Portability	portable
Maintainer	libraries@haskell.org

Distribution.Compat.ReadP

Contents

The ReadP type
Primitive operations
Other operations
Running a parser
Properties

Description

This is a library of parser combinators, originally written by Koen Claessen. It parses all alternatives in parallel, so it never keeps hold of the beginning of the input string, a common source of space leaks with other parsers. The '(+++)' choice combinator is genuinely commutative; it makes no difference which branch is "shorter".

See also Koen's paper Parallel Parsing Processes (http://www.cs.chalmers.se/~koen/publications.html).

This version of ReadP has been locally hacked to make it H98, by Martin Sjögren mailto:msjogren@gmail.com

Synopsis

The `ReadP` type

type ReadP r a = Parser r Char a

Primitive operations

get :: ReadP r Char

Consumes and returns the next character. Fails if there is no input left.

look :: ReadP r String

Look-ahead: returns the part of the input that is left, without consuming it.

(+++) :: ReadP r a -> ReadP r a -> ReadP r a

Symmetric choice.

(<++) :: ReadP a a -> ReadP r a -> ReadP r a

Local, exclusive, left-biased choice: If left parser locally produces any result at all, then right parser is not used.

gather :: ReadP (String -> P Char r) a -> ReadP r (String, a)

Transforms a parser into one that does the same, but in addition returns the exact characters read. IMPORTANT NOTE: gather gives a runtime error if its first argument is built using any occurrences of readS_to_P.

Other operations

pfail :: ReadP r a

Always fails.

satisfy :: (Char -> Bool) -> ReadP r Char

Consumes and returns the next character, if it satisfies the specified predicate.

char :: Char -> ReadP r Char

Parses and returns the specified character.

string :: String -> ReadP r String

Parses and returns the specified string.

munch :: (Char -> Bool) -> ReadP r String

Parses the first zero or more characters satisfying the predicate.

munch1 :: (Char -> Bool) -> ReadP r String

Parses the first one or more characters satisfying the predicate.

skipSpaces :: ReadP r ()

Skips all whitespace.

choice :: [ReadP r a] -> ReadP r a

Combines all parsers in the specified list.

count :: Int -> ReadP r a -> ReadP r [a]

count n p parses n occurrences of p in sequence. A list of results is returned.

between :: ReadP r open -> ReadP r close -> ReadP r a -> ReadP r a

between open close p parses open, followed by p and finally close. Only the value of p is returned.

option :: a -> ReadP r a -> ReadP r a

option x p will either parse p or return x without consuming any input.

optional :: ReadP r a -> ReadP r ()

optional p optionally parses p and always returns ().

many :: ReadP r a -> ReadP r [a]

Parses zero or more occurrences of the given parser.

many1 :: ReadP r a -> ReadP r [a]

Parses one or more occurrences of the given parser.

skipMany :: ReadP r a -> ReadP r ()

Like many, but discards the result.

skipMany1 :: ReadP r a -> ReadP r ()

Like many1, but discards the result.

sepBy :: ReadP r a -> ReadP r sep -> ReadP r [a]

sepBy p sep parses zero or more occurrences of p, separated by sep. Returns a list of values returned by p.

sepBy1 :: ReadP r a -> ReadP r sep -> ReadP r [a]

sepBy1 p sep parses one or more occurrences of p, separated by sep. Returns a list of values returned by p.

endBy :: ReadP r a -> ReadP r sep -> ReadP r [a]

endBy p sep parses zero or more occurrences of p, separated and ended by sep.

endBy1 :: ReadP r a -> ReadP r sep -> ReadP r [a]

endBy p sep parses one or more occurrences of p, separated and ended by sep.

chainr :: ReadP r a -> ReadP r (a -> a -> a) -> a -> ReadP r a

chainr p op x parses zero or more occurrences of p, separated by op. Returns a value produced by a right associative application of all functions returned by op. If there are no occurrences of p, x is returned.

