Copyright	(c) The University of Glasgow 2002
License	BSD-style (see the file libraries/base/LICENSE)
Maintainer	[email protected]
Stability	provisional
Portability	non-portable (local universal quantification)
Safe Haskell	Trustworthy
Language	Haskell2010

The ReadP type

data ReadP a Source

Primitive operations

get :: ReadP Char Source

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

look :: ReadP String Source

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

(+++) :: ReadP a -> ReadP a -> ReadP a infixr 5 Source

Symmetric choice.

(<++) :: ReadP a -> ReadP a -> ReadP a infixr 5 Source

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

gather :: ReadP a -> ReadP (String, a) Source

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 a Source

Always fails.

eof :: ReadP () Source

Succeeds iff we are at the end of input

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

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

char :: Char -> ReadP Char Source

Parses and returns the specified character.

string :: String -> ReadP String Source

Parses and returns the specified string.

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

Parses the first zero or more characters satisfying the predicate. Always succeeds, exactly once having consumed all the characters Hence NOT the same as (many (satisfy p))

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

Parses the first one or more characters satisfying the predicate. Fails if none, else succeeds exactly once having consumed all the characters Hence NOT the same as (many1 (satisfy p))

skipSpaces :: ReadP () Source

Skips all whitespace.

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

Combines all parsers in the specified list.

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

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

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

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

option :: a -> ReadP a -> ReadP a Source

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

optional :: ReadP a -> ReadP () Source

optional p optionally parses p and always returns ().

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

Parses zero or more occurrences of the given parser.

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

Parses one or more occurrences of the given parser.

skipMany :: ReadP a -> ReadP () Source

Like many, but discards the result.

skipMany1 :: ReadP a -> ReadP () Source

Like many1, but discards the result.

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

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

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

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

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

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

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

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

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

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 a -> ReadP (a -> a -> a) -> a -> ReadP a Source

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 a -> ReadP (a -> a -> a) -> ReadP a Source

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

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

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

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

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 -> ReadS a Source

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 a Source

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.

For some values, we only care about the lists contents, not their order,

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

Here follow the properties:

>>> readP_to_S get []
[]

\c str -> readP_to_S get (c:str) == [(c, str)]

\str -> readP_to_S look str == [(str, str)]

\str -> readP_to_S pfail str == []

\x str -> 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
      ]

\this str -> readP_to_S (string this) (this ++ str) == [(this,str)]

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