Re: [GHC] #14694: Can't coerce given assumptions

23 Jan 2018

      #14694: Can't coerce given assumptions
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  Roles
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 Iceland_jack):

 I don't remember a place where we collect them, we can make one & I can go
 through my old tickets in search for examples.

 Here is another one maybe:

 {{{#!hs
 {-# Language FlexibleContexts #-}

 import Data.Coerce

 class Exp repr where
   int :: Int -> repr Int

 newtype Coerce f a = Coerce (f a)

 -- /tmp/Test.hs:13:9: error:
 --     • Couldn't match representation of type ‘f Int’ with that of ‘Int’
 --         arising from a use of ‘coerce’
 --     • In the expression: coerce
 --       In an equation for ‘int’: int = coerce
 --       In the instance declaration for ‘Exp (Coerce f)’
 --     • Relevant bindings include
 --         int :: Int -> Coerce f Int (bound at /tmp/Test.hs:13:3)
 --    |
 -- 13 |   int = coerce
 --    |         ^^^^^^
 instance Coercible Int (f Int) => Exp (Coerce f) where
   int = coerce
 }}}

 but it works with `int = Coerce . coerce`.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14694#comment:3
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