sexp/parse.go

package main

import (
	"errors"
	"fmt"
	"strings"
)

var (
	noMatch = errors.New("no match")
	zero    = &Value[int]{0}
)

type Function func([]Expression) (Expression, error)

type Expression interface {
	Step() (Expression, error)
	Replace(Id, Expression)
}

type List struct {
	els []Expression
}

type Value[T interface{ int | float64 | Function }] struct {
	value T
	// rng   Range
}

type Quoted struct {
	expr Expression
}

type Declaration struct {
	variables []*Symbol
	body      []Expression
}

type Lambda struct {
	args []*Symbol
	body []Expression
}

type Symbol struct {
	id   *Identifier
	expr Expression
}

// type Progn struct {
// 	*List
// }

func (v Value[T]) String() string {
	return fmt.Sprint(v.value)
}

func (v Quoted) String() string {
	return fmt.Sprintf("'%s", v.expr)
}

func (v List) String() string {
	var s []string
	for _, e := range v.els {
		s = append(s, fmt.Sprint(e))
	}
	return fmt.Sprintf("(%s)", strings.Join(s, " "))
}

func (v Lambda) String() string {
	var args List
	for _, arg := range v.args {
		args.els = append(args.els, arg)
	}
	var s []string
	for _, e := range v.body {
		s = append(s, fmt.Sprint(e))
	}
	return fmt.Sprintf("(lambda %v %v)", &args, strings.Join(s, " "))
}

func (v Function) String() string {
	return "(lambda (..) (..))"
}

func (t Symbol) String() string {
	return fmt.Sprintf("%v", t.id.value)
}

func parseQuotedObject(in *Input) (Expression, error) {
	l, ok := in.Pop()
	if !ok {
		return nil, noMatch
	}
	switch {
	case l.Equals(SingleQuote):
		n, err := consume(in)
		if err != nil {
			return nil, err
		}
		return &Quoted{n}, nil
	default:
		return nil, noMatch
	}
}

func parseSymbol(in *Input) (Expression, error) {
	l, ok := in.Pop()
	if !ok {
		return nil, noMatch
	}
	switch x := l.(type) {
	case *Identifier:
		return &Symbol{id: x}, nil
	default:
		return nil, noMatch
	}
}

func parseValue(in *Input) (Expression, error) {
	l, ok := in.Pop()
	if !ok {
		return nil, noMatch
	}
	switch x := l.(type) {
	case *Int:
		return &Value[int]{x.value}, nil
	case *Float:
		return &Value[float64]{x.value}, nil
	default:
		return nil, noMatch
	}
}

func parseList(in *Input) (Expression, error) {
	openingParens, ok := in.Pop()
	if !ok {
		return nil, noMatch
	}
	if !openingParens.Equals(Lparen) {
		return nil, noMatch
	}
	idx, ok := in.Find(Lparen, Rparen)
	if !ok {
		return nil, fmt.Errorf("unmatched parenthesis starting at %s", openingParens.Position())
	}
	list := in.Take(idx)
	var args = new(List)
	for list.Len() > 0 {
		arg, err := consume(list)
		if err != nil {
			return nil, err
		}
		args.els = append(args.els, arg)
	}
	_, ok = in.Pop()
	if !ok {
		return nil, noMatch
	}
	return args, nil
}

func parseLet(in *Input) (Expression, error) {
	openingParens, ok := in.Pop()
	if !ok {
		return nil, noMatch
	}
	if !openingParens.Equals(Lparen) {
		return nil, noMatch
	}
	idx, ok := in.Find(Lparen, Rparen)
	if !ok {
		return nil, fmt.Errorf("unmatched parenthesis starting at %s", openingParens.Position())
	}
	decl := in.Take(idx)
	tok, ok := decl.Pop()
	if !ok {
		return nil, noMatch
	}
	if !tok.Equals(Let) {
		return nil, noMatch
	}
	l, err := parseList(decl)
	if err != nil {
		return nil, err
	}
	list, ok := l.(*List)
	if !ok {
		return nil, noMatch
	}
	var variables []*Symbol
	for _, e := range list.els {
		l, ok := e.(*List)
		if !ok {
			return nil, noMatch
		}
		if len(l.els) != 2 {
			return nil, noMatch
		}
		d, ok := l.els[0].(*Symbol)
		if !ok {
			return nil, noMatch
		}
		d.expr = l.els[1]
		variables = append(variables, d)
	}
	body, err := consumeAll(decl)
	if err != nil {
		return nil, err
	}
	return &Declaration{variables: variables, body: body}, nil
}

func parseLambda(in *Input) (Expression, error) {
	openingParens, ok := in.Pop()
	if !ok {
		return nil, noMatch
	}
	if !openingParens.Equals(Lparen) {
		return nil, noMatch
	}
	idx, ok := in.Find(Lparen, Rparen)
	if !ok {
		return nil, fmt.Errorf("unmatched parenthesis starting at %s", openingParens.Position())
	}
	decl := in.Take(idx)
	tok, ok := decl.Pop()
	if !ok {
		return nil, noMatch
	}
	if !tok.Equals(LambdaTok) {
		return nil, noMatch
	}
	l, err := parseList(decl)
	if err != nil {
		return nil, err
	}
	list, ok := l.(*List)
	if !ok {
		return nil, noMatch
	}
	var args []*Symbol
	for _, e := range list.els {
		id, ok := e.(*Symbol)
		if !ok {
			return nil, noMatch
		}
		args = append(args, id)
	}
	body, err := consumeAll(decl)
	if err != nil {
		return nil, err
	}
	_, ok = in.Pop()
	if !ok {
		return nil, noMatch
	}
	return &Lambda{args: args, body: body}, nil
}

type ParserFunc func(*Input) (Expression, error)

func consume(in *Input) (n Expression, err error) {
	var parseFunctions = []ParserFunc{
		parseQuotedObject,
		parseLambda,
		parseLet,
		parseList,
		parseSymbol,
		parseValue,
	}
	for _, f := range parseFunctions {
		start := in.Index()
		n, err := f(in)
		if err != nil {
			in.Seek(start)
			if errors.Is(err, noMatch) {
				continue
			} else {
				return nil, err
			}
		}
		return n, nil
	}
	return nil, fmt.Errorf("unrecognized construction: %s", in)
}

func consumeAll(in *Input) (e []Expression, err error) {
	for in.Len() > 0 {
		start := in.Index()
		n, err := consume(in)
		if err != nil {
			in.Seek(start)
			return nil, err
		}
		e = append(e, n)
	}
	return e, err
}