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|
// Package ebnf contains the AST and parser for parsing the form of EBNF produced by Participle.
//
// The self-referential EBNF is:
//
// EBNF = Production* .
// Production = <ident> "=" Expression "." .
// Expression = Sequence ("|" Sequence)* .
// SubExpression = "(" ("?!" | "?=")? Expression ")" .
// Sequence = Term+ .
// Term = "~"? (<ident> | <string> | ("<" <ident> ">") | SubExpression) ("*" | "+" | "?" | "!")? .
package ebnf
import (
"fmt"
"io"
"github.com/alecthomas/participle/v2"
)
var parser = participle.MustBuild[EBNF]()
// A Node in the EBNF grammar.
type Node interface {
sealed()
}
var _ Node = &Term{}
// Term in the EBNF grammar.
type Term struct {
Negation bool `@("~")?`
Name string `( @Ident`
Literal string ` | @String`
Token string ` | "<" @Ident ">"`
Group *SubExpression ` | @@ )`
Repetition string `@("*" | "+" | "?" | "!")?`
}
func (t *Term) sealed() {}
func (t *Term) String() string {
switch {
case t.Name != "":
return t.Name + t.Repetition
case t.Literal != "":
return t.Literal + t.Repetition
case t.Token != "":
return "<" + t.Token + ">" + t.Repetition
case t.Group != nil:
return t.Group.String() + t.Repetition
default:
panic("??")
}
}
// LookaheadAssertion enum.
type LookaheadAssertion rune
func (l *LookaheadAssertion) sealed() {}
func (l *LookaheadAssertion) Capture(tokens []string) error { // nolint
rn := tokens[0][0]
switch rn {
case '!', '=':
*l = LookaheadAssertion(rn)
default:
panic(rn)
}
return nil
}
// Lookahead assertion enums.
const (
LookaheadAssertionNone LookaheadAssertion = 0
LookaheadAssertionNegative LookaheadAssertion = '!'
LookaheadAssertionPositive LookaheadAssertion = '='
)
var _ Node = &SubExpression{}
// SubExpression is an expression inside parentheses ( ... )
type SubExpression struct {
Lookahead LookaheadAssertion `"(" ("?" @("!" | "="))?`
Expr *Expression `@@ ")"`
}
func (s *SubExpression) sealed() {}
func (s *SubExpression) String() string {
out := "("
if s.Lookahead != LookaheadAssertionNone {
out += "?" + string(s.Lookahead)
}
out += s.Expr.String() + ")"
return out
}
var _ Node = &Sequence{}
// A Sequence of terms.
type Sequence struct {
Terms []*Term `@@+`
}
func (s *Sequence) sealed() {}
func (s *Sequence) String() (out string) {
for i, term := range s.Terms {
if i > 0 {
out += " "
}
out += term.String()
}
return
}
var _ Node = &Expression{}
// Expression is a set of alternatives separated by "|" in the EBNF.
type Expression struct {
Alternatives []*Sequence `@@ ( "|" @@ )*`
}
func (e *Expression) sealed() {}
func (e *Expression) String() (out string) {
for i, seq := range e.Alternatives {
if i > 0 {
out += " | "
}
out += seq.String()
}
return
}
var _ Node = &Production{}
// Production of the grammar.
type Production struct {
Production string `@Ident "="`
Expression *Expression `@@ "."`
}
func (p *Production) sealed() {}
var _ Node = &EBNF{}
// EBNF itself.
type EBNF struct {
Productions []*Production `@@*`
}
func (e *EBNF) sealed() {}
func (e *EBNF) String() (out string) {
for i, production := range e.Productions {
out += fmt.Sprintf("%s = %s .", production.Production, production.Expression)
if i < len(e.Productions)-1 {
out += "\n"
}
}
return
}
// ParseString string into EBNF.
func ParseString(ebnf string) (*EBNF, error) {
return parser.ParseString("", ebnf)
}
// Parse io.Reader into EBNF.
func Parse(r io.Reader) (*EBNF, error) {
return parser.Parse("", r)
}
|
