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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
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

Instances

Functor ReadP
Monad ReadP
MonadPlus ReadP

Primitive operations
get :: ReadP Char
Consumes and returns the next character. Fails if there is no input left.
look :: ReadP String
Look-ahead: returns the part of the input that is left, without consuming it.
(+++) :: ReadP a -> ReadP a -> ReadP a
Symmetric choice.
(<++) :: ReadP a -> ReadP a -> ReadP a
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)
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
Always fails.
satisfy :: (Char -> Bool) -> ReadP Char
Consumes and returns the next character, if it satisfies the specified predicate.
char :: Char -> ReadP Char
Parses and returns the specified character.
string :: String -> ReadP String
Parses and returns the specified string.
munch :: (Char -> Bool) -> ReadP String
Parses the first zero or more characters satisfying the predicate.
munch1 :: (Char -> Bool) -> ReadP String
Parses the first one or more characters satisfying the predicate.
skipSpaces :: ReadP ()
Skips all whitespace.
choice :: [ReadP a] -> ReadP a
Combines all parsers in the specified list.
Running a parser
type ReadS a = String -> [(a, String)]
A parser for a type `a`, represented as a function that takes a `String` and returns a list of possible parses `(a,`String`)` pairs.
readP_to_S :: ReadP 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 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

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