chainl :: ReadP r a -> ReadP r (a -> a -> a) -> a -> ReadP r a

chainl p op x parses zero or more occurrences of p, separated by op. Returns a value produced by a left associative application of all functions returned by op. If there are no occurrences of p, x is returned.

chainl1 :: ReadP r a -> ReadP r (a -> a -> a) -> ReadP r a

Like chainl, but parses one or more occurrences of p.

chainr1 :: ReadP r a -> ReadP r (a -> a -> a) -> ReadP r a

Like chainr, but parses one or more occurrences of p.

manyTill :: ReadP r a -> ReadP [a] end -> ReadP r [a]

manyTill p end parses zero or more occurrences of p, until end succeeds. Returns a list of values returned by p.

Running a parser

type ReadS a = String -> [(a, String)]Source

A parser for a type a, represented as a function that takes a String and returns a list of possible parses as (a,String) pairs.

Note that this kind of backtracking parser is very inefficient; reading a large structure may be quite slow (cf ReadP).

readP_to_S :: ReadP a a -> ReadS a

Converts a parser into a Haskell ReadS-style function. This is the main way in which you can "run" a ReadP parser: the expanded type is readP_to_S :: ReadP a -> String -> [(a,String)]

readS_to_P :: ReadS a -> ReadP r a

Converts a Haskell ReadS-style function into a parser. Warning: This introduces local backtracking in the resulting parser, and therefore a possible inefficiency.

Properties

The following are QuickCheck specifications of what the combinators do. These can be seen as formal specifications of the behavior of the combinators.

We use bags to give semantics to the combinators.

  type Bag a = [a]

Equality on bags does not care about the order of elements.

  (=~) :: Ord a => Bag a -> Bag a -> Bool
  xs =~ ys = sort xs == sort ys

A special equality operator to avoid unresolved overloading when testing the properties.

  (=~.) :: Bag (Int,String) -> Bag (Int,String) -> Bool
  (=~.) = (=~)

Here follow the properties:

  prop_Get_Nil =
    readP_to_S get [] =~ []

  prop_Get_Cons c s =
    readP_to_S get (c:s) =~ [(c,s)]

  prop_Look s =
    readP_to_S look s =~ [(s,s)]

  prop_Fail s =
    readP_to_S pfail s =~. []

  prop_Return x s =
    readP_to_S (return x) s =~. [(x,s)]

  prop_Bind p k s =
    readP_to_S (p >>= k) s =~.
      [ ys''
      | (x,s') <- readP_to_S p s
      , ys''   <- readP_to_S (k (x::Int)) s'
      ]

  prop_Plus p q s =
    readP_to_S (p +++ q) s =~.
      (readP_to_S p s ++ readP_to_S q s)

  prop_LeftPlus p q s =
    readP_to_S (p <++ q) s =~.
      (readP_to_S p s +<+ readP_to_S q s)
   where
    [] +<+ ys = ys
    xs +<+ _  = xs

  prop_Gather s =
    forAll readPWithoutReadS $ \p ->
      readP_to_S (gather p) s =~
	 [ ((pre,x::Int),s')
	 | (x,s') <- readP_to_S p s
	 , let pre = take (length s - length s') s
	 ]

  prop_String_Yes this s =
    readP_to_S (string this) (this ++ s) =~
      [(this,s)]

  prop_String_Maybe this s =
    readP_to_S (string this) s =~
      [(this, drop (length this) s) | this `isPrefixOf` s]

  prop_Munch p s =
    readP_to_S (munch p) s =~
      [(takeWhile p s, dropWhile p s)]

  prop_Munch1 p s =
    readP_to_S (munch1 p) s =~
      [(res,s') | let (res,s') = (takeWhile p s, dropWhile p s), not (null res)]

  prop_Choice ps s =
    readP_to_S (choice ps) s =~.
      readP_to_S (foldr (+++) pfail ps) s

  prop_ReadS r s =
    readP_to_S (readS_to_P r) s =~. r s

The ReadP type

Primitive operations

Other operations

Running a parser

Properties

The `ReadP` type