[GHC] #14983: Have custom type errors imply Void

[GHC]

#14983: Have custom type errors imply Void
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  CustomTypeErrors
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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Description changed by Iceland_jack:

Old description:

> This is a very minor issue, but `TypeError` (CustomTypeErrors) should
> entail a constraint like
>
> {{{#!hs
> import Data.Void
>
> class (forall x. x) => No where
>   no :: Void
> }}}
>
> so users don't have to use `undefined` or `error ..`:
>
> {{{#!hs
> instance TypeError (Text "Can't show functions) => Show (a -> b) where
>   show :: (a -> b) -> String
>   show = no & absurd
> }}}

New description:

 This is a very minor issue, but `TypeError` (CustomTypeErrors) should
 entail a constraint like

 {{{#!hs
 import Data.Void

 class (forall x. x) => No where
   no :: Void
 }}}

 so users don't have to use `undefined` or `error ..`:

 {{{#!hs
 instance TypeError (Text "Can't show functions") => Show (a -> b) where
   show :: (a -> b) -> String
   show = no & absurd
 }}}

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14983#comment:1>
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