scala-parser-combinators/scala.util.parsing.combinator/Parsers

Parsers

trait Parsers

Parsers is a component that ''provides'' generic parser combinators.

Parsers is a component that ''provides'' generic parser combinators.

There are two abstract members that must be defined in order to produce parsers: the type Elem and scala.util.parsing.combinator.Parsers.Parser. There are helper methods that produce concrete Parser implementations -- see ''primitive parser'' below.

A Parsers may define multiple Parser instances, which are combined to produced the desired parser.

The type of the elements these parsers should parse must be defined by declaring Elem (each parser is polymorphic in the type of result it produces).

There are two aspects to the result of a parser:

  1. success or failure
  2. the result.

A scala.util.parsing.combinator.Parsers.Parser produces both kinds of information, by returning a scala.util.parsing.combinator.Parsers.ParseResult when its apply method is called on an input.

The term ''parser combinator'' refers to the fact that these parsers are constructed from primitive parsers and composition operators, such as sequencing, alternation, optionality, repetition, lifting, and so on. For example, given p1 and p2 of type scala.util.parsing.combinator.Parsers.Parser:

p1 ~ p2 // sequencing: must match p1 followed by p2
p1 | p2 // alternation: must match either p1 or p2, with preference given to p1
p1.?    // optionality: may match p1 or not
p1.*    // repetition: matches any number of repetitions of p1

These combinators are provided as methods on scala.util.parsing.combinator.Parsers.Parser, or as methods taking one or more Parsers and returning a Parser provided in this class.

A ''primitive parser'' is a parser that accepts or rejects a single piece of input, based on a certain criterion, such as whether the input...

  • is equal to some given object (see method accept),
  • satisfies a certain predicate (see method acceptIf),
  • is in the domain of a given partial function (see method acceptMatch)
  • or other conditions, by using one of the other methods available, or subclassing Parser

Even more primitive parsers always produce the same result, irrespective of the input. See methods success, err and failure as examples.

See also

scala.util.parsing.combinator.RegexParsers and other known subclasses for practical examples.

class Object
trait Matchable
class Any
trait Scanners
class Lexical

Type members

Classlikes

case class Error(msg: String, next: Input) extends NoSuccess

The fatal failure case of ParseResult: contains an error-message and the remaining input. No back-tracking is done when a parser returns an Error.

The fatal failure case of ParseResult: contains an error-message and the remaining input. No back-tracking is done when a parser returns an Error.

Value Params
msg

An error message string describing the error.

next

The parser's unconsumed input at the point where the error occurred.

case class Failure(msg: String, next: Input) extends NoSuccess

The failure case of ParseResult: contains an error-message and the remaining input. Parsing will back-track when a failure occurs.

The failure case of ParseResult: contains an error-message and the remaining input. Parsing will back-track when a failure occurs.

Value Params
msg

An error message string describing the failure.

next

The parser's unconsumed input at the point where the failure occurred.

sealed abstract class NoSuccess(val msg: String, val next: Input) extends ParseResult[Nothing]

A common super-class for unsuccessful parse results.

A common super-class for unsuccessful parse results.

Companion
object
object NoSuccess

An extractor so NoSuccess(msg, next) can be used in matches.

An extractor so NoSuccess(msg, next) can be used in matches.

Companion
class
trait OnceParser[+T] extends Parser[T]

A parser whose ~ combinator disallows back-tracking.

A parser whose ~ combinator disallows back-tracking.

sealed abstract class ParseResult[+T]

A base class for parser results. A result is either successful or not (failure may be fatal, i.e., an Error, or not, i.e., a Failure). On success, provides a result of type T which consists of some result (and the rest of the input).

A base class for parser results. A result is either successful or not (failure may be fatal, i.e., an Error, or not, i.e., a Failure). On success, provides a result of type T which consists of some result (and the rest of the input).

abstract class Parser[+T] extends Input => ParseResult[T]

The root class of parsers. Parsers are functions from the Input type to ParseResult.

The root class of parsers. Parsers are functions from the Input type to ParseResult.

case class Success[+T](result: T, next: Input) extends ParseResult[T]

The success case of ParseResult: contains the result and the remaining input.

The success case of ParseResult: contains the result and the remaining input.

Value Params
next

The parser's remaining input

result

The parser's output

case class ~[+a, +b](_1: a, _2: b)

A wrapper over sequence of matches.

A wrapper over sequence of matches.

Given p1: Parser[A] and p2: Parser[B], a parser composed with p1 ~ p2 will have type Parser[~[A, B]]. The successful result of the parser can be extracted from this case class.

