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  • #13731

Closed
Open
Opened May 20, 2017 by Sophie Taylor@spacekittehReporter

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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Reference: ghc/ghc#13731