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
|
-- The behavior of type-inference and OverlappingInstances has changed
-- between GHC 6.12 and GHC 7.0 such that the following code
-- type-checks under 6.12, but not 7.0rc2. I assume this change has
-- something to do with the new type checker in GHC 7, but it is not
-- clear to me if this change in behavior is intended. Nor am I clear
-- how to achieve something similar to the old behavior. This is
-- preventing HSP (and by extension, happstack) from migrating to GHC
-- 7. I reported this earlier on the mailing lists, but I have further
-- simplied the test case here.
{-# LANGUAGE TypeFamilies, MultiParamTypeClasses
, FlexibleContexts, FlexibleInstances, UndecidableInstances
, TypeSynonymInstances, GeneralizedNewtypeDeriving
, OverlappingInstances
#-}
module XMLGenerator where
newtype XMLGenT m a = XMLGenT (m a)
deriving (Functor, Monad)
class Monad m => XMLGen m where
type XML m
data Child m
genElement :: String -> XMLGenT m (XML m)
class XMLGen m => EmbedAsChild m c where
asChild :: c -> XMLGenT m [Child m]
instance (EmbedAsChild m c, m1 ~ m) => EmbedAsChild m (XMLGenT m1 c)
instance (XMLGen m, XML m ~ x) => EmbedAsChild m x
data Xml = Xml
data IdentityT m a = IdentityT (m a)
instance Monad (IdentityT m)
instance XMLGen (IdentityT m) where
type XML (IdentityT m) = Xml
data Identity a = Identity a
instance Monad Identity
instance EmbedAsChild (IdentityT IO) (XMLGenT Identity ())
data FooBar = FooBar
instance EmbedAsChild (IdentityT IO) FooBar where
asChild b = asChild $ (genElement "foo")
-- asChild :: FooBar -> XMLGenT (XMLGenT (IdentityT IO) [Child (IdentitiyT IO)])
{- ---------- Deriving the constraints ----------
asChild :: EmbedAsChild m c => c -> XMLGenT m [Child m]
genElement :: XMLGen m => String -> XMLGenT m (XML m)
Wanted: EmbedAsChild m c, with m = IdentityT IO
c = XMLGenT meta (XML meta)
XMLGen meta
ie EmbedAsChild (IdentityT IO) (XMLGen meta (XML meta)
XMLGen meta
We have instances
EmbedAsChild (IdentityT IO) FooBar
EmbedAsChild (IdentityT IO) (XMLGenT Identity ())
EmbedAsChild m (XMLGenT m1 c)
EmbedAsChild m x
-}
|
