Text.ParserCombinators.ReadP (original) (raw)

The `[ReadP](Text-ParserCombinators-ReadP.html#t:ReadP)` type

Primitive operations

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

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

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](Text-ParserCombinators-ReadP.html#v:gather) gives a runtime error if its first argument is built using any occurrences of readS_to_P.

Other operations

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

Parses the first zero or more characters satisfying the predicate. Always succeds, 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))

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

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

option :: a -> ReadP a -> ReadP aSource

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

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.

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

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 aSource

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.

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](Data-String.html#t:String) and returns a list of possible parses as (a,`[String](Data-String.html#t:String)`) pairs.

Note that this kind of backtracking parser is very inefficient; reading a large structure may be quite slow (cf [ReadP](Text-ParserCombinators-ReadP.html#t:ReadP)).

readP_to_S :: ReadP a -> ReadS aSource

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

readS_to_P :: ReadS a -> ReadP aSource

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