|Portability|| non-portable (local universal quantification)|
|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".|
|The ReadP type|
|data ReadP a|
|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
|(+++) :: ReadP a -> ReadP a -> ReadP a|
|(<++) :: ReadP a -> ReadP a -> ReadP a|
|Local, exclusive, left-biased choice: If left parser
locally produces any result at all, then right parser is
|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. |
|pfail :: ReadP a|
|satisfy :: (Char -> Bool) -> ReadP Char|
|Consumes and returns the next character, if it satisfies the
|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.|
The following are QuickCheck specifications of what the combinators do.
These can be seen as formal specifications of the behavior of the
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:
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 =~.
| (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)
 +<+ ys = ys
xs +<+ _ = xs
prop_Gather s =
forAll readPWithoutReadS $ \p ->
readP_to_S (gather p) 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) =~
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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