blob: 41754b4a07018ff6bd66188575ee21ee4ecc5487 (
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
40
41
42
|
{-# LANGUAGE
MonoLocalBinds,
PolyKinds,
FunctionalDependencies,
FlexibleContexts #-}
-- reduced version of
-- http://code.haskell.org/~aavogt/HList_dredge_ghc802/Data/HList/Dredge.hs
--
-- F ~ EnsureLabel
-- g_f ~ dredge
-- g_f' ~ dredge'
{- ghc-8.0.2
too_eager.hs:25:14: warning: [-Wdeferred-type-errors]
• No instance for (F t0 (Proxy b0)) arising from a use of ‘g_f’
• In the first argument of ‘id’, namely ‘(g_f a)’
In the expression: id (g_f a)
In an equation for ‘g_f'’: g_f' a = id (g_f a)
ghc-7.10.3 can wait to select the instance for F
-}
module T13371 where
import Data.Proxy
class F a b | a -> b where
f :: a -> b
g :: Proxy b -> c
g _ = undefined
-- a type signature (what ghc infers) makes it work
-- g_f :: F a (Proxy b) => a -> c
g_f a = g (f a)
g_f' a = id (g_f a)
|
