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Text.ParserCombinators.ReadP |

Portability | non-portable (local universal quantification)
Stability | provisional
Maintainer | libraries@haskell.org

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".
Synopsis

data ReadP a
get :: ReadP Char
look :: ReadP String
(+++) :: ReadP a -> ReadP a -> ReadP a
(<++) :: ReadP a -> ReadP a -> ReadP a
gather :: ReadP a -> ReadP (String, a)
pfail :: ReadP a
satisfy :: (Char -> Bool) -> ReadP Char
char :: Char -> ReadP Char
string :: String -> ReadP String
munch :: (Char -> Bool) -> ReadP String
munch1 :: (Char -> Bool) -> ReadP String
skipSpaces :: ReadP ()
choice :: [ReadP a] -> ReadP a
count :: Int -> ReadP a -> ReadP [a]
between :: ReadP open -> ReadP close -> ReadP a -> ReadP a
option :: a -> ReadP a -> ReadP a
optional :: ReadP a -> ReadP ()
many :: ReadP a -> ReadP [a]
many1 :: ReadP a -> ReadP [a]
skipMany :: ReadP a -> ReadP ()
skipMany1 :: ReadP a -> ReadP ()
sepBy :: ReadP a -> ReadP sep -> ReadP [a]
sepBy1 :: ReadP a -> ReadP sep -> ReadP [a]
endBy :: ReadP a -> ReadP sep -> ReadP [a]
endBy1 :: ReadP a -> ReadP sep -> ReadP [a]
chainr :: ReadP a -> ReadP (a -> a -> a) -> a -> ReadP a
chainl :: ReadP a -> ReadP (a -> a -> a) -> a -> ReadP a
chainl1 :: ReadP a -> ReadP (a -> a -> a) -> ReadP a
chainr1 :: ReadP a -> ReadP (a -> a -> a) -> ReadP a
manyTill :: ReadP a -> ReadP end -> ReadP [a]
type ReadS a = String -> [(a, String)]
readP_to_S :: ReadP a -> ReadS a
readS_to_P :: ReadS a -> ReadP a

The `ReadP` type

data ReadP a | Source

Instances

Functor ReadP
Monad ReadP
MonadPlus ReadP

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 | Source

Symmetric choice.

(<++) :: ReadP a -> ReadP a -> ReadP a | 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.

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.

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

Parses the first one or more characters satisfying the predicate.

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. 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
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