[