[GHC] #12680: Permit type equality instances in signatures

[GHC]

#12680: Permit type equality instances in signatures
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        Reporter:  ezyang            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  backpack
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 goldfire):

 Replying to [comment:4 ezyang]:
 > > type S is much better, because users can't make any assumptions about
 a type declaration that they might for a data declaration.
 >
 > As far as I could tell, GHC actually doesn't make any assumptions when
 `data S` denotes an abstract type; so it's just a syntactic wart. It's a
 bit bothersome to fix because hs-boot syntax only supports data.

 I don't agree. In GHC 8:

 {{{
 -- A.hs-boot
 module A where

 data T a
 }}}

 {{{
 -- B.hs
 {-# LANGUAGE TypeFamilies #-}

 module B where

 import {-# SOURCE #-} A

 foo :: (T a ~ T b) => a -> b
 foo x = x
 }}}

 These compile fine. GHC must be assuming that `T` is injective. So we must
 proceed cautiously.

 >
 > > It would be super awesome if the user could prove the equality
 manually.
 >
 > Do you have an example of a case where you could do this?

 Nope! :)

 Hence the level of awesomeness if we could do this. Admittedly well beyond
 the scope of Backpack.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12680#comment:5>
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