Text/ParserCombinators/ReadPrec.hs

{-# 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

-- | @since 2.01
instance Functor ReadPrec where
 fmap h (P f) = P (\n -> fmap h (f n))

-- | @since 4.6.0.0
instance Applicative ReadPrec where
 pure x = P (\_ -> pure x)
 (<*>) = ap
 liftA2 = liftM2

-- | @since 2.01
instance Monad ReadPrec where
 P f >>= k = P (\n -> do a <- f n; let P f' = k a in f' n)

-- | @since 4.9.0.0
instance MonadFail.MonadFail ReadPrec where
 fail s = P (\_ -> MonadFail.fail s)

-- | @since 2.01
instance MonadPlus ReadPrec

-- | @since 4.6.0.0
instance Alternative ReadPrec where
 empty = pfail
 (<|>) = (+++)

-- 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))