mirror of http://darcs.net/screened (fork of darcs's darcs-reviewed)  (http://darcs.net/Development/GettingStarted)

root / src / Darcs / Patch / Witnesses / Sealed.hs

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
-- Copyright (C) 2007 David Roundy, 2009 Ganesh Sittampalam
--
-- This program is free software; you can redistribute it and/or modify
-- it under the terms of the GNU General Public License as published by
-- the Free Software Foundation; either version 2, or (at your option)
-- any later version.
--
-- This program is distributed in the hope that it will be useful,
-- but WITHOUT ANY WARRANTY; without even the implied warranty of
-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-- GNU General Public License for more details.
--
-- You should have received a copy of the GNU General Public License
-- along with this program; see the file COPYING.  If not, write to
-- the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
-- Boston, MA 02110-1301, USA.

{-# LANGUAGE CPP, FlexibleInstances #-}
{-# OPTIONS_HADDOCK ignore-exports #-}

module Darcs.Patch.Witnesses.Sealed
    ( Sealed(..)
    , seal
    , unseal
    , mapSeal
    , unsafeUnseal
    , unsafeUnsealFlipped
    , unsafeUnseal2
    , Sealed2(..)
    , seal2
    , unseal2
    , mapSeal2
    , FlippedSeal(..)
    , flipSeal
    , unsealFlipped
    , mapFlipped
    , unsealM
    , liftSM
    , Gap(..)
    , FreeLeft
    , unFreeLeft
    , FreeRight
    , unFreeRight
    ) where

import Prelude ()
import Darcs.Prelude

import Darcs.Patch.Witnesses.Eq ( MyEq, EqCheck(..) )
import Darcs.Patch.Witnesses.Show
import Darcs.Patch.Witnesses.Eq ( (=\/=) )
import Darcs.Patch.Witnesses.Unsafe ( unsafeCoerceP1, unsafeCoerceP )

-- |A 'Sealed' type is a way of hide an existentially quantified type parameter,
-- in this case wX, inside the type. Note that the only thing we can currently
-- recover about the existentially quantified type wX is that it exists.
data Sealed a where
    Sealed :: a wX -> Sealed a

seal :: a wX -> Sealed a
seal = Sealed

instance MyEq a => Eq (Sealed (a wX)) where
    Sealed x == Sealed y | IsEq <- x =\/= y = True
                         | otherwise = False

-- |The same as 'Sealed' but for two parameters (wX and wY).
data Sealed2 a where
    Sealed2 :: !(a wX wY) -> Sealed2 a

seal2 :: a wX wY -> Sealed2 a
seal2 = Sealed2

data FlippedSeal a wY where
    FlippedSeal :: !(a wX wY) -> FlippedSeal a wY

flipSeal :: a wX wY -> FlippedSeal a wY
flipSeal = FlippedSeal

unsafeUnseal :: Sealed a -> a wX
unsafeUnseal (Sealed a) = unsafeCoerceP1 a

unsafeUnsealFlipped :: FlippedSeal a wY -> a wX wY
unsafeUnsealFlipped (FlippedSeal a) = unsafeCoerceP a

unsafeUnseal2 :: Sealed2 a -> a wX wY
unsafeUnseal2 (Sealed2 a) = unsafeCoerceP a

unseal :: (forall wX . a wX -> b) -> Sealed a -> b
unseal f x = f (unsafeUnseal x)

-- laziness property:
-- unseal (const True) undefined == True

unsealM :: Monad m => m (Sealed a) -> (forall wX . a wX -> m b) -> m b
unsealM m1 m2 = do sx <- m1
                   unseal m2 sx

liftSM :: Monad m => (forall wX . a wX -> b) -> m (Sealed a) -> m b
liftSM f m = do sx <- m
                return (unseal f sx)

mapSeal :: (forall wX . a wX -> b wX) -> Sealed a -> Sealed b
mapSeal f = unseal (seal . f)

mapFlipped :: (forall wX . a wX wY -> b wX wZ) -> FlippedSeal a wY -> FlippedSeal b wZ
mapFlipped f (FlippedSeal x) = FlippedSeal (f x)

