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grammar.go
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grammar.go
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package participle
import (
"reflect"
"text/scanner"
"github.com/alecthomas/participle/lexer"
)
type generatorContext struct {
lexer.Definition
typeNodes map[reflect.Type]node
}
func newGeneratorContext(lex lexer.Definition) *generatorContext {
return &generatorContext{Definition: lex, typeNodes: map[reflect.Type]node{}}
}
// Takes a type and builds a tree of nodes out of it.
func (g *generatorContext) parseType(t reflect.Type) node {
rt := t
t = indirectType(t)
defer decorate(t.Name())
if n, ok := g.typeNodes[t]; ok {
return n
}
switch t.Kind() {
case reflect.Slice, reflect.Ptr:
t = indirectType(t.Elem())
fallthrough
case reflect.Struct:
if rt.Implements(parseableType) {
return &parseable{rt}
}
out := &strct{typ: t}
g.typeNodes[t] = out
slexer := lexStruct(t)
defer func() {
if msg := recover(); msg != nil {
panic(slexer.Field().Name + ": " + msg.(string))
}
}()
e := g.parseExpression(slexer)
if !slexer.Peek().EOF() {
panic("unexpected input " + slexer.Peek().Value)
}
out.expr = e
return out
}
panic("expected struct type but got " + t.String())
}
func (g *generatorContext) parseExpression(slexer *structLexer) node {
out := disjunction{}
for {
out = append(out, g.parseAlternative(slexer))
if slexer.Peek().Type != '|' {
break
}
slexer.Next() // |
}
if len(out) == 1 {
return out[0]
}
return out
}
func (g *generatorContext) parseAlternative(slexer *structLexer) node {
elements := sequence{}
loop:
for {
switch slexer.Peek().Type {
case lexer.EOF:
break loop
default:
term := g.parseTerm(slexer)
if term == nil {
break loop
}
elements = append(elements, term)
}
}
if len(elements) == 1 {
return elements[0]
}
return elements
}
func (g *generatorContext) parseTerm(slexer *structLexer) node {
r := slexer.Peek()
switch r.Type {
case '@':
return g.parseCapture(slexer)
case scanner.String, scanner.RawString, scanner.Char:
return g.parseLiteral(slexer)
case '[':
return g.parseOptional(slexer)
case '{':
return g.parseRepetition(slexer)
case '(':
return g.parseGroup(slexer)
case scanner.Ident:
return g.parseTokenReference(slexer)
case lexer.EOF:
slexer.Next()
return nil
default:
return nil
}
}
// @<expression> captures <expression> into the current field.
func (g *generatorContext) parseCapture(slexer *structLexer) node {
slexer.Next()
token := slexer.Peek()
field := slexer.Field()
if token.Type == '@' {
slexer.Next()
return &reference{field, g.parseType(field.Type)}
}
if indirectType(field.Type).Kind() == reflect.Struct && !field.Type.Implements(captureType) {
panic("structs can only be parsed with @@ or by implementing the Capture interface")
}
return &reference{field, g.parseTerm(slexer)}
}
// A reference in the form <identifier> refers to a named token from the lexer.
func (g *generatorContext) parseTokenReference(slexer *structLexer) node {
token := slexer.Next()
if token.Type != scanner.Ident {
panic("expected identifier")
}
typ, ok := g.Symbols()[token.Value]
if !ok {
panicf("unknown token type %q", token.String())
}
return &tokenReference{typ, token.Value}
}
// [ <expression> ] optionally matches <expression>.
func (g *generatorContext) parseOptional(slexer *structLexer) node {
slexer.Next() // [
optional := &optional{g.parseExpression(slexer)}
next := slexer.Peek()
if next.Type != ']' {
panic("expected ] but got " + next.String())
}
slexer.Next()
return optional
}
// { <expression> } matches 0 or more repititions of <expression>
func (g *generatorContext) parseRepetition(slexer *structLexer) node {
slexer.Next() // {
n := &repetition{
node: g.parseExpression(slexer),
}
next := slexer.Next()
if next.Type != '}' {
panic("expected } but got " + next.String())
}
return n
}
// ( <expression> ) groups a sub-expression
func (g *generatorContext) parseGroup(slexer *structLexer) node {
slexer.Next() // (
n := g.parseExpression(slexer)
next := slexer.Peek() // )
if next.Type != ')' {
panic("expected ) but got " + next.Value)
}
slexer.Next() // )
return n
}
// A literal string.
//
// Note that for this to match, the tokeniser must be able to produce this string. For example,
// if the tokeniser only produces individual charactersk but the literal is "hello", or vice versa.
func (g *generatorContext) parseLiteral(lex *structLexer) node { // nolint: interfacer
token := lex.Next()
if token.Type != scanner.String && token.Type != scanner.RawString && token.Type != scanner.Char {
panic("expected quoted string but got " + token.String())
}
s := token.Value
t := rune(-1)
token = lex.Peek()
if token.Value == ":" {
lex.Next()
token = lex.Next()
if token.Type != scanner.Ident {
panic("expected identifier for literal type constraint but got " + token.String())
}
var ok bool
t, ok = g.Symbols()[token.Value]
if !ok {
panic("unknown token type " + token.String() + " in literal type constraint")
}
}
return &literal{s: s, t: t}
}