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|
{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE NoImplicitPrelude #-}
-----------------------------------------------------------------------------
-- |
-- Module : Text.ParserCombinators.ReadPrec
-- 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 (uses Text.ParserCombinators.ReadP)
--
-- This library defines parser combinators for precedence parsing.
-----------------------------------------------------------------------------
module Text.ParserCombinators.ReadPrec
(
ReadPrec,
-- * Precedences
Prec,
minPrec,
-- * Precedence operations
lift,
prec,
step,
reset,
-- * Other operations
-- | All are based directly on their similarly-named 'ReadP' counterparts.
get,
look,
(+++),
(<++),
pfail,
choice,
-- * Converters
readPrec_to_P,
readP_to_Prec,
readPrec_to_S,
readS_to_Prec,
)
where
import Text.ParserCombinators.ReadP
( ReadP
, ReadS
, readP_to_S
, readS_to_P
)
import qualified Text.ParserCombinators.ReadP as ReadP
( get
, look
, (+++), (<++)
, pfail
)
import GHC.Num( Num(..) )
import GHC.Base
import qualified Control.Monad.Fail as MonadFail
-- ---------------------------------------------------------------------------
-- The readPrec type
newtype ReadPrec a = P (Prec -> ReadP a)
-- Functor, Monad, MonadPlus
instance Functor ReadPrec where
fmap h (P f) = P (\n -> fmap h (f n))
instance Applicative ReadPrec where
pure x = P (\_ -> pure x)
(<*>) = ap
instance Monad ReadPrec where
fail s = P (\_ -> fail s)
P f >>= k = P (\n -> do a <- f n; let P f' = k a in f' n)
instance MonadFail.MonadFail ReadPrec where
fail s = P (\_ -> fail s)
instance MonadPlus ReadPrec where
mzero = pfail
mplus = (+++)
instance Alternative ReadPrec where
empty = mzero
(<|>) = mplus
-- precedences
type Prec = Int
minPrec :: Prec
minPrec = 0
-- ---------------------------------------------------------------------------
-- Operations over ReadPrec
lift :: ReadP a -> ReadPrec a
-- ^ Lift a precedence-insensitive 'ReadP' to a 'ReadPrec'.
lift m = P (\_ -> m)
step :: ReadPrec a -> ReadPrec a
-- ^ Increases the precedence context by one.
step (P f) = P (\n -> f (n+1))
reset :: ReadPrec a -> ReadPrec a
-- ^ Resets the precedence context to zero.
reset (P f) = P (\_ -> f minPrec)
prec :: Prec -> ReadPrec a -> ReadPrec a
-- ^ @(prec n p)@ checks whether the precedence context is
-- less than or equal to @n@, and
--
-- * if not, fails
--
-- * if so, parses @p@ in context @n@.
prec n (P f) = P (\c -> if c <= n then f n else ReadP.pfail)
-- ---------------------------------------------------------------------------
-- Derived operations
get :: ReadPrec Char
-- ^ Consumes and returns the next character.
-- Fails if there is no input left.
get = lift ReadP.get
look :: ReadPrec String
-- ^ Look-ahead: returns the part of the input that is left, without
-- consuming it.
look = lift ReadP.look
(+++) :: ReadPrec a -> ReadPrec a -> ReadPrec a
-- ^ Symmetric choice.
P f1 +++ P f2 = P (\n -> f1 n ReadP.+++ f2 n)
(<++) :: ReadPrec a -> ReadPrec a -> ReadPrec a
-- ^ Local, exclusive, left-biased choice: If left parser
-- locally produces any result at all, then right parser is
-- not used.
P f1 <++ P f2 = P (\n -> f1 n ReadP.<++ f2 n)
pfail :: ReadPrec a
-- ^ Always fails.
pfail = lift ReadP.pfail
choice :: [ReadPrec a] -> ReadPrec a
-- ^ Combines all parsers in the specified list.
choice ps = foldr (+++) pfail ps
-- ---------------------------------------------------------------------------
-- Converting between ReadPrec and Read
readPrec_to_P :: ReadPrec a -> (Int -> ReadP a)
readPrec_to_P (P f) = f
readP_to_Prec :: (Int -> ReadP a) -> ReadPrec a
readP_to_Prec f = P f
readPrec_to_S :: ReadPrec a -> (Int -> ReadS a)
readPrec_to_S (P f) n = readP_to_S (f n)
readS_to_Prec :: (Int -> ReadS a) -> ReadPrec a
readS_to_Prec f = P (\n -> readS_to_P (f n))
|
