Copyright | (c) The University of Glasgow 2002 |
---|---|
License | BSD-style (see the file libraries/base/LICENSE) |
Maintainer | libraries@haskell.org |
Stability | provisional |
Portability | non-portable (local universal quantification) |
Safe Haskell | Trustworthy |
Language | Haskell2010 |
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".
- 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
- eof :: ReadP ()
- 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
Primitive operations
Look-ahead: returns the part of the input that is left, without consuming it.
(<++) :: 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
satisfy :: (Char -> Bool) -> ReadP Char Source
Consumes and returns the next character, if it satisfies the specified predicate.
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))
skipSpaces :: ReadP () Source
Skips all whitespace.
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.
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
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