unseal2 :: (forall wX wY . a wX wY -> b) -> Sealed2 a -> b
unseal2 f a = f (unsafeUnseal2 a)

mapSeal2 :: (forall wX wY . a wX wY -> b wX wY) -> Sealed2 a -> Sealed2 b
mapSeal2 f = unseal2 (seal2 . f)

unsealFlipped :: (forall wX wY . a wX wY -> b) -> FlippedSeal a wZ -> b
unsealFlipped f (FlippedSeal a) = f a

instance Show1 a => Show (Sealed a) where
    showsPrec d (Sealed x) = showParen (d > appPrec) $ showString "Sealed " . showsPrec1 (appPrec + 1) x
instance Show2 a => Show (Sealed2 a) where
    showsPrec d (Sealed2 x) = showParen (d > appPrec) $ showString "Sealed2 " . showsPrec2 (appPrec + 1) x

-- |'Poly' is similar to 'Sealed', but the type argument is
-- universally quantified instead of being existentially quantified.
newtype Poly a = Poly { unPoly :: forall wX . a wX }

-- |'Stepped' is a type level composition operator.
-- For example, @ 'Stepped' ('Sealed' p) @ is equivalent to 
-- @ \\x -> 'Sealed' (p x) @
newtype Stepped (f :: (* -> *) -> *) a wX = Stepped { unStepped :: f (a wX) }

-- |'FreeLeft' p is @ \forall x . \exists y . p x y @
-- In other words the caller is free to specify the left witness,
-- and then the right witness is an existential.
-- Note that the order of the type constructors is important for ensuring
-- that @ y @ is dependent on the @ x @ that is supplied.
-- This is why 'Stepped' is needed, rather than writing the more obvious
-- 'Sealed' ('Poly' p) which would notionally have the same quantification
-- of the type witnesses.
newtype FreeLeft p = FLInternal (Poly (Stepped Sealed p))

-- |'FreeLeft' p is @ \forall y . \exists x . p x y @
-- In other words the caller is free to specify the right witness,
-- and then the left witness is an existential.
-- Note that the order of the type constructors is important for ensuring
-- that @ x @ is dependent on the @ y @ that is supplied.
newtype FreeRight p = FRInternal (Poly (FlippedSeal p))

-- |Unwrap a 'FreeLeft' value
unFreeLeft :: FreeLeft p -> Sealed (p wX)
unFreeLeft (FLInternal x) = unStepped (unPoly x)

-- |Unwrap a 'FreeRight' value
unFreeRight :: FreeRight p -> FlippedSeal p wX
unFreeRight (FRInternal x) = unPoly x

-- |'Gap' abstracts over 'FreeLeft' and 'FreeRight' for code constructing these values
class Gap w where
  -- |An empty 'Gap', e.g. 'NilFL' or 'NilRL'
  emptyGap :: (forall wX . p wX wX) -> w p
  -- |A 'Gap' constructed from a completely polymorphic value, for example the constructors
  -- for primitive patches
  freeGap :: (forall wX wY . p wX wY) -> w p

  -- |Compose two 'Gap' values together in series, e.g. 'joinGap (+>+)' or 'joinGap (:>:)'
  joinGap :: (forall wX wY wZ . p wX wY -> q wY wZ -> r wX wZ) -> w p -> w q -> w r

instance Gap FreeLeft where
  emptyGap e = FLInternal (Poly (Stepped (Sealed e)))
  freeGap e =  FLInternal (Poly (Stepped (Sealed e)))
  joinGap op (FLInternal p) (FLInternal q)
    = FLInternal (Poly (case unPoly p of Stepped (Sealed p') -> case unPoly q of Stepped (Sealed q') -> Stepped (Sealed (p' `op` q'))))

instance Gap FreeRight where
  emptyGap e = FRInternal (Poly (FlippedSeal e))
  freeGap e =  FRInternal (Poly (FlippedSeal e))
  joinGap op (FRInternal p) (FRInternal q)
    = FRInternal (Poly (case unPoly q of FlippedSeal q' -> case unPoly p of FlippedSeal p' -> FlippedSeal (p' `op` q')))