Re: [GHC] #14332: Deriving clauses can have forall types

9 Oct 2017

      #14332: Deriving clauses can have forall types
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1
  checker)                           |
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Replying to [comment:7 simonpj]:
...
I'd say this is an outright bug.  You should say
{{{
data D = D deriving (C Type)
}}}
Would you like to open a ticket?
No, because this is behaving as I would expect it to! Kind unification is
 fundamental to the way `deriving` works, and I'm leery of any design which
 doesn't incorporate it as a guiding principle. Here is another example
 where `deriving` //must// unify kinds:

 {{{#!hs
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TypeInType #-}
 {-# OPTIONS_GHC -ddump-deriv #-}

 data Proxy k (a :: k) = Proxy deriving Functor
 }}}

 {{{
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/ryanglscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 ==================== Derived instances ====================
 Derived class instances:
   instance GHC.Base.Functor (Main.Proxy *) where
 }}}

 Here, if `k` weren't unified with `*`, then the instance simply wouldn't
 be well kinded.

 How about another example?

 {{{#!hs
 {-# LANGUAGE DeriveAnyClass #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE TypeInType #-}
 {-# OPTIONS_GHC -ddump-deriv #-}

 import Data.Kind

 class C k (f :: k -> *)

 data T j (a :: j) deriving (C k)
 }}}
 {{{
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/ryanglscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 ==================== Derived instances ====================
 Derived class instances:
   instance Main.C k (Main.T k) where
 }}}

 Notice that GHC didn't attempt to emit an instance of the form `forall k
 j. C k (T j)`—instead, it deliberately unified `k` and `j`! This is a good
 thing, because otherwise GHC would spit out utter nonsense that wouldn't
 pass muster.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14332#comment:8
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