Ambiguous type variables in GeneralisedNewtypeDeriving
When deriving the C
instance in the following code:
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE GeneralisedNewtypeDeriving #-}
module Test where
class C a where f :: String
instance C () where f = "foo"
newtype T = T () deriving C
The following error occurs:
Test.hs:10:27: error:
• Ambiguous type variable ‘a0’ arising from a use of ‘f’
prevents the constraint ‘(C a0)’ from being solved.
Probable fix: use a type annotation to specify what ‘a0’ should be.
These potential instances exist:
instance C T -- Defined at Test.hs:10:27
instance C () -- Defined at Test.hs:8:10
• In the third argument of ‘GHC.Prim.coerce’, namely ‘f’
In the expression: GHC.Prim.coerce @String @String f :: String
In an equation for ‘f’:
f = GHC.Prim.coerce @String @String f :: String
When typechecking the code for ‘f’
in a derived instance for ‘C T’:
To see the code I am typechecking, use -ddump-deriv
|
10 | newtype T = T () deriving C
| ^
- .. and the following core is produced:
==================== Derived instances ====================
Derived class instances:
instance Test.C Test.T where
Test.f
= GHC.Prim.coerce @GHC.Base.String @GHC.Base.String Test.f ::
GHC.Base.String
Derived type family instances:
The problem seems to be that the a
should have been set to ()
within the coerced instance. I've been working round this with a newtype X a = X String
as the result value so that the a
is present in the signature, but I think this is a bug; should a more specialised instance be generated? I hope this is enough of an explanation!
Thanks, Tom
Trac metadata
Trac field | Value |
---|---|
Version | 8.4.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |