Incompleteness in the Coercible constraint solver
$ ghci -ignore-dot-ghci
GHCi, version 8.5.20180105: http://www.haskell.org/ghc/ :? for help
Prelude> newtype WF f a = WF (f a)
Prelude> import Data.Coerce
Prelude Data.Coerce> :set -XFlexibleContexts
Prelude Data.Coerce> :t coerce :: Coercible (cat a b) (a -> f b) => cat a b -> (a -> WF f b)
<interactive>:1:1: error:
• Couldn't match representation of type ‘cat1 a1 b1’
with that of ‘a1 -> WF f1 b1’
arising from a use of ‘coerce’
• In the expression:
coerce ::
Coercible (cat a b) (a -> f b) => cat a b -> (a -> WF f b)
I'm not sure if I've filed this before or if it's even a bug.
But we know that Coercible (a -> f b) (a -> WF f b)
so why doesn't this work?
Trac metadata
Trac field | Value |
---|---|
Version | 8.2.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |