[GHC] #8477: Allow inferring ambiguous types

[GHC]

#8477: Allow inferring ambiguous types
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        Reporter:  aavogt            |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  8390
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Changes (by simonpj):

 * cc: goldfire, diatchki (added)


Comment:

 Richard Eisenberg (goldfire) writes: I'm here at MSR Redmond talking with
 Nik and Daan about type inference, among other things. We're wondering
 about ambiguity checks. When we type check
 {{{
 > showRead = show . read
 }}}
 (with `-XAllowAmbiguousTypes`) we are told that term is ill-typed because
 of a failed constraint due to an ambiguous type variable. But, there is a
 perfectly good type for this: `(Show a, Read a) => String -> String`. This
 type is, of course, ambiguous, but I thought we wanted to ''allow''
 ambiguous types. Our only theory is that GHC has to check the RHS
 ''twice'' -- once to infer a type and then once to check that it has the
 inferred type. It is during this second pass that we discover that the
 inferred type is no good.

 What has intrigued us is that
 {{{
 > showRead :: forall a. (Show a, Read a) => String -> String
 > showRead = show . (read :: String -> a)
 }}}
 works just fine (with `-XAllowAmbiguousTypes` and
 `-XScopedTypeVariables`). So, the analysis is ''not'' just looking at the
 types; it has to be examining the RHS also.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8477#comment:5>
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