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options.go
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options.go
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package participle
import (
"fmt"
"reflect"
"github.com/alecthomas/participle/v2/lexer"
)
// MaxLookahead can be used with UseLookahead to get pseudo-infinite
// lookahead without the risk of pathological cases causing a stack
// overflow.
const MaxLookahead = 99999
// An Option to modify the behaviour of the Parser.
type Option func(p *Parser) error
// Lexer is an Option that sets the lexer to use with the given grammar.
func Lexer(def lexer.Definition) Option {
return func(p *Parser) error {
p.lex = def
return nil
}
}
// UseLookahead allows branch lookahead up to "n" tokens.
//
// If parsing cannot be disambiguated before "n" tokens of lookahead, parsing will fail.
//
// Note that increasing lookahead has a minor performance impact, but also
// reduces the accuracy of error reporting.
//
// If "n" is negative, it will be treated as "infinite" lookahead.
// This can have a large impact on performance, and does not provide any
// protection against stack overflow during parsing.
// In most cases, using MaxLookahead will achieve the same results in practice,
// but with a concrete upper bound to prevent pathological behavior in the parser.
// Using infinite lookahead can be useful for testing, or for parsing especially
// ambiguous grammars. Use at your own risk!
func UseLookahead(n int) Option {
return func(p *Parser) error {
p.useLookahead = n
return nil
}
}
// CaseInsensitive allows the specified token types to be matched case-insensitively.
//
// Note that the lexer itself will also have to be case-insensitive; this option
// just controls whether literals in the grammar are matched case insensitively.
func CaseInsensitive(tokens ...string) Option {
return func(p *Parser) error {
for _, token := range tokens {
p.caseInsensitive[token] = true
}
return nil
}
}
// ParseTypeWith associates a custom parsing function with some interface type T.
// When the parser encounters a value of type T, it will use the given parse function to
// parse a value from the input.
//
// The parse function may return anything it wishes as long as that value satisfies the interface T.
// However, only a single function can be defined for any type T.
// If you want to have multiple parse functions returning types that satisfy the same interface, you'll
// need to define new wrapper types for each one.
//
// This can be useful if you want to parse a DSL within the larger grammar, or if you want
// to implement an optimized parsing scheme for some portion of the grammar.
func ParseTypeWith[T any](parseFn func(*lexer.PeekingLexer) (T, error)) Option {
return func(p *Parser) error {
parseFnVal := reflect.ValueOf(parseFn)
parseFnType := parseFnVal.Type()
if parseFnType.Out(0).Kind() != reflect.Interface {
return fmt.Errorf("ParseTypeWith: T must be an interface type (got %s)", parseFnType.Out(0))
}
prodType := parseFnType.Out(0)
p.customDefs = append(p.customDefs, customDef{prodType, parseFnVal})
return nil
}
}
// Union associates several member productions with some interface type T.
// Given members X, Y, Z, and W for a union type U, the the EBNF rule is:
// U = X | Y | Z | W .
// When the parser encounters a field of type T, it will attempt to parse each member
// in sequence and take the first matche. Because of this, the order in which the
// members are defined is important. You must be careful to order your members appropriately.
//
// An example of a bad parse that can happen if members are out of order:
//
// If the first member matches A, and the second member matches A B,
// and he source string is "AB", then the parser will only match A, and will not
// try to parse the second member at all.
func Union[T any](members ...T) Option {
return func(p *Parser) error {
unionType := reflect.TypeOf((*T)(nil)).Elem()
if unionType.Kind() != reflect.Interface {
return fmt.Errorf("ParseUnion: union type must be an interface (got %s)", unionType)
}
memberTypes := make([]reflect.Type, 0, len(members))
for _, m := range members {
memberTypes = append(memberTypes, reflect.TypeOf(m))
}
p.unionDefs = append(p.unionDefs, unionDef{unionType, memberTypes})
return nil
}
}
// ParseOption modifies how an individual parse is applied.
type ParseOption func(p *parseContext)
// AllowTrailing tokens without erroring.
//
// That is, do not error if a full parse completes but additional tokens remain.
func AllowTrailing(ok bool) ParseOption {
return func(p *parseContext) {
p.allowTrailing = ok
}
}