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

[GHC]

#9893: Switching on TypeFamilies extension stops code from typechecking
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              Reporter:  phischu     |            Owner:
                  Type:  bug         |           Status:  closed
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:  invalid     |         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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Changes (by simonpj):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 This is confusing, but it's by design.

 With `TypeFamilies` we also get `MonoLocalBinds`, so that `f`'s definition
 (which is local) is not generalised.  But `runST` requires a polymorphic
 `f`; and a monomorphic `f` isn't good enough. Hence the error.

 Giving a type signature to `f` is enough.  (Giving a type signature to its
 right hand side, on the other hand, is not enough

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9893#comment:2>
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