Deriving Functor-like classes should unify kind variables

While the deriving machinery always unifies the kind of the typeclass
argument with the kind of the datatype, this proves not to be sufficient
to produce well kinded instances for some poly-kinded datatypes. For
example:

```
newtype Compose (f :: k2 -> *) (g :: k1 -> k2) (a :: k1)
  = Compose (f (g a)) deriving Functor
```

would fail because only `k1` would get unified with `*`, causing the
following
ill kinded instance to be generated:

```
instance (Functor (f :: k2 -> *), Functor (g :: * -> k2)) =>
  Functor (Compose f g) where ...
```

To prevent this, we need to take the subtypes and unify their kinds with
`* -> *`.

Fixes #10524 for good.

Test Plan: ./validate

Reviewers: simonpj, hvr, austin, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2097

GHC Trac Issues: #10524, #10561

1 files changed, 1 insertions, 6 deletions

diff --git a/libraries/base/Data/Functor/Compose.hs b/libraries/base/Data/Functor/Compose.hs
index 230f4e77de..d548836820 100644
--- a/libraries/base/Data/Functor/Compose.hs
+++ b/libraries/base/Data/Functor/Compose.hs
@@ -2,7 +2,6 @@
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE Safe #-}
-{-# LANGUAGE StandaloneDeriving #-}
 -----------------------------------------------------------------------------
 -- |
 -- Module      :  Data.Functor.Compose
@@ -36,11 +35,7 @@ infixr 9 `Compose`
 -- The composition of applicative functors is always applicative,
 -- but the composition of monads is not always a monad.
 newtype Compose f g a = Compose { getCompose :: f (g a) }
-  deriving (Data, Generic)
-
--- We must use standalone deriving here due to a bad interaction between
--- PolyKinds and GHC generics
-deriving instance Functor f => Generic1 (Compose f g)
+  deriving (Data, Generic, Generic1)
 
 -- Instances of lifted Prelude classes