[GHC] #16050: Instance resolution error message unclear, because of missing kind information

14 Dec 2018

      #16050: Instance resolution error message unclear, because of missing kind
information
-------------------------------------+-------------------------------------
           Reporter:  chessai        |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 consider the following modules:

 {{{#!hs
 module A where

 (.) :: forall (a :: TYPE 'UnliftedRep) (b :: TYPE 'UnliftedRep) (c :: TYPE
 'UnliftedRep).
      (b -> c)
   -> (a -> b)
   -> (a -> c)
 (.) f g = \x -> f (g x)

 data UList (a :: TYPE 'UnliftedRep) where
   UNil :: UList a
   UCons :: a -> UList a -> UList a

 mapFB :: forall (a :: TYPE 'UnliftedRep)
                 (elt :: TYPE 'UnliftedRep)
                 (lst :: Type).
   (elt -> lst -> lst) -> (a -> elt) -> a -> lst -> lst
 mapFB c f = \x ys -> c (f x) ys

 {-# RULES
 "mapFB" forall c f g. mapFB (mapFB c f) g = mapFB c (f . g)
    #-}
 }}}

 {{{#!hs
 module B where

 import Control.Category ((.))

 data UList (a :: TYPE 'UnliftedRep) where
   UNil :: UList a
   UCons :: a -> UList a -> UList a

 mapFB :: forall (a :: TYPE 'UnliftedRep)
                 (elt :: TYPE 'UnliftedRep)
                 (lst :: Type).
   (elt -> lst -> lst) -> (a -> elt) -> a -> lst -> lst
 mapFB c f = \x ys -> c (f x) ys

 {-# RULES
 "mapFB" forall c f g. mapFB (mapFB c f) g = mapFB c (f . g)
    #-}
 }}}

 Module 'A' works fine. Module 'B', fails with the following error:

 {{{#!hs
     • No instance for (Category (->)) arising from a use of ‘.’
     • In the second argument of ‘mapFB’, namely ‘(f . g)’
       In the expression: mapFB c (f . g)
       When checking the transformation rule "mapFB"
     |
 line| "mapFB" forall c f g. mapFB (mapFB c f) g = mapFB c (f . g)
     |                                                      ^^^
 }}}

 I expected this failure because of the kind mismatch; the category
 instance for `(->)` obviously requires that it be kinded `Type -> Type ->
 Type`. However, it confused someone I am teaching, who said to me that
 they didn't understand the error, since they expected it to work as `(->)`
 does indeed have a Category instance. (They are very unfamiliar with
 Levity-Polymorphism).

 My question is this: Would it be preferable to include such kind
 information in the error message?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16050
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