[GHC] #9893: Switching on TypeFamilies extension stops code from typechecking

[GHC]

#9893: Switching on TypeFamilies extension stops code from typechecking
-------------------------------------+-------------------------------------
              Reporter:  phischu     |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  x86_64 (amd64)
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:  GHC         |       Blocked By:
  rejects valid program              |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Description changed by phischu:

Old description:

> The following code has the `TypeFamilies` extension enabled but does not
> use it. It does not typecheck with GHC 7.8.3. If we do not enable
> `TypeFamilies` (i.e. delete the first line) it typechecks.
>
> {{{
> {-# LANGUAGE TypeFamilies #-}
> module ExistentialFamilies where
>
> import Control.Monad.ST (runST,ST)
>
> un :: ()
> un = runST f where
>     f = return un :: ST s ()
> }}}
> This happens when trying to compile the program as well as when trying to
> load it into ghci.

New description:

 The following code has the `TypeFamilies` extension enabled but does not
 use it. It does not typecheck with GHC 7.8.3. If we do not enable
 `TypeFamilies` (i.e. delete the first line) it typechecks.

 {{{
 {-# LANGUAGE TypeFamilies #-}
 module ExistentialFamilies where

 import Control.Monad.ST (runST,ST)

 un :: ()
 un = runST f where
     f = return un :: ST s ()
 }}}
 This happens when trying to compile the program as well as when trying to
 load it into ghci.

 The error message when `TypeFamilies` is on is
 {{{
 > ghc ExistentialFamilies.hs
 [1 of 1] Compiling ExistentialFamilies ( ExistentialFamilies.hs,
 ExistentialFamilies.o )

 ExistentialFamilies.hs:7:12:
     Couldn't match type ‘s0’ with ‘s’
       because type variable ‘s’ would escape its scope
     This (rigid, skolem) type variable is bound by
       a type expected by the context: ST s ()
       at ExistentialFamilies.hs:7:6-12
     Expected type: ST s ()
       Actual type: ST s0 ()
     Relevant bindings include
       f :: ST s0 () (bound at ExistentialFamilies.hs:8:5)
     In the first argument of ‘runST’, namely ‘f’
     In the expression: runST f
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9893#comment:1>
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