It also enables pattern matching, so something like this is possible:

def concat(p1: Parser[String], p2: Parser[String]): Parser[String] =
  p1 ~ p2 ^^ { case a ~ b => a + b }

Types

type Elem

the type of input elements the provided parsers consume (When consuming invidual characters, a parser is typically called a ''scanner'', which produces ''tokens'' that are consumed by what is normally called a ''parser''. Nonetheless, the same principles apply, regardless of the input type.)

the type of input elements the provided parsers consume (When consuming invidual characters, a parser is typically called a ''scanner'', which produces ''tokens'' that are consumed by what is normally called a ''parser''. Nonetheless, the same principles apply, regardless of the input type.)

type Input = Reader[Elem]

The parser input is an abstract reader of input elements, i.e. the type of input the parsers in this component expect.

The parser input is an abstract reader of input elements, i.e. the type of input the parsers in this component expect.

Value members

Concrete methods

def OnceParser[T](f: Input => ParseResult[T]): Parser[T] & OnceParser[T]
def Parser[T](f: Input => ParseResult[T]): Parser[T]
def accept[ES](es: ES)(f: ES => List[Elem]): Parser[List[Elem]]

A parser that matches only the given list of element es.

A parser that matches only the given list of element es.

accept(es) succeeds if the input subsequently provides the elements in the list es.

Value Params
es

the list of expected elements

Returns

a Parser that recognizes a specified list of elements

def accept[U](expected: String, f: PartialFunction[Elem, U]): Parser[U]

The parser that matches an element in the domain of the partial function f.

The parser that matches an element in the domain of the partial function f.

If f is defined on the first element in the input, f is applied to it to produce this parser's result.

Example: The parser accept("name", {case Identifier(n) => Name(n)}) accepts an Identifier(n) and returns a Name(n)

Value Params
expected

a description of the kind of element this parser expects (for error messages)

f

a partial function that determines when this parser is successful and what its output is

Returns

A parser that succeeds if f is applicable to the first element of the input, applying f to it to produce the result.

def acceptIf(p: Elem => Boolean)(err: Elem => String): Parser[Elem]

A parser matching input elements that satisfy a given predicate.

A parser matching input elements that satisfy a given predicate.

acceptIf(p)(el => "Unexpected "+el) succeeds if the input starts with an element e for which p(e) is true.

Value Params
err

A function from the received element into an error message.

p

A predicate that determines which elements match.

Returns

A parser for elements satisfying p(e).

def acceptMatch[U](expected: String, f: PartialFunction[Elem, U]): Parser[U]

The parser that matches an element in the domain of the partial function f.

The parser that matches an element in the domain of the partial function f.

If f is defined on the first element in the input, f is applied to it to produce this parser's result.

Example: The parser acceptMatch("name", {case Identifier(n) => Name(n)}) accepts an Identifier(n) and returns a Name(n)

Value Params
expected

a description of the kind of element this parser expects (for error messages)

f

a partial function that determines when this parser is successful and what its output is

Returns

A parser that succeeds if f is applicable to the first element of the input, applying f to it to produce the result.

def acceptSeq[ES](es: ES)(f: ES => Iterable[Elem]): Parser[List[Elem]]

A parser that matches only the given scala.collection.Iterable collection of elements es.

A parser that matches only the given scala.collection.Iterable collection of elements es.

acceptSeq(es) succeeds if the input subsequently provides the elements in the iterable es.

Value Params
es

the list of expected elements

Returns

a Parser that recognizes a specified list of elements

def chainl1[T](p: => Parser[T], q: => Parser[(T, T) => T]): Parser[T]

A parser generator that, roughly, generalises the rep1sep generator so that q, which parses the separator, produces a left-associative function that combines the elements it separates.

A parser generator that, roughly, generalises the rep1sep generator so that q, which parses the separator, produces a left-associative function that combines the elements it separates.

''From: J. Fokker. Functional parsers. In J. Jeuring and E. Meijer, editors, Advanced Functional Programming, volume 925 of Lecture Notes in Computer Science, pages 1--23. Springer, 1995.''

Value Params
p

a parser that parses the elements

q

a parser that parses the token(s) separating the elements, yielding a left-associative function that combines two elements into one

def chainl1[T, U](first: => Parser[T], p: => Parser[U], q: => Parser[(T, U) => T]): Parser[T]

A parser generator that, roughly, generalises the rep1sep generator so that q, which parses the separator, produces a left-associative function that combines the elements it separates.

A parser generator that, roughly, generalises the rep1sep generator so that q, which parses the separator, produces a left-associative function that combines the elements it separates.

