blob: f2407ac623e97718d462d18348c12bcad817386a (
plain)
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
|
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE EmptyDataDecls #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
-- #2238
-- Notice that class CTF has just one value field, but
-- it also has an equality predicate.
-- See Note [Class newtypes and equality predicates] in BuildTyCl
module Foo where
data A
data B
-- via functional dependencies
class HowFD a how | a -> how
class HowFD a how => CFD a how where
cfd :: a -> String
cfd _ = "cfd"
instance HowFD a how => CFD a how
instance HowFD Bool A
-- via type families
type family HowTF a
class how ~ HowTF a => CTF a how where
ctf :: a -> String
ctf _ = "ctf"
instance how ~ HowTF a => CTF a how
type instance HowTF Bool = A
|
