Fold integer-simple.git into ghc.git (re #8545)
[ghc.git] / libraries / base / Text / ParserCombinators / ReadPrec.hs
1 {-# LANGUAGE Trustworthy #-}
2 {-# LANGUAGE NoImplicitPrelude #-}
3
4 -----------------------------------------------------------------------------
5 -- |
6 -- Module : Text.ParserCombinators.ReadPrec
7 -- Copyright : (c) The University of Glasgow 2002
8 -- License : BSD-style (see the file libraries/base/LICENSE)
9 --
10 -- Maintainer : libraries@haskell.org
11 -- Stability : provisional
12 -- Portability : non-portable (uses Text.ParserCombinators.ReadP)
13 --
14 -- This library defines parser combinators for precedence parsing.
15
16 -----------------------------------------------------------------------------
17
18 module Text.ParserCombinators.ReadPrec
19 (
20 ReadPrec,
21
22 -- * Precedences
23 Prec,
24 minPrec,
25
26 -- * Precedence operations
27 lift,
28 prec,
29 step,
30 reset,
31
32 -- * Other operations
33 -- | All are based directly on their similarly-named 'ReadP' counterparts.
34 get,
35 look,
36 (+++),
37 (<++),
38 pfail,
39 choice,
40
41 -- * Converters
42 readPrec_to_P,
43 readP_to_Prec,
44 readPrec_to_S,
45 readS_to_Prec,
46 )
47 where
48
49
50 import Text.ParserCombinators.ReadP
51 ( ReadP
52 , ReadS
53 , readP_to_S
54 , readS_to_P
55 )
56
57 import qualified Text.ParserCombinators.ReadP as ReadP
58 ( get
59 , look
60 , (+++), (<++)
61 , pfail
62 )
63
64 import Control.Monad( MonadPlus(..) )
65 import GHC.Num( Num(..) )
66 import GHC.Base
67
68 -- ---------------------------------------------------------------------------
69 -- The readPrec type
70
71 newtype ReadPrec a = P (Prec -> ReadP a)
72
73 -- Functor, Monad, MonadPlus
74
75 instance Functor ReadPrec where
76 fmap h (P f) = P (\n -> fmap h (f n))
77
78 instance Monad ReadPrec where
79 return x = P (\_ -> return x)
80 fail s = P (\_ -> fail s)
81 P f >>= k = P (\n -> do a <- f n; let P f' = k a in f' n)
82
83 instance MonadPlus ReadPrec where
84 mzero = pfail
85 mplus = (+++)
86
87 -- precedences
88
89 type Prec = Int
90
91 minPrec :: Prec
92 minPrec = 0
93
94 -- ---------------------------------------------------------------------------
95 -- Operations over ReadPrec
96
97 lift :: ReadP a -> ReadPrec a
98 -- ^ Lift a precedence-insensitive 'ReadP' to a 'ReadPrec'.
99 lift m = P (\_ -> m)
100
101 step :: ReadPrec a -> ReadPrec a
102 -- ^ Increases the precedence context by one.
103 step (P f) = P (\n -> f (n+1))
104
105 reset :: ReadPrec a -> ReadPrec a
106 -- ^ Resets the precedence context to zero.
107 reset (P f) = P (\_ -> f minPrec)
108
109 prec :: Prec -> ReadPrec a -> ReadPrec a
110 -- ^ @(prec n p)@ checks whether the precedence context is
111 -- less than or equal to @n@, and
112 --
113 -- * if not, fails
114 --
115 -- * if so, parses @p@ in context @n@.
116 prec n (P f) = P (\c -> if c <= n then f n else ReadP.pfail)
117
118 -- ---------------------------------------------------------------------------
119 -- Derived operations
120
121 get :: ReadPrec Char
122 -- ^ Consumes and returns the next character.
123 -- Fails if there is no input left.
124 get = lift ReadP.get
125
126 look :: ReadPrec String
127 -- ^ Look-ahead: returns the part of the input that is left, without
128 -- consuming it.
129 look = lift ReadP.look
130
131 (+++) :: ReadPrec a -> ReadPrec a -> ReadPrec a
132 -- ^ Symmetric choice.
133 P f1 +++ P f2 = P (\n -> f1 n ReadP.+++ f2 n)
134
135 (<++) :: ReadPrec a -> ReadPrec a -> ReadPrec a
136 -- ^ Local, exclusive, left-biased choice: If left parser
137 -- locally produces any result at all, then right parser is
138 -- not used.
139 P f1 <++ P f2 = P (\n -> f1 n ReadP.<++ f2 n)
140
141 pfail :: ReadPrec a
142 -- ^ Always fails.
143 pfail = lift ReadP.pfail
144
145 choice :: [ReadPrec a] -> ReadPrec a
146 -- ^ Combines all parsers in the specified list.
147 choice ps = foldr (+++) pfail ps
148
149 -- ---------------------------------------------------------------------------
150 -- Converting between ReadPrec and Read
151
152 readPrec_to_P :: ReadPrec a -> (Int -> ReadP a)
153 readPrec_to_P (P f) = f
154
155 readP_to_Prec :: (Int -> ReadP a) -> ReadPrec a
156 readP_to_Prec f = P f
157
158 readPrec_to_S :: ReadPrec a -> (Int -> ReadS a)
159 readPrec_to_S (P f) n = readP_to_S (f n)
160
161 readS_to_Prec :: (Int -> ReadS a) -> ReadPrec a
162 readS_to_Prec f = P (\n -> readS_to_P (f n))
163