Value Params
first

a parser that parses the first element

p

a parser that parses the subsequent elements

q

a parser that parses the token(s) separating the elements, yielding a left-associative function that combines two elements into one

def chainr1[T, U](p: => Parser[T], q: => Parser[(T, U) => U], combine: (T, U) => U, first: U): Parser[U]

A parser generator that generalises the rep1sep generator so that q, which parses the separator, produces a right-associative function that combines the elements it separates. Additionally, the right-most (last) element and the left-most combining function have to be supplied.

A parser generator that generalises the rep1sep generator so that q, which parses the separator, produces a right-associative function that combines the elements it separates. Additionally, the right-most (last) element and the left-most combining function have to be supplied.

rep1sep(p: Parser[T], q) corresponds to chainr1(p, q ^^ cons, cons, Nil) (where val cons = (x: T, y: List[T]) => x :: y)

Value Params
combine

the "last" (left-most) combination function to be applied

first

the "first" (right-most) element to be combined

p

a parser that parses the elements

q

a parser that parses the token(s) separating the elements, yielding a right-associative function that combines two elements into one

def commit[T](p: => Parser[T]): Parser[T]

Wrap a parser so that its failures become errors (the | combinator will give up as soon as it encounters an error, on failure it simply tries the next alternative).

Wrap a parser so that its failures become errors (the | combinator will give up as soon as it encounters an error, on failure it simply tries the next alternative).

def elem(kind: String, p: Elem => Boolean): Parser[Elem]

A parser matching input elements that satisfy a given predicate.

A parser matching input elements that satisfy a given predicate.

elem(kind, p) succeeds if the input starts with an element e for which p(e) is true.

Value Params
kind

The element kind, used for error messages

p

A predicate that determines which elements match.

def elem(e: Elem): Parser[Elem]

A parser that matches only the given element e.

A parser that matches only the given element e.

elem(e) succeeds if the input starts with an element e.

Value Params
e

the Elem that must be the next piece of input for the returned parser to succeed

Returns

a Parser that succeeds if e is the next available input (and returns it).

def err(msg: String): Parser[Nothing]

A parser that results in an error.

A parser that results in an error.

Value Params
msg

The error message describing the failure.

Returns

A parser that always fails with the specified error message.

def failure(msg: String): Parser[Nothing]

A parser that always fails.

A parser that always fails.

Value Params
msg

The error message describing the failure.

Returns

A parser that always fails with the specified error message.

def guard[T](p: => Parser[T]): Parser[T]

A parser generator for guard expressions. The resulting parser will fail or succeed just like the one given as parameter but it will not consume any input.

A parser generator for guard expressions. The resulting parser will fail or succeed just like the one given as parameter but it will not consume any input.

Value Params
p

a Parser that is to be applied to the input

Returns

A parser that returns success if and only if p succeeds but never consumes any input

def log[T](p: => Parser[T])(name: String): Parser[T]

A helper method that turns a Parser into one that will print debugging information to stdout before and after being applied.

A helper method that turns a Parser into one that will print debugging information to stdout before and after being applied.

def mkList[T]: T ~ List[T] => List[T]

Given a concatenation with a repetition (list), move the concatenated element into the list

Given a concatenation with a repetition (list), move the concatenated element into the list

def not[T](p: => Parser[T]): Parser[Unit]

Wrap a parser so that its failures and errors become success and vice versa -- it never consumes any input.

Wrap a parser so that its failures and errors become success and vice versa -- it never consumes any input.

def opt[T](p: => Parser[T]): Parser[Option[T]]

A parser generator for optional sub-phrases.

A parser generator for optional sub-phrases.

opt(p) is a parser that returns Some(x) if p returns x and None if p fails.

Value Params
p

A Parser that is tried on the input

Returns

a Parser that always succeeds: either with the result provided by p or with the empty result

def phrase[T](p: Parser[T]): Parser[T]

A parser generator delimiting whole phrases (i.e. programs).

A parser generator delimiting whole phrases (i.e. programs).

phrase(p) succeeds if p succeeds and no input is left over after p.

Value Params
p

the parser that must consume all input for the resulting parser to succeed.

Returns

a parser that has the same result as p, but that only succeeds if p consumed all the input.

def positioned[T <: Positional](p: => Parser[T]): Parser[T]

positioned decorates a parser's result with the start position of the input it consumed.

positioned decorates a parser's result with the start position of the input it consumed.

Value Params
p

a Parser whose result conforms to Positional.

Returns

A parser that has the same behaviour as p, but which marks its result with the start position of the input it consumed, if it didn't already have a position.

def rep[T](p: => Parser[T]): Parser[List[T]]

A parser generator for repetitions.

