1 {-# OPTIONS -fno-implicit-prelude #-}
2 -----------------------------------------------------------------------------
4 -- Module : Text.ParserCombinators.ReadPrec
5 -- Copyright : (c) The University of Glasgow 2002
6 -- License : BSD-style (see the file libraries/base/LICENSE)
8 -- Maintainer : libraries@haskell.org
9 -- Stability : provisional
10 -- Portability : non-portable (uses Text.ParserCombinators.ReadP)
12 -- This library defines parser combinators for precedence parsing.
14 -----------------------------------------------------------------------------
16 module Text.ParserCombinators.ReadPrec
18 ReadPrec, -- :: * -> *; instance Functor, Monad, MonadPlus
22 minPrec, -- :: Prec; = 0
24 -- * Precedence operations
25 lift, -- :: ReadP a -> ReadPrec a
26 prec, -- :: Prec -> ReadPrec a -> ReadPrec a
27 step, -- :: ReadPrec a -> ReadPrec a
28 reset, -- :: ReadPrec a -> ReadPrec a
31 -- All are based directly on their similarly-naned 'ReadP' counterparts.
32 get, -- :: ReadPrec Char
33 look, -- :: ReadPrec String
34 (+++), -- :: ReadPrec a -> ReadPrec a -> ReadPrec a
35 (<++), -- :: ReadPrec a -> ReadPrec a -> ReadPrec a
36 pfail, -- :: ReadPrec a
37 choice, -- :: [ReadPrec a] -> ReadPrec a
40 readPrec_to_P, -- :: ReadPrec a -> (Int -> ReadP a)
41 readP_to_Prec, -- :: (Int -> ReadP a) -> ReadPrec a
42 readPrec_to_S, -- :: ReadPrec a -> (Int -> ReadS a)
43 readS_to_Prec, -- :: (Int -> ReadS a) -> ReadPrec a
48 import Text.ParserCombinators.ReadP
54 import qualified Text.ParserCombinators.ReadP as ReadP
61 import Control.Monad( MonadPlus(..) )
62 #ifdef __GLASGOW_HASKELL__
63 import GHC.Num( Num(..) )
67 -- ---------------------------------------------------------------------------
70 newtype ReadPrec a = P { unP :: Prec -> ReadP a }
72 -- Functor, Monad, MonadPlus
74 instance Functor ReadPrec where
75 fmap h (P f) = P (\n -> fmap h (f n))
77 instance Monad ReadPrec where
78 return x = P (\_ -> return x)
79 fail s = P (\_ -> fail s)
80 P f >>= k = P (\n -> do a <- f n; let P f' = k a in f' n)
82 instance MonadPlus ReadPrec where
93 -- ---------------------------------------------------------------------------
94 -- Operations over ReadPrec
96 lift :: ReadP a -> ReadPrec a
97 -- ^ Lift a predence-insensitive 'ReadP' to a 'ReadPrec'
100 step :: ReadPrec a -> ReadPrec a
101 -- ^ Increases the precedence context by one
102 step (P f) = P (\n -> f (n+1))
104 reset :: ReadPrec a -> ReadPrec a
105 -- ^ Resets the precedence context to zero
106 reset (P f) = P (\n -> f minPrec)
108 prec :: Prec -> ReadPrec a -> ReadPrec a
109 -- ^ @(prec n p)@ checks that the precedence context is
110 -- less than or equal to n,
112 -- * if so, parses p in context n
113 prec n (P f) = P (\c -> if c <= n then f n else ReadP.pfail)
115 -- ---------------------------------------------------------------------------
116 -- Derived operations
121 look :: ReadPrec String
122 look = lift ReadP.look
124 (+++) :: ReadPrec a -> ReadPrec a -> ReadPrec a
125 P f1 +++ P f2 = P (\n -> f1 n ReadP.+++ f2 n)
127 (<++) :: ReadPrec a -> ReadPrec a -> ReadPrec a
128 P f1 <++ P f2 = P (\n -> f1 n ReadP.<++ f2 n)
131 pfail = lift ReadP.pfail
133 choice :: [ReadPrec a] -> ReadPrec a
134 choice ps = foldr (+++) pfail ps
136 -- ---------------------------------------------------------------------------
137 -- Converting between ReadPrec and Read
139 #ifdef __GLASGOW_HASKELL__
140 -- We define a local version of ReadS here,
141 -- because its "real" definition site is in GHC.Read
142 type ReadS a = String -> [(a,String)]
145 readPrec_to_P :: ReadPrec a -> (Int -> ReadP a)
146 readPrec_to_P (P f) = f
148 readP_to_Prec :: (Int -> ReadP a) -> ReadPrec a
149 readP_to_Prec f = P f
151 readPrec_to_S :: ReadPrec a -> (Int -> ReadS a)
152 readPrec_to_S (P f) n = readP_to_S (f n)
154 readS_to_Prec :: (Int -> ReadS a) -> ReadPrec a
155 readS_to_Prec f = P (\n -> readS_to_P (f n))