lambda/pkg/saccharine/token/parse.go

package token

import (
	"errors"
	"fmt"
	"unicode"

	"git.maximhutz.com/max/lambda/pkg/iterator"
	"git.maximhutz.com/max/lambda/pkg/trace"
)

// isVariables determines whether a rune can be a valid variable.
func isVariable(r rune) bool {
	return unicode.IsLetter(r) || unicode.IsNumber(r)
}

func parseRune(i *iterator.Iterator[rune], expected func(rune) bool) (rune, error) {
	i2 := i.Copy()

	if r, err := i2.Next(); err != nil {
		return r, err
	} else if !expected(r) {
		return r, fmt.Errorf("got unexpected rune %v'", r)
	} else {
		i.Sync(i2)
		return r, nil
	}
}

func parseCharacter(i *iterator.Iterator[rune], expected rune) (rune, error) {
	i2 := i.Copy()

	if r, err := i2.Next(); err != nil {
		return r, err
	} else if r != expected {
		return r, fmt.Errorf("got unexpected rune %v'", r)
	} else {
		i.Sync(i2)
		return r, nil
	}
}

// Pulls the next token from an iterator over runes. If it cannot, it will
// return nil. If an error occurs, it will return that.
func getToken(i *iterator.Iterator[rune]) (*Token, error) {
	index := i.Index()

	if i.Done() {
		return nil, nil
	}

	letter, err := i.Next()
	if err != nil {
		return nil, trace.Wrap(err, "cannot produce next token")
	}

	switch {
	case letter == '(':
		return NewOpenParen(index), nil
	case letter == ')':
		return NewCloseParen(index), nil
	case letter == '.':
		return NewDot(index), nil
	case letter == '\\':
		return NewSlash(index), nil
	case letter == '\n':
		return NewSoftBreak(index), nil
	case letter == '{':
		return NewOpenBrace(index), nil
	case letter == '}':
		return NewCloseBrace(index), nil
	case letter == ':':
		if _, err := parseCharacter(i, '='); err != nil {
			return nil, err
		} else {
			return NewAssign(index), nil
		}
	case letter == ';':
		return NewHardBreak(index), nil
	case letter == '#':
		// Skip everything until the next newline or EOF.
		for !i.Done() {
			r, err := i.Next()
			if err != nil {
				return nil, trace.Wrap(err, "error while parsing comment")
			}

			if r == '\n' {
				// Put the newline back so it can be processed as a soft break.
				i.Back()
				break
			}
		}
		return nil, nil
	case unicode.IsSpace(letter):
		return nil, nil
	case isVariable(letter):
		atom := []rune{letter}

		for {
			if r, err := parseRune(i, isVariable); err != nil {
				break
			} else {
				atom = append(atom, r)
			}
		}

		return NewAtom(string(atom), index), nil
	}

	return nil, fmt.Errorf("unknown character '%v'", string(letter))
}

// Parse a string into tokens.
func Parse(input string) ([]Token, error) {
	i := iterator.Of([]rune(input))
	tokens := []Token{}
	errorList := []error{}

	for !i.Done() {
		token, err := getToken(i)
		if err != nil {
			errorList = append(errorList, err)
		} else if token != nil {
			tokens = append(tokens, *token)
		}
	}

	return tokens, errors.Join(errorList...)
}