Re: [GHC] #13060: Visible type application doesn't work for functions with inferred types

3 Jan 2017

      #13060: Visible type application doesn't work for functions with inferred types
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        Reporter:  Feuerbach         |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:  invalid           |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by Feuerbach):

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

Comment:

 Thanks Richard, this makes sense.

 I agree that depending on the inferred order of type variables in actual
 code would be a bad idea.

 In my case, I was just trying to evaluate something quickly in ghci like
 this:

 {{{
 *Main> description decoGrid

 <interactive>:64:1: error:
     • Ambiguous type variable ‘a0’ arising from a use of ‘description’
       prevents the constraint ‘(LayoutModifier
                                   (Decoration DefaultDecoration
 DefaultShrinker)
                                   a0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘a0’ should be.
       These potential instance exist:
         one instance involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the expression: description decoGrid
       In an equation for ‘it’: it = description decoGrid

 <interactive>:64:13: error:
     • Ambiguous type variable ‘a0’ arising from a use of ‘decoGrid’
       prevents the constraint ‘(Eq a0)’ from being solved.
       Probable fix: use a type annotation to specify what ‘a0’ should be.
       These potential instances exist:
         instance Eq XEvent -- Defined in ‘Graphics.X11.Xlib.Event’
         instance Eq FontSet -- Defined in ‘Graphics.X11.Xlib.Extras’
         instance Eq FontStruct -- Defined in ‘Graphics.X11.Xlib.Font’
         ...plus 61 others
         ...plus 138 instances involving out-of-scope types
         (use -fprint-potential-instances to see them all)
     • In the first argument of ‘description’, namely ‘decoGrid’
       In the expression: description decoGrid
       In an equation for ‘it’: it = description decoGrid
 *Main> :t decoGrid
 decoGrid
   :: forall {a}.
      Eq a =>
      ModifiedLayout
        (Decoration DefaultDecoration DefaultShrinker) Grid a
 *Main> -- ok, so I need to apply decoGrid to the right type param
 *Main> description (decoGrid @Window)

 <interactive>:67:14: error:
     • Cannot apply expression of type ‘ModifiedLayout
                                          (Decoration DefaultDecoration
 DefaultShrinker) Grid a0’
       to a visible type argument ‘Window’
     • In the first argument of ‘description’, namely
         ‘(decoGrid @Window)’
       In the expression: description (decoGrid @Window)
       In an equation for ‘it’: it = description (decoGrid @Window)
 *Main> -- this works:
 *Main> :set -XPartialTypeSignatures -Wno-partial-type-signatures
 *Main> description (decoGrid :: _ Window)
 "DefaultDecoration Grid"
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13060#comment:2
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