GHCi suggestion for instance signature (InstanceSigs) is wrong
With InstanceSigs
and PolyKinds
enabled and the following type with kind ĸ -> *
:
data NullableInterp a = NullI Bool
I want to make it an instance of Alternative
and I decide to supply a signature for empty
:
instance Alternative NullableInterp where
empty :: NullableInterp a
empty = undefined
(<|>) = undefined
but GHCi complains with the following error:
% ghci -ignore-dot-ghci error.hs
GHCi, version 7.8.2: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main ( error.hs, interpreted )
error.hs:14:12:
Method signature does not match class; it should be
empty :: forall a. NullableInterp a
In the instance declaration for ‘Alternative NullableInterp’
Failed, modules loaded: none.
Prelude>
Trying GHCi's suggestion (with appropriate extensions) doesn't help and I have to write empty :: NullableInterp (a :: *)
for it to work. I'm not sure which behaviour is the right one but either it should have compiled or GHCi's suggestion should be empty :: forall a. NullableInterp (a :: *)
.
Trac metadata
Trac field | Value |
---|---|
Version | 7.8.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | GHCi |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | hvr |
Operating system | |
Architecture |