/
sexpr.go
458 lines (419 loc) · 11.6 KB
/
sexpr.go
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// Package sexpr parses s-expressions in a manner similar to the Common Lisp
// reader. It supports typical s-expression syntax and customization.
//
// The elements of S-Expressions are called "forms" by this package and
// represented with the Form interface in the "form" subpackage. Form provides a
// Value() method for obtaining the underlying value. There are specific
// interfaces like `String`, `Number`, `Comment`, and `SymbolForm` corresponding
// to typical s-expression types. Type switches and assertions may be used to
// cast a Form to a suitable type.
package sexpr
import (
"fmt"
"io"
"strconv"
"strings"
"github.com/google/xtoproto/sexpr/form"
)
type Form = form.Form
type StringForm = form.String
type ListForm = form.List
type CommentForm = form.Comment
type WhitespaceForm = form.Whitespace
type ValuelessForm = form.Valueless
type SourceSpan = form.SourcePosition
// readerRequiredSymbol specifies one of the symbols needed by the the s-expression parser
// to handle typical lisp-style s-expressions
type readerRequiredSymbol string
const (
quoteSymbol readerRequiredSymbol = "QUOTE" // '
quasiquoteSymbol readerRequiredSymbol = "QUASIQUOTE" // `
unquoteSymbol readerRequiredSymbol = "UNQUOTE" // ,
unquoteSplicingSymbol readerRequiredSymbol = "UNQUOTE-SPLICING" // ,@
)
type readTable map[rune]func(fr *FormReader) (ReaderMacroResult, error)
// ReaderMacroResult is returned by custom reader macro functions.
type ReaderMacroResult interface {
// Skip returns true if the reader macro declined to read from the stream
// and returned the cursor back to the original location. If true is
// rturned, the Form() function will not be called.
Skip() bool
// The form read by the reader macro.
Form() form.Form
}
// FormReader reads a stream of S-Expressions.
type FormReader struct {
src sourceFile
readTable readTable
factory FormFactory
}
// Option is used to configure a FormReader.
type Option interface {
apply(*FormReader)
}
type simpleOption func(fr *FormReader)
func (opt simpleOption) apply(fr *FormReader) {
opt(fr)
}
// CustomFormFactory returns an Option that may be passed to NewFileReader
// that uses a non-default FormFactory for constructing Forms.
//
// The passed in function will be called with a newly created *FormReader
// as an argument, allowing the factory to depend on the reader to implement
// its functionality.
func CustomFormFactory(factoryProvider func(*FormReader) FormFactory) Option {
return simpleOption(func(fr *FormReader) {
fr.factory = factoryProvider(fr)
})
}
// NewFileReader returns an object for reading Forms from a source file, which
// is provided as a string.
//
// The filename value is used to print error messages and will not be accessed
// by the reader, so it does not need to be a real file at all.
func NewFileReader(fileName, contents string, opts ...Option) *FormReader {
fr := newFormReader(fileName, contents)
for _, opt := range opts {
opt.apply(fr)
}
return fr
}
func newFormReader(name, code string) *FormReader {
sf := newStrSourceFile(name, code)
fr := &FormReader{sf, make(readTable), nil}
fr.factory = DefaultFormFactory(fr)
// To give a flavor of macro characters, we use a non-builtin method of
// defining quote, quasiquote, and unquasiquote:
registerQuoteMacroChars(fr)
return fr
}
// ReadForm reads the next form in the input stream.
//
// If the end of the file is encountered, the second value will be io.EOF.
func (fr *FormReader) ReadForm() (Form, error) {
for {
form, err := fr.readFormEvenTrivial()
if err != nil {
return nil, err
}
if _, ok := form.(nonSubstantiveForm); ok {
continue
}
return form, nil
}
}
// readFormEvenTrivial returns a (Form, error), just like ReadForm(), but it
// also reads comments and whitespace.
func (fr *FormReader) readFormEvenTrivial() (Form, error) {
r, err := fr.src.peekRune()
if err != nil {
return nil, err
}
handler := fr.readTable[r]
if handler != nil {
result, err := handler(fr)
if err != nil {
return nil, err
}
if !result.Skip() {
return result.Form(), nil
}
}
switch r {
case ' ', '\t', '\n':
return fr.readWhitespace()
case '"':
return fr.readString()
case '(':
return fr.readList()
default:
return fr.readNumberSymbolOrComment()
}
}
func (fr *FormReader) errorfWithPosition(format string, arg ...interface{}) error {
return fmt.Errorf("%s:%s: %w",
fr.src.fileName(),
fr.src.offsetToRowCol(fr.src.cursorOffset()).String(),
fmt.Errorf(format, arg...))
