Constraint doesn't reduce in the presence of quantified type variables
{-# OPTIONS -fglasgow-exts #-}
{-# OPTIONS -fallow-undecidable-instances #-}
module Problem where
data Z
data S a
class HPrefix l
instance (NSub (S Z) ndiff, HDrop ndiff l l) => HPrefix l
class NSub n1 n3 | n1 -> n3
instance NSub Z Z
instance NSub n1 n3 => NSub (S n1) n3
class HDrop n l1 l2 | n l1 -> l2
instance HDrop Z l l
t_hPrefix :: HPrefix l => l -> ()
t_hPrefix = undefined
-- This works...
thr' :: (forall r. l -> a) -> a
thr' f = f undefined
thP4' = thr' t_hPrefix
-- ... but this doesn't work...?
thr :: (forall r. r -> a) -> a
thr f = f undefined
thP4 = thr t_hPrefix
$ ghci GHCProblem.hs
___ ___ _
/ _ \ /\ /\/ __(_)
/ /_\// /_/ / / | | GHC Interactive, version 6.6.1, for Haskell 98.
/ /_\\/ __ / /___| | http://www.haskell.org/ghc/
\____/\/ /_/\____/|_| Type :? for help.
Loading package base ... linking ... done.
[1 of 1] Compiling Problem ( GHCProblem.hs, interpreted )
GHCProblem.hs:30:11:
No instance for (HDrop ndiff r r)
arising from use of `t_hPrefix' at GHCProblem.hs:30:11-19
Possible fix:
add (HDrop ndiff r r) to the expected type of an expression
In the first argument of `thr', namely `t_hPrefix'
In the expression: thr t_hPrefix
In the definition of `thP4': thP4 = thr t_hPrefix
Failed, modules loaded: none.
Trac metadata
Trac field |
Value |
Version |
6.6.1 |
Type |
Bug |
TypeOfFailure |
OtherFailure |
Priority |
normal |
Resolution |
Unresolved |
Component |
Compiler (Type checker) |
Test case |
|
Differential revisions |
|
BlockedBy |
|
Related |
|
Blocking |
|
CC |
|
Operating system |
Unknown |
Architecture |
|