A parser generator for repetitions.

rep(p) repeatedly uses p to parse the input until p fails (the result is a List of the consecutive results of p).

Value Params
p

a Parser that is to be applied successively to the input

Returns

A parser that returns a list of results produced by repeatedly applying p to the input.

def rep1[T](p: => Parser[T]): Parser[List[T]]

A parser generator for non-empty repetitions.

A parser generator for non-empty repetitions.

rep1(p) repeatedly uses p to parse the input until p fails -- p must succeed at least once (the result is a List of the consecutive results of p)

Value Params
p

a Parser that is to be applied successively to the input

Returns

A parser that returns a list of results produced by repeatedly applying p to the input (and that only succeeds if p matches at least once).

@migration("The `p0` call-by-name arguments is evaluated at most once per constructed Parser object, instead of on every need that arises during parsing.", "2.9.0")
def rep1[T](first: => Parser[T], p0: => Parser[T]): Parser[List[T]]

A parser generator for non-empty repetitions.

A parser generator for non-empty repetitions.

rep1(f, p) first uses f (which must succeed) and then repeatedly uses p to parse the input until p fails (the result is a List of the consecutive results of f and p)

Value Params
first

a Parser that parses the first piece of input

p0

a Parser that is to be applied successively to the rest of the input (if any) -- evaluated at most once, and only when necessary

Returns

A parser that returns a list of results produced by first applying f and then repeatedly p to the input (it only succeeds if f matches).

def rep1sep[T](p: => Parser[T], q: => Parser[Any]): Parser[List[T]]

A parser generator for non-empty repetitions.

A parser generator for non-empty repetitions.

rep1sep(p, q) repeatedly applies p interleaved with q to parse the input, until p fails. The parser p must succeed at least once.

Value Params
p

a Parser that is to be applied successively to the input

q

a Parser that parses the elements that separate the elements parsed by p (interleaved with q)

Returns

A parser that returns a list of results produced by repeatedly applying p to the input (and that only succeeds if p matches at least once). The results of p are collected in a list. The results of q are discarded.

def repN[T](num: Int, p: => Parser[T]): Parser[List[T]]

A parser generator for a specified number of repetitions.

A parser generator for a specified number of repetitions.

repN(n, p) uses p exactly n time to parse the input (the result is a List of the n consecutive results of p).

Value Params
num

the exact number of times p must succeed

p

a Parser that is to be applied successively to the input

Returns

A parser that returns a list of results produced by repeatedly applying p to the input (and that only succeeds if p matches exactly n times).

def repNM[T](n: Int, m: Int, p: Parser[T], sep: Parser[Any]): Parser[List[T]]

A parser generator for a specified range of repetitions interleaved by a separator.

A parser generator for a specified range of repetitions interleaved by a separator.

repNM(n, m, p, s) uses p at least n times and up to m times, interleaved with separator s, to parse the input (the result is a List of at least n consecutive results of p and up to m results).

Value Params
m

maximum number of repetitions

n

minimum number of repetitions

p

a Parser that is to be applied successively to the input

sep

a Parser that interleaves with p

Returns

A parser that returns a list of results produced by repeatedly applying p interleaved with sep to the input. The list has a size between n and up to m (and that only succeeds if p matches at least n times).

def repsep[T](p: => Parser[T], q: => Parser[Any]): Parser[List[T]]

A parser generator for interleaved repetitions.

A parser generator for interleaved repetitions.

repsep(p, q) repeatedly uses p interleaved with q to parse the input, until p fails. (The result is a List of the results of p.)

Example: repsep(term, ",") parses a comma-separated list of term's, yielding a list of these terms.

Value Params
p

a Parser that is to be applied successively to the input

q

a Parser that parses the elements that separate the elements parsed by p

Returns

A parser that returns a list of results produced by repeatedly applying p (interleaved with q) to the input. The results of p are collected in a list. The results of q are discarded.

def success[T](v: T): Parser[T]

A parser that always succeeds.

A parser that always succeeds.

Value Params
v

The result for the parser

Returns

A parser that always succeeds, with the given result v

Implicits

Implicits

implicit def accept(e: Elem): Parser[Elem]

A parser that matches only the given element e.

A parser that matches only the given element e.

The method is implicit so that elements can automatically be lifted to their parsers. For example, when parsing Tokens, Identifier("new") (which is a Token) can be used directly, instead of first creating a Parser using accept(Identifier("new")).

Value Params
e

the Elem that must be the next piece of input for the returned parser to succeed

Returns

a tParser that succeeds if e is the next available input.