DeriveFunctor and friends don't understand type families
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeFamilies #-}
data Test ext a where
Foo :: a -> Test ext a
Extend :: (ExtensionType ext a) -> Test ext a
type family ExtensionType ext a
data ListExtension
type instance ExtensionType ListExtension a = [a]
deriving instance Functor (Test ListExtension)
{-
a.hs:15:1: error:
• Can't make a derived instance of ‘Functor (Test ListExtension)’:
Constructor ‘Extend’ must use the type variable only as the last argument of a data type
• In the stand-alone deriving instance for
‘Functor (Test ListExtension)’
|
15 | deriving instance Functor (Test ListExtension)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Failed, modules loaded: none.
-}
Trac metadata
Trac field |
Value |
Version |
8.2.1-rc2 |
Type |
Bug |
TypeOfFailure |
OtherFailure |
Priority |
normal |
Resolution |
Unresolved |
Component |
Compiler |
Test case |
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Differential revisions |
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BlockedBy |
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Related |
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Blocking |
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CC |
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Operating system |
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Architecture |
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