Unhelpful IncoherentInstances suggestion with FunctionalDependencies
Consider the following program (simplified from http://hpaste.org/86928):
{-# LANGUAGE
FlexibleContexts
, FlexibleInstances
, FunctionalDependencies
, MultiParamTypeClasses
, KindSignatures
, UndecidableInstances #-}
class Het a b | a -> b where
het :: m (f c) -> a -> m b
class GHet (a :: * -> *) (b :: * -> *) | a -> b
instance GHet (K a) (K [a])
instance Het a b => GHet (K a) (K b)
data A a = A (A a)
data K x a = K x
instance Het (A a) (A [a]) where het = het1
het1 :: (GHet (K a) (K b)) => m (f c) -> a -> m b
het1 = undefined
In HEAD, it gives rise to the following error:
Overlapping instances for GHet (K (A a)) (K (A [a]))
arising from a use of ‛het1’
Matching instances:
instance Het a b => GHet (K a) (K b) -- Defined at Bug.hs:14:10
There exists a (perhaps superclass) match:
(The choice depends on the instantiation of ‛a’
To pick the first instance above, use -XIncoherentInstances
when compiling the other instance declarations)
In the expression: het1
In an equation for ‛het’: het = het1
In the instance declaration for ‛Het (A a) (A [a])’
It's already strange that it says multiple instances match, but it only lists one. Furthermore, enabling -XIncoherentInstances
, as suggested, does not change the error message.
#7150 (closed) and #7171 (closed) might be related.
Trac metadata
Trac field | Value |
---|---|
Version | 7.7 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |