[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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Comment (by simonpj):

 I'm dubious about this

  * Accepting ''inferred'' ambiguous types is a recipe for planting land-
 mines that will only show up later.  Requiring a type signature in such
 cases seems quite reasonable to me.

  * I tried [http://okmij.org/ftp/Haskell/TypeLambdaVal.hs].  The instance
 on line 199 seems problematic; it fails both the ambiguity check and the
 coverage condition.  However I realise that the ambiguity check on the
 instance declaration should really subsume the (liberal) coverage
 condition. I'll fix that.

  * Then the `TypeLambdaVal` example gives
 {{{
 T8477.hs:218:1:
     Could not deduce (Apply
                         (HIF' (b0 -> HNothing) (HList (a0 :* xs0) -> HJust
 a0))
                         (HBool b, x)
                         res)
       arising from the ambiguity check for ‛hsafe_head’
     from the context (Apply HNull x (HBool b),
                       Apply
                         (HIF' (b1 -> HNothing) (HList (a :* xs) -> HJust
 a))
                         (HBool b, x)
                         res)
       bound by the inferred type for ‛hsafe_head’:
                  (Apply HNull x (HBool b),
                   Apply
                     (HIF' (b1 -> HNothing) (HList (a :* xs) -> HJust a))
                     (HBool b, x)
                     res) =>
                  x -> res
       at T8477.hs:(218,1)-(221,24)
     The type variables ‛b0’, ‛a0’, ‛xs0’ are ambiguous
     When checking that ‛hsafe_head’
       has the inferred type ‛forall x res b b1 a xs.
                              (Apply HNull x (HBool b),
                               Apply
                                 (HIF' (b1 -> HNothing) (HList (a :* xs) ->
 HJust a))
                                 (HBool b, x)
                                 res) =>
                              x -> res’
     Probable cause: the inferred type is ambiguous
 }}}
   and one other.  Indeed, writing out that type may be a bit onerous, but
 it really does look ambiguous to me!

 So I'm inclined to "won't fix" for now.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8477#comment:3>
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