GHC mistakenly believes datatype with type synonym in its type can't be eta-reduced
I uncovered this when playing around with -XTypeInType
:
{-# LANGUAGE DeriveFunctor, TypeInType #-}
module CantEtaReduce1 where
import Data.Kind
type ConstantT a b = a
newtype T (f :: * -> *) (a :: ConstantT * f) = T (f a) deriving Functor
This fails because it thinks that you can't reduce the last type variable of T
, since it mentions another type variable (f
):
• Cannot eta-reduce to an instance of form
instance (...) => Functor (T f)
• In the newtype declaration for ‘T
But it should be able to, since ConstantT * f
reduces to *
, and the equivalent declaration newtype T (f :: * -> *) (a :: *) = ...
eta-reduces just fine.
I marked this as appearing in GHC 8.1 since you need -XTypeInType
to have kind synonyms, but this can also happen in earlier GHC versions with data families:
{-# LANGUAGE DeriveFunctor, PolyKinds, TypeFamilies #-}
module CantEtaReduce2 where
type ConstantT a b = a
data family T (f :: * -> *) (a :: *)
newtype instance T f (ConstantT a f) = T (f a) deriving Functor
I believe the fix will be to apply coreView
with precision to parts of the code which typecheck deriving
statements so that these type synonyms are peeled off.
Trac metadata
Trac field | Value |
---|---|
Version | 8.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | goldfire |
Operating system | |
Architecture |