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| Text.ParserCombinators.ReadP | | Portability | non-portable (local universal quantification) | | Stability | provisional | | Maintainer | libraries@haskell.org |
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| 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".
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| Synopsis |
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| The ReadP type
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 Instances | |
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| Primitive operations
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| Consumes and returns the next character.
Fails if there is no input left.
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| Look-ahead: returns the part of the input that is left, without
consuming it.
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| Symmetric choice.
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| Local, exclusive, left-biased choice: If left parser
locally produces any result at all, then right parser is
not used.
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| 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.
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| Other operations
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| Always fails.
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| Consumes and returns the next character, if it satisfies the
specified predicate.
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| Parses and returns the specified character.
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| Parses and returns the specified string.
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| Parses the first zero or more characters satisfying the predicate.
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| Parses the first one or more characters satisfying the predicate.
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| Skips all whitespace.
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| Combines all parsers in the specified list.
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| count n p parses n occurrences of p in sequence. A list of
results is returned.
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| between open close p parses open, followed by p and finally
close. Only the value of p is returned.
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| option x p will either parse p or return x without consuming
any input.
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| optional p optionally parses p and always returns ().
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| Parses zero or more occurrences of the given parser.
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| Parses one or more occurrences of the given parser.
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| Like many, but discards the result.
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| Like many1, but discards the result.
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| sepBy p sep parses zero or more occurrences of p, separated by sep.
Returns a list of values returned by p.
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| sepBy1 p sep parses one or more occurrences of p, separated by sep.
Returns a list of values returned by p.
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| endBy p sep parses zero or more occurrences of p, separated and ended
by sep.
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| endBy p sep parses one or more occurrences of p, separated and ended
by sep.
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| 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.
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| 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.
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| Like chainl, but parses one or more occurrences of p.
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| Like chainr, but parses one or more occurrences of p.
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| manyTill p end parses zero or more occurrences of p, until end
succeeds. Returns a list of values returned by p.
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| Running a parser
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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).
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| 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)]
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| Converts a Haskell ReadS-style function into a parser.
Warning: This introduces local backtracking in the resulting
parser, and therefore a possible inefficiency.
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| Properties
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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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| Produced by Haddock version 0.9 |
