skeam.go

package main

import (
	"errors"
	"flag"
	"fmt"
	"io"
	"os"
	"reflect"
	"strconv"
	"strings"
)

var DEBUG bool

type sexp struct {
	items    []interface{}
	quotelvl int
}

func (s *sexp) eval(env *environment) (interface{}, error) {
	debugPrint("eval sexp")
	if s.len() == 0 {
		return nil, errors.New("illegal evaluation of empty sexp ()")
	}

	if s.quotelvl > 0 {
		return s, nil
	}

	// eval the first item
	v, err := eval(s.items[0], env)
	if err != nil {
		return nil, err
	}

	c, ok := v.(callable)
	if !ok {
		return nil, fmt.Errorf(`expected special form or builtin procedure, received %v`, reflect.TypeOf(v))
	}
	if len(s.items) > 1 {
		return c.call(env, s.items[1:])
	}
	return c.call(env, nil)
}

type callable interface {
	call(*environment, []interface{}) (interface{}, error)
}

func newSexp() *sexp {
	return &sexp{
		items:    make([]interface{}, 0, 8),
		quotelvl: 0,
	}
}

func (s sexp) String() string {
	parts := make([]string, len(s.items))
	for i, _ := range s.items {
		parts[i] = fmt.Sprint(s.items[i])
	}
	return "(" + strings.Join(parts, " ") + ")"
}

func (s *sexp) append(item interface{}) {
	s.items = append(s.items, item)
}

func (s sexp) len() int {
	return len(s.items)
}

type symbol string

func (s symbol) eval(env *environment) (interface{}, error) {
	debugPrint("eval symbol")
	v, err := env.get(s)
	if err != nil {
		return nil, err
	}
	return eval(v, env)
}

var universe = &environment{map[symbol]interface{}{
	// predefined values
	"#t":   true,
	"#f":   false,
	"null": nil,

	// builtin functions
	symbol(add.name):      add,
	symbol(sub.name):      sub,
	symbol(mul.name):      mul,
	symbol(div.name):      div,
	symbol(gt.name):       gt,
	symbol(gte.name):      gte,
	symbol(lt.name):       lt,
	symbol(lte.name):      lte,
	symbol(equals.name):   equals,
	symbol(and.name):      and,
	symbol(or.name):       or,
	symbol(cons.name):     cons,
	symbol(car.name):      car,
	symbol(cdr.name):      cdr,
	symbol(length.name):   length,
	symbol(lst.name):      lst,
	symbol(islist.name):   islist,
	symbol(not.name):      not,
	symbol(isnull.name):   isnull,
	symbol(issymbol.name): issymbol,
	// "=":       builtin(equal),
	// "equal?":  builtin(equal),
	// "eq?"
	// "append"

	// special forms
	symbol(begin.name):    begin,
	symbol(define.name):   define,
	symbol(_if.name):      _if,
	symbol(mklambda.name): mklambda,
	symbol(quote.name):    quote,
	symbol(set.name):      set,
}, nil}

func init() {
	universe.set(symbol(names.name), names)
}

// parses the string lexeme into a value that can be eval'd
func atom(t token) (interface{}, error) {
	switch t.t {
	case integerToken:
		val, err := strconv.ParseInt(t.lexeme, 10, 64)
		if err != nil {
			return nil, err
		}
		return val, nil

	case floatToken:
		val, err := strconv.ParseFloat(t.lexeme, 64)
		if err != nil {
			return nil, err
		}
		return val, nil

	case stringToken:
		return t.lexeme, nil

	case symbolToken:
		return symbol(t.lexeme), nil
	}

	return nil, fmt.Errorf("unable to atomize token: %v", t)
}

// reads in tokens on the channel until a matching close paren is found.
func (s *sexp) readIn(c chan token) error {
	for t := range c {
		switch t.t {
		case closeParenToken:
			return nil
		case openParenToken:
			child := newSexp()
			if err := child.readIn(c); err != nil {
				return err
			}
			s.append(child)
		default:
			v, err := atom(t)
			if err != nil {
				return err
			}
			s.append(v)
		}
	}
	return errors.New("unexpected EOF in sexp.readIn")
}

// parses one value that can be evaled from the channel
func parse(c chan token) (interface{}, error) {
	for t := range c {
		switch t.t {
		case closeParenToken:
			return nil, errors.New("unexpected close paren in read")
		case openParenToken:
			s := newSexp()
			if err := s.readIn(c); err != nil {
				return nil, err
			}
			return s, nil
		default:
			return atom(t)
		}
	}
	return nil, io.EOF
}

func main() {
	flag.BoolVar(&DEBUG, "debug", false, "puts the interpreter in debug mode")
	flag.Parse()
	if DEBUG {
		fmt.Println(universe)
	}
	if *tcpAddr != "" {
		runTCPServer()
	}
	if *httpAddr != "" {
		runHTTPServer()
	}
	if len(flag.Args()) > 0 {
		runfile()
		return
	}

	i := newInterpreter(os.Stdin, os.Stdout, os.Stderr)
	i.run(universe)
}