{-# LANGUAGE Trustworthy #-} {-# LANGUAGE NoImplicitPrelude #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE MagicHash #-} ----------------------------------------------------------------------------- -- | -- Module : Text.ParserCombinators.ReadP -- 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) -- -- 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\". ----------------------------------------------------------------------------- module Text.ParserCombinators.ReadP ( -- * The 'ReadP' type ReadP, -- * Primitive operations get, look, (+++), (<++), gather, -- * Other operations pfail, eof, satisfy, char, string, munch, munch1, skipSpaces, choice, count, between, option, optional, many, many1, skipMany, skipMany1, sepBy, sepBy1, endBy, endBy1, chainr, chainl, chainl1, chainr1, manyTill, -- * Running a parser ReadS, readP_to_S, readS_to_P, -- * Properties -- $properties ) where import Control.Monad( MonadPlus(..), sequence, liftM2 ) import {-# SOURCE #-} GHC.Unicode ( isSpace ) import GHC.List ( replicate, null ) import GHC.Base infixr 5 +++, <++ ------------------------------------------------------------------------ -- ReadS -- | 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'). type ReadS a = String -> [(a,String)] -- --------------------------------------------------------------------------- -- The P type -- is representation type -- should be kept abstract data P a = Get (Char -> P a) | Look (String -> P a) | Fail | Result a (P a) | Final [(a,String)] -- invariant: list is non-empty! -- Monad, MonadPlus instance Monad P where return x = Result x Fail (Get f) >>= k = Get (\c -> f c >>= k) (Look f) >>= k = Look (\s -> f s >>= k) Fail >>= _ = Fail (Result x p) >>= k = k x `mplus` (p >>= k) (Final r) >>= k = final [ys' | (x,s) <- r, ys' <- run (k x) s] fail _ = Fail instance MonadPlus P where mzero = Fail -- most common case: two gets are combined Get f1 `mplus` Get f2 = Get (\c -> f1 c `mplus` f2 c) -- results are delivered as soon as possible Result x p `mplus` q = Result x (p `mplus` q) p `mplus` Result x q = Result x (p `mplus` q) -- fail disappears Fail `mplus` p = p p `mplus` Fail = p -- two finals are combined -- final + look becomes one look and one final (=optimization) -- final + sthg else becomes one look and one final Final r `mplus` Final t = Final (r ++ t) Final r `mplus` Look f = Look (\s -> Final (r ++ run (f s) s)) Final r `mplus` p = Look (\s -> Final (r ++ run p s)) Look f `mplus` Final r = Look (\s -> Final (run (f s) s ++ r)) p `mplus` Final r = Look (\s -> Final (run p s ++ r)) -- two looks are combined (=optimization) -- look + sthg else floats upwards Look f `mplus` Look g = Look (\s -> f s `mplus` g s) Look f `mplus` p = Look (\s -> f s `mplus` p) p `mplus` Look f = Look (\s -> p `mplus` f s) -- --------------------------------------------------------------------------- -- The ReadP type newtype ReadP a = R (forall b . (a -> P b) -> P b) -- Functor, Monad, MonadPlus instance Functor ReadP where fmap h (R f) = R (\k -> f (k . h)) instance Monad ReadP where return x = R (\k -> k x) fail _ = R (\_ -> Fail) R m >>= f = R (\k -> m (\a -> let R m' = f a in m' k)) instance MonadPlus ReadP where mzero = pfail mplus = (+++) -- --------------------------------------------------------------------------- -- Operations over P final :: [(a,String)] -> P a -- Maintains invariant for Final constructor final [] = Fail final r = Final r run :: P a -> ReadS a run (Get f) (c:s) = run (f c) s run (Look f) s = run (f s) s run (Result x p) s = (x,s) : run p s run (Final r) _ = r run _ _ = [] -- --------------------------------------------------------------------------- -- Operations over ReadP get :: ReadP Char -- ^ Consumes and returns the next character. -- Fails if there is no input left. get = R Get look :: ReadP String -- ^ Look-ahead: returns the part of the input that is left, without -- consuming it. look = R Look pfail :: ReadP a -- ^ Always fails. pfail = R (\_ -> Fail) (+++) :: ReadP a -> ReadP a -> ReadP a -- ^ Symmetric choice. R f1 +++ R f2 = R (\k -> f1 k `mplus` f2 k) (<++) :: 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. R f0 <++ q = do s <- look probe (f0 return) s 0# where probe (Get f) (c:s) n = probe (f c) s (n+#1#) probe (Look f) s n = probe (f s) s n probe p@(Result _ _) _ n = discard n >> R (p >>=) probe (Final r) _ _ = R (Final r >>=) probe _ _ _ = q discard 0# = return () discard n = get >> discard (n-#1#) 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. gather (R m) = R (\k -> gath id (m (\a -> return (\s -> k (s,a))))) where gath :: (String -> String) -> P (String -> P b) -> P b gath l (Get f) = Get (\c -> gath (l.