monna/evaluator/evaluator.go

package evaluator

import (
	"fmt"
	"monna/ast"
	"monna/object"
)

var (
	NULL  = &object.Null{}
	TRUE  = &object.Boolean{Value: true}
	FALSE = &object.Boolean{Value: false}
)

func Eval(node ast.Node, env *object.Environment) object.Object {
	switch node := node.(type) {
	// Statements
	case *ast.Program:
		return eval_program(node, env)

	case *ast.BlockStatement:
		return eval_block_statement(node, env)

	case *ast.ExpressionStatement:
		return Eval(node.Expression, env)

	case *ast.ReturnStatement:
		val := Eval(node.ReturnValue, env)
		if is_error(val) {
			return val
		}
		return &object.ReturnValue{Value: val}

	case *ast.LetStatement:
		val := Eval(node.Value, env)
		if is_error(val) {
			return val
		}
		env.Set(node.Name.Value, val)

		// Expressions
	case *ast.IntegerLiteral:
		return &object.Integer{Value: node.Value}

	case *ast.Boolean:
		return native_bool_to_boolean_object(node.Value)

	case *ast.PrefixExpression:
		right := Eval(node.Right, env)
		if is_error(right) {
			return right
		}
		return eval_prefix_expression(node.Operator, right)

	case *ast.InfixExpression:
		left := Eval(node.Left, env)
		if is_error(left) {
			return left
		}

		right := Eval(node.Right, env)
		if is_error(right) {
			return right
		}
		return eval_infix_expression(node.Operator, left, right)

	case *ast.IfExpression:
		return eval_if_expression(node, env)

	case *ast.Identifier:
		return eval_identifier(node, env)

	case *ast.FunctionLiteral:
		params := node.Parameters
		body := node.Body
		return &object.Function{Parameters: params, Env: env, Body: body}

	case *ast.CallExpression:
		function := Eval(node.Function, env)
		if is_error(function) {
			return function
		}
		args := eval_expression(node.Arguments, env)
		if len(args) == 1 && is_error(args[0]) {
			return args[0]
		}
		return apply_function(function, args)

	case *ast.StringLiteral:
		return &object.String{Value: node.Value}
	}

	return nil
}

func eval_program(program *ast.Program, env *object.Environment) object.Object {
	var result object.Object

	for _, statement := range program.Statements {
		result = Eval(statement, env)

		switch result := result.(type) {
		case *object.ReturnValue:
			return result.Value

		case *object.Error:
			return result
		}
	}
	return result
}

func apply_function(fn object.Object, args []object.Object) object.Object {
	switch fn := fn.(type) {
	case *object.Function:
		extended_env := extend_function_env(fn, args)
		evaluated := Eval(fn.Body, extended_env)
		return unwrap_return_value(evaluated)

	case *object.Builtin:
		return fn.Fn(args...)

	default:
		return new_error("not a funciton: %s", fn.Type())
	}

}

func extend_function_env(fn *object.Function, args []object.Object) *object.Environment {
	env := object.NewEnclosedEnvironment(fn.Env)

	for param_index, param := range fn.Parameters {
		env.Set(param.Value, args[param_index])
	}
	return env
}

func unwrap_return_value(obj object.Object) object.Object {
	if return_value, ok := obj.(*object.ReturnValue); ok {
		return return_value.Value
	}
	return obj
}

func eval_expression(expressions []ast.Expression, env *object.Environment) []object.Object {
	var result []object.Object

	for _, e := range expressions {
		evaluated := Eval(e, env)
		if is_error(evaluated) {
			return []object.Object{evaluated}
		}
		result = append(result, evaluated)
	}
	return result
}

func eval_identifier(node *ast.Identifier, env *object.Environment) object.Object {
	if val, ok := env.Get(node.Value); ok {
		return val
	}
	if builtin, ok := builtins[node.Value]; ok {
		return builtin
	}

	return new_error("identifier not found: " + node.Value)
}

func eval_block_statement(block *ast.BlockStatement, env *object.Environment) object.Object {
	var result object.Object

	for _, statement := range block.Statements {
		result = Eval(statement, env)

		if result != nil {
			rt := result.Type()
			if rt == object.RETURN_VALUE_OBJECT || rt == object.ERROR_OBJECT {
				return result
			}
		}
	}

	return result
}

func native_bool_to_boolean_object(input bool) *object.Boolean {
	if input {
		return TRUE
	}
	return FALSE
}

func eval_prefix_expression(operator string, right object.Object) object.Object {
	switch operator {
	case "!":
		return eval_bang_operator_expression(right)
	case "-":
		return eval_minus_prefix_operator_expression(right)

	default:
		return new_error("unknown operator: %s%s", operator, right.Type())
	}
}

func eval_bang_operator_expression(right object.Object) object.Object {
	switch right {
	case TRUE:
		return FALSE

	case FALSE:
		return TRUE

	case NULL:
		return TRUE

	default:
		return FALSE
	}
}

func eval_minus_prefix_operator_expression(right object.Object) object.Object {
	if right.Type() != object.INTEGER_OBJECT {
		return new_error("unknown operator: -%s", right.Type())
	}
	value := right.(*object.Integer).Value
	return &object.Integer{Value: -value}
}

func eval_infix_expression(operator string, left object.Object, right object.Object) object.Object {
	switch {
	case left.Type() == object.INTEGER_OBJECT && right.Type() == object.INTEGER_OBJECT:
		return eval_integer_infix_expression(operator, left, right)

	case operator == "==":
		return native_bool_to_boolean_object(left == right)

	case operator == "!=":
		return native_bool_to_boolean_object(left != right)

	case left.Type() != right.Type():
		return new_error("type mismatch: %s %s %s", left.Type(), operator, right.Type())

	case left.Type() == object.STRING_OBJECT && right.Type() == object.STRING_OBJECT:
		return eval_string_infix_expression(operator, left, right)
	default:
		return new_error("unknown operator: %s %s %s", left.Type(), operator, right.Type())
	}
}

func eval_integer_infix_expression(operator string, left object.Object, right object.Object) object.Object {
	left_value := left.(*object.Integer).Value
	right_value := right.(*object.Integer).Value

	switch operator {
	case "+":
		return &object.Integer{Value: left_value + right_value}

	case "-":
		return &object.Integer{Value: left_value - right_value}

	case "*":
		return &object.Integer{Value: left_value * right_value}

	case "/":
		return &object.Integer{Value: left_value / right_value}

	case "<":
		return native_bool_to_boolean_object(left_value < right_value)

	case ">":
		return native_bool_to_boolean_object(left_value > right_value)

	case "==":
		return native_bool_to_boolean_object(left_value == right_value)

	case "!=":
		return native_bool_to_boolean_object(left_value != right_value)

	default:
		return new_error("unknown operator: %s %s %s", left.Type(), operator, right.Type())
	}
}

func eval_if_expression(ie *ast.IfExpression, env *object.Environment) object.Object {
	condition := Eval(ie.Condition, env)

	if is_error(condition) {
		return condition
	}

	if is_truthy(condition) {
		return Eval(ie.Consequence, env)
	} else if ie.Alternative != nil {
		return Eval(ie.Alternative, env)
	} else {
		return NULL
	}
}

func eval_string_infix_expression(operator string, left, right object.Object) object.Object {
	if operator != "+" {
		return new_error("unknown operator: %s %s %s", left.Type(), operator, right.Type())
	}

	left_value := left.(*object.String).Value
	right_value := right.(*object.String).Value
	return &object.String{Value: left_value + right_value}
}

func is_truthy(object object.Object) bool {
	switch object {
	case NULL:
		return false

	case TRUE:
		return true

	case FALSE:
		return false

	default:
		return true
	}
}

func new_error(format string, a ...interface{}) *object.Error {
	return &object.Error{Message: fmt.Sprintf(format, a...)}
}

func is_error(l_object object.Object) bool {
	if l_object != nil {
		return l_object.Type() == object.ERROR_OBJECT
	}
	return false
}