Re: [GHC] #8865: Cannot derive well-kinded instance of form ‘Category

2 May 2016

      #8865: Cannot derive well-kinded instance of form ‘Category
-------------------------------------+-------------------------------------
        Reporter:  adinapoli         |         Owner:
            Type:  bug               |        Status:  closed
        Priority:  normal            |     Milestone:  7.8.1
       Component:  Compiler          |       Version:  7.8.1-rc2
      Resolution:  fixed             |      Keywords:
Operating System:  MacOS X           |  Architecture:  x86_64 (amd64)
 Type of failure:  GHC rejects       |     Test Case:
  valid program                      |  deriving/should_compile/T8865
      Blocked By:                    |      Blocking:
 Related Tickets:                    |
-------------------------------------+-------------------------------------

Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"fa86ac7c14b67f27017d795811265c3a9750024b/ghc"
 fa86ac7c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="fa86ac7c14b67f27017d795811265c3a9750024b"
 Make validDerivPred ignore non-visible arguments to a class type
 constructor

 Summary:
 GHC choked when trying to derive the following:

 ```
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE PolyKinds #-}
 module Example where

 class Category (cat :: k -> k -> *) where
   catId   :: cat a a
   catComp :: cat b c -> cat a b -> cat a c

 newtype T (c :: * -> * -> *) a b = MkT (c a b) deriving Category
 ```

 Unlike in #8865, where we were deriving `Category` for a concrete type
 like
 `Either`, in the above example we are attempting to derive an instance of
 the
 form:

 ```
 instance Category * c => Category (T * c) where ...
 ```

 (using `-fprint-explicit-kinds` syntax). But `validDerivPred` is checking
 if
 `sizePred (Category * c)` equals the number of free type variables in
 `Category * c`. But note that `sizePred` counts both type variables
 //and//
 type constructors, and `*` is a type constructor! So `validDerivPred`
 erroneously rejects the above instance.

 The fix is to make `validDerivPred` ignore non-visible arguments to the
 class
 type constructor (e.g., ignore `*` is `Category * c`) by using
 `filterOutInvisibleTypes`.

 Fixes #11833.

 Test Plan: ./validate

 Reviewers: goldfire, hvr, simonpj, austin, bgamari

 Reviewed By: bgamari

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2112

 GHC Trac Issues: #11833
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8865#comment:14>
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