Re: [GHC] #14317: Solve Coercible constraints over type constructors

4 Oct 2017

      #14317: Solve Coercible constraints over type constructors
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  Roles,
                                     |  QuantifiedContexts
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by adamse):

 Slightly on a tangent but:

 This program typechecks in 8.0.2 but fails in 8.2.1

 {{{#!haskell
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE GADTs #-}

 import Data.Coerce

 newtype (f ... g) a = C (f (g a))

 newtype I a = I a

 data F f a b where
   MkF :: Coercible (f b) (f' b) => (a -> f' b) -> F f a b

 id1 :: a -> I a
 id1 = I

 id1F :: Coercible b (f b) => F f b b
 id1F = MkF I
 }}}

 {{{
 /Users/adam/src/doodles/co.hs:17:8: error:
     • Could not deduce: Coercible b (f b) arising from a use of ‘MkF’
       from the context: Coercible b (f b)
         bound by the type signature for:
                    id1F :: forall b (f :: * -> *). Coercible b (f b) => F
 f b b
         at /Users/adam/src/doodles/co.hs:16:1-36
       ‘b’ is a rigid type variable bound by
         the type signature for:
           id1F :: forall b (f :: * -> *). Coercible b (f b) => F f b b
         at /Users/adam/src/doodles/co.hs:16:1-36
     • In the expression: MkF I
       In an equation for ‘id1F’: id1F = MkF I
     • Relevant bindings include
         id1F :: F f b b (bound at /Users/adam/src/doodles/co.hs:17:1)
    |
 17 | id1F = MkF I
    |        ^^^^^
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14317#comment:3
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