Evaluate function literals and function calls.

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

pkg/evaluator/evaluator.go

@@ -59,6 +59,20 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 		env.Set(node.Name.Value, val)
 	case *ast.Identifier:
 		return evalIdentifier(node, env)
+	case *ast.FunctionLiteral:
+		params := node.Parameters
+		body := node.Body
+		return &object.Function{Parameters: params, Body: body, Env: env}
+	case *ast.CallExpression:
+		fn := Eval(node.Function, env)
+		if isError(fn) {
+			return fn
+		}
+		args := evalExpressions(node.Arguments, env)
+		if len(args) == 1 && isError(args[0]) {
+			return args[0]
+		}
+		return applyFunction(fn, args)
 	}
 	return nil
 }
@@ -184,6 +198,46 @@ func evalIdentifier(exp *ast.Identifier, env *object.Environment) object.Object
 	return val
 }
 
+func evalExpressions(
+	exps []ast.Expression,
+	env *object.Environment,
+) []object.Object {
+	var res []object.Object
+	for _, exp := range exps {
+		ev := Eval(exp, env)
+		if isError(ev) {
+			return []object.Object{ev}
+		}
+		res = append(res, ev)
+	}
+	return res
+}
+
+func applyFunction(fnObj object.Object, args []object.Object) object.Object {
+	fn, ok := fnObj.(*object.Function)
+	if !ok {
+		return newError("not a function: %s", fn.Type())
+	}
+	env := extendFunctionEnv(fn, args)
+	ret := Eval(fn.Body, env)
+	return unwrapReturnValue(ret)
+}
+
+func extendFunctionEnv(fn *object.Function, args []object.Object) *object.Environment {
+	env := object.NewEnclosedEnvironment(fn.Env)
+	for pi, param := range fn.Parameters {
+		env.Set(param.Value, args[pi])
+	}
+	return env
+}
+
+func unwrapReturnValue(obj object.Object) object.Object {
+	if ret, ok := obj.(*object.ReturnValue); ok {
+		return ret.Value
+	}
+	return obj
+}
+
 func isTruthy(obj object.Object) bool {
 	switch obj {
 	case _TRUE: