skeam/special.go

package main

import (
	"fmt"
	"reflect"
)

type special func(*environment, ...interface{}) (interface{}, error)

type nargsInvalidError struct {
	expected int
	received int
	name     string
}

func (n nargsInvalidError) Error() string {
	return fmt.Sprintf(`received %d arguments in *%v*, expected %d`, n.received, n.name, n.expected)
}

func define(env *environment, args ...interface{}) (interface{}, error) {
	if len(args) != 2 {
		return nil, nargsInvalidError{2, len(args), "define"}
	}
	s, ok := args[0].(symbol)
	if !ok {
		return nil, fmt.Errorf(`first argument to *define* must be symbol, received %v`, reflect.TypeOf(args[0]))
	}
	v, err := eval(args[1], env)
	if err != nil {
		return nil, err
	}
	env.set(s, v)
	return nil, nil
}

func quote(_ *environment, args ...interface{}) (interface{}, error) {
	if len(args) != 1 {
		return nil, nargsInvalidError{1, len(args), "quote"}
	}
	return args[0], nil
}

func _if(env *environment, args ...interface{}) (interface{}, error) {
	if len(args) != 3 {
		return nil, nargsInvalidError{3, len(args), "if"}
	}
	v, err := eval(args[0], env)
	if err != nil {
		return nil, err
	}
	if b, ok := v.(bool); ok && !b {
		return eval(args[2], env)
	}
	return eval(args[1], env)
}

func set(env *environment, args ...interface{}) (interface{}, error) {
	if len(args) != 2 {
		return nil, nargsInvalidError{2, len(args), "set!"}
	}
	s, ok := args[0].(symbol)
	if !ok {
		return nil, fmt.Errorf(`first argument to *set!* must be symbol, received %v`, reflect.TypeOf(args[0]))
	}
	if !env.defined(s) {
		return nil, fmt.Errorf(`cannot *set!* undefined symbol %v`, s)
	}
	v, err := eval(args[1], env)
	if err != nil {
		return nil, err
	}
	env.set(s, v)
	return nil, nil
}