil(saved_cursor, saved_token) unless @current.kind == Tokenizer::TokenKind::Identifier
+            next
+          end
+
+          break
+        end
+
+        if @current.kind == Tokenizer::TokenKind::RParen
+          advance_token
+          if @current.kind == Tokenizer::TokenKind::Arrow
+            advance_token
+            return parse_arrow_body(parameters)
+          end
+        end
+      end
+
+      @tokenizer.cursor = saved_cursor
+      @current = saved_token
+      nil
+    end
+
+    private def try_parse_identifier_arrow_function : ArrowFunctionExpr?
+      return nil unless @current.kind == Tokenizer::TokenKind::Identifier
+
+      saved_cursor = @tokenizer.cursor
+      saved_token = @current
+
+      param = @current.lexeme
+      advance_token
+
+      if @current.kind == Tokenizer::TokenKind::Arrow
+        advance_token
+        return parse_arrow_body([param])
+      end
+
+      @tokenizer.cursor = saved_cursor
+      @current = saved_token
+      nil
+    end
+
+    private def restore_and_nil(saved_cursor : Int32, saved_token : Tokenizer::Token) : Nil
+      @tokenizer.cursor = saved_cursor
+      @current = saved_token
+      nil
+    end
+
+    private def parse_arrow_body(parameters : Array(String)) : ArrowFunctionExpr
+      if @current.kind == Tokenizer::TokenKind::LBrace
+        body_start = @tokenizer.cursor
+        body_end = find_matching_brace_end_index(body_start)
+        body_source = @source[body_start...body_end]
+
+        @tokenizer.cursor = body_end + 1
+        advance_token
+
+        return ArrowFunctionExpr.new(parameters, body_source)
+      end
+
+      body_start = @tokenizer.cursor - @current.lexeme.size
+      parse_expression
+      body_end = @tokenizer.cursor - @current.lexeme.size
+      body_source = "return " + @source[body_start...body_end].strip + ";"
+      ArrowFunctionExpr.new(parameters, body_source)
+    end
+
     private def parse_identifier_expression : Expr
       identifier = @current.lexeme
       advance_token

src/giavascript/tokenizer.cr

@@ -17,6 +17,7 @@ module GiavaScript
       Void
       New
       Function
+      Arrow
       Plus
       Minus
       Star
@@ -96,7 +97,10 @@ module GiavaScript
         end
       when '='
         advance
-        if current_char == '='
+        if current_char == '>'
+          advance
+          Token.new(TokenKind::Arrow, "=>")
+        elsif current_char == '='
           advance
           if current_char == '='
             advance