}
func (fr *FormReader) errorfWithRangePosition(start, end cursorOffset, format string, arg ...interface{}) error {
return fmt.Errorf("%s: %w",
fr.makeSpan(start, end).String(),
fmt.Errorf(format, arg...))
}
func (fr *FormReader) makeSpan(start, end cursorOffset) *simpleSourceSpan {
return &simpleSourceSpan{fr.src.fileName(), fr.src.offsetToRowCol(start), fr.src.offsetToRowCol(end)}
}
func (fr *FormReader) readWhitespace() (Form, error) {
start := fr.src.cursorOffset()
literal := strings.Builder{}
loop:
for {
r, err := fr.src.readRune()
if err == io.EOF {
break
}
if err != nil {
return nil, err
}
switch r {
case ' ', '\t', '\n':
literal.WriteRune(r)
default:
if err := fr.src.unreadRune(); err != nil {
return nil, err
}
break loop
}
}
if literal.Len() == 0 {
return nil, fr.errorfWithPosition("failed to consume any whitespace")
}
end := fr.src.cursorOffset()
return fr.factory.NewWhitespaceForm(literal.String(), fr.makeSpan(start, end))
}
func (fr *FormReader) readString() (StringForm, error) {
start := fr.src.cursorOffset()
r, err := fr.src.readRune()
if err != nil {
return nil, err
}
if r != '"' {
return nil, fr.errorfWithPosition("expected opening \", got %q", r)
}
value := strings.Builder{}
value.WriteRune(r)
prevCharWasEscape := false
loop:
for {
r, err := fr.src.readRune()
if err == io.EOF {
return nil, fr.errorfWithPosition("did not find end of string token '\"'")
}
if err != nil {
return nil, err
}
switch r {
case '\n':
value.WriteString(`\n`) // Replace newline literals with the literal `\n`, which is supported by strconv.Unquote.
//return nil, fr.errorfWithPosition("unexpected newline before terminating \" character", src)
case '"':
if !prevCharWasEscape {
value.WriteRune('"')
break loop
}
fallthrough
default:
value.WriteRune(r)
}
prevCharWasEscape = (r == '\\')
}
parsedValue, err := strconv.Unquote(value.String())
if err != nil {
return nil, fr.errorfWithPosition("error parsing string - %w", err)
}
end := fr.src.cursorOffset()
return fr.factory.NewStringForm(parsedValue, fr.makeSpan(start, end))
}
func (fr *FormReader) readList() (ListForm, error) {
start := fr.src.cursorOffset()
r, err := fr.src.readRune()
if err != nil {
return nil, err
}
if r != '(' {
return nil, fr.errorfWithPosition("expected opening paren, got %q", r)
}
var forms []Form
loop:
for {
r, err := fr.src.readRune()
if err == io.EOF {
return nil, fr.errorfWithPosition("did not find end of list token ')'")
}
if err != nil {
return nil, err
}
switch r {
case ')':
break loop
default:
if err := fr.src.unreadRune(); err != nil {
return nil, err
}
f, err := fr.readFormEvenTrivial()
if err != nil {
return nil, err
}
forms = append(forms, f)
}
}
end := fr.src.cursorOffset()
return fr.factory.NewList(forms, fr.makeSpan(start, end))
}
func (fr *FormReader) readNumberSymbolOrComment() (Form, error) {
start := fr.src.cursorOffset()
literal := strings.Builder{}
loop:
for {
r, err := fr.src.readRune()
if err == io.EOF {
break loop
}
if err != nil {
return nil, err
}
switch r {
case ' ', '\t', '\n', ')', '(', '"':
if err := fr.src.unreadRune(); err != nil {
return nil, err
}
break loop
case '/':
if literal.Len() == 0 {
// could be a comment
commentForm, isComment, err := fr.readPossibleComment(start)
if err != nil {
return nil, err
}
if isComment {
return commentForm, nil
}
}
// Could have come to the end of a symbol and start of a comment, or a /
// is in the middle of the symbol.