(c:)) (f c)) gath _ Fail = Fail gath l (Look f) = Look (\s -> gath l (f s)) gath l (Result k p) = k (l []) `mplus` gath l p gath _ (Final _) = error "do not use readS_to_P in gather!" -- --------------------------------------------------------------------------- -- Derived operations satisfy :: (Char -> Bool) -> ReadP Char -- ^ Consumes and returns the next character, if it satisfies the -- specified predicate. satisfy p = do c <- get; if p c then return c else pfail char :: Char -> ReadP Char -- ^ Parses and returns the specified character. char c = satisfy (c ==) eof :: ReadP () -- ^ Succeeds iff we are at the end of input eof = do { s <- look ; if null s then return () else pfail } string :: String -> ReadP String -- ^ Parses and returns the specified string. string this = do s <- look; scan this s where scan [] _ = do return this scan (x:xs) (y:ys) | x == y = do _ <- get; scan xs ys scan _ _ = do pfail munch :: (Char -> Bool) -> ReadP String -- ^ 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)) munch p = do s <- look scan s where scan (c:cs) | p c = do _ <- get; s <- scan cs; return (c:s) scan _ = do return "" munch1 :: (Char -> Bool) -> ReadP String -- ^ 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)) munch1 p = do c <- get if p c then do s <- munch p; return (c:s) else pfail choice :: [ReadP a] -> ReadP a -- ^ Combines all parsers in the specified list. choice [] = pfail choice [p] = p choice (p:ps) = p +++ choice ps skipSpaces :: ReadP () -- ^ Skips all whitespace. skipSpaces = do s <- look skip s where skip (c:s) | isSpace c = do _ <- get; skip s skip _ = do return () count :: Int -> ReadP a -> ReadP [a] -- ^ @count n p@ parses @n@ occurrences of @p@ in sequence. A list of -- results is returned. count n p = sequence (replicate n p) between :: ReadP open -> ReadP close -> ReadP a -> ReadP a -- ^ @between open close p@ parses @open@, followed by @p@ and finally -- @close@. Only the value of @p@ is returned. between open close p = do _ <- open x <- p _ <- close return x option :: a -> ReadP a -> ReadP a -- ^ @option x p@ will either parse @p@ or return @x@ without consuming -- any input. option x p = p +++ return x optional :: ReadP a -> ReadP () -- ^ @optional p@ optionally parses @p@ and always returns @()@. optional p = (p >> return ()) +++ return () many :: ReadP a -> ReadP [a] -- ^ Parses zero or more occurrences of the given parser. many p = return [] +++ many1 p many1 :: ReadP a -> ReadP [a] -- ^ Parses one or more occurrences of the given parser. many1 p = liftM2 (:) p (many p) skipMany :: ReadP a -> ReadP () -- ^ Like 'many', but discards the result. skipMany p = many p >> return () skipMany1 :: ReadP a -> ReadP () -- ^ Like 'many1', but discards the result. skipMany1 p = p >> skipMany p sepBy :: ReadP a -> ReadP sep -> ReadP [a] -- ^ @sepBy p sep@ parses zero or more occurrences of @p@, separated by @sep@. -- Returns a list of values returned by @p@. sepBy p sep = sepBy1 p sep +++ return [] sepBy1 :: ReadP a -> ReadP sep -> ReadP [a] -- ^ @sepBy1 p sep@ parses one or more occurrences of @p@, separated by @sep@. -- Returns a list of values returned by @p@. sepBy1 p sep = liftM2 (:) p (many (sep >> p)) endBy :: ReadP a -> ReadP sep -> ReadP [a] -- ^ @endBy p sep@ parses zero or more occurrences of @p@, separated and ended -- by @sep@. endBy p sep = many (do x <- p ; _ <- sep ; return x) endBy1 :: ReadP a -> ReadP sep -> ReadP [a] -- ^ @endBy p sep@ parses one or more occurrences of @p@, separated and ended -- by @sep@. endBy1 p sep = many1 (do x <- p ; _ <- sep ; return x) chainr :: ReadP a -> ReadP (a -> a -> a) -> a -> ReadP a -- ^ @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. chainr p op x = chainr1 p op +++ return x chainl :: ReadP a -> ReadP (a -> a -> a) -> a -> ReadP a -- ^ @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. chainl p op x = chainl1 p op +++ return x chainr1 :: ReadP a -> ReadP (a -> a -> a) -> ReadP a -- ^ Like 'chainr', but parses one or more occurrences of @p@. chainr1 p op = scan where scan = p >>= rest rest x = do f <- op y <- scan return (f x y) +++ return x chainl1 :: ReadP a -> ReadP (a -> a -> a) -> ReadP a -- ^ Like 'chainl', but parses one or more occurrences of @p@. chainl1 p op = p >>= rest where rest x = do f <- op y <- p rest (f x y) +++ return x manyTill :: ReadP a -> ReadP end -> ReadP [a] -- ^ @manyTill p end@ parses zero or more occurrences of @p@, until @end@ -- succeeds. Returns a list of values returned by @p@. manyTill p end = scan where scan = (end >> return []) <++ (liftM2 (:) p scan) -- --------------------------------------------------------------------------- -- Converting between ReadP and Read 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)] @ readP_to_S (R f) = run (f return) 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. readS_to_P r = R (\k -> Look (\s -> final [bs'' | (a,s') <- r s, bs'' <- run (k a) s'])) -- --------------------------------------------------------------------------- -- QuickCheck properties that hold for the combinators {- $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 -}