r2, err := fr.src.readRune()
if err == io.EOF {
break loop
}
if err := fr.src.unreadRune(); err != nil {
return nil, err
}
startOfComment := r2 == '/' || r2 == '*'
if startOfComment {
// Start of comment... unread r as well and return.
if err := fr.src.unreadRune(); err != nil {
return nil, err
}
break loop
}
default:
literal.WriteRune(r)
}
}
if literal.Len() == 0 {
return nil, fr.errorfWithPosition("failed to consume a token")
}
end := fr.src.cursorOffset()
return fr.makeFormFromSymbolOrNumberToken(literal.String(), start, end)
}
func (fr *FormReader) makeFormFromSymbolOrNumberToken(token string, start, end cursorOffset) (Form, error) {
num, err := parseNumber(token)
if err != nil {
return nil, fr.errorfWithRangePosition(start, end, "bad number: %w", err)
}
if num == nil {
return fr.factory.NewSymbolForm(token, fr.makeSpan(start, end))
}
return fr.factory.NewNumberForm(num.constValue(), fr.makeSpan(start, end))
}
func (fr *FormReader) makeFormFromReaderRequiredSymbol(rr readerRequiredSymbol, start, end cursorOffset) (Form, error) {
// This could be swapped out for some other symbol construction method.
return fr.factory.NewSymbolForm(string(rr), fr.makeSpan(start, end))
}
// Callsed after reading a '/' rune.
func (fr *FormReader) readPossibleComment(start cursorOffset) (CommentForm, bool, error) {
contents := strings.Builder{}
contents.WriteRune('/')
secondCommentRune, err := fr.src.readRune()
if err != nil {
return nil, false, err
}
contents.WriteRune(secondCommentRune)
switch secondCommentRune {
case '*':
// Consume the contents of the multi-line comment.
for {
r, err := fr.src.readRune()
if err == io.EOF {
break
}
if err != nil {
return nil, false, err
}
contents.WriteRune(r)
if r != '*' {
continue
}
r2, err := fr.src.readRune()
contents.WriteRune(r2)
if err == io.EOF {
break
}
if err != nil {
return nil, false, err
}
if r2 == '/' {
break
}
}
end := fr.src.cursorOffset()
c, err := fr.factory.NewCommentForm(contents.String(), fr.makeSpan(start, end))
if err != nil {
return nil, false, err
}
return c, true, nil
case '/':
for {
r, err := fr.src.readRune()
if err == io.EOF {
break
}
if err != nil {
return nil, false, err
}
if r == '\n' {
break
}
contents.WriteRune(r)
}
end := fr.src.cursorOffset()
c, err := fr.factory.NewCommentForm(contents.String(), fr.makeSpan(start, end))
if err != nil {
return nil, false, err
}
return c, true, nil
default:
// Unread last character.
return nil, false, fr.src.unreadRune()
}
}
func registerQuoteMacroChars(fr *FormReader) {
fr.readTable['\''] = func(fr *FormReader) (ReaderMacroResult, error) {
start := fr.src.cursorOffset()
if _, err := fr.src.readRune(); err != nil {
return nil, fr.errorfWithPosition("I/O failure reading quote character after peek: %w", err)
}
afterQuote := fr.src.cursorOffset()
quoteSym, err := fr.makeFormFromReaderRequiredSymbol(quoteSymbol, start, afterQuote)
if err != nil {
return nil, err
}
quotedValue, err := fr.ReadForm()
if err == io.EOF {
return nil, fr.errorfWithPosition("expecting form after quote character, got EOF")
}
if err != nil {
return nil, err
}
end := fr.src.cursorOffset()
f, err := fr.factory.NewList([]Form{quoteSym, quotedValue}, fr.makeSpan(start, end))
if err != nil {
return nil, err
}
return &simpleReaderMacroResult{
skip: false,
form: f,
}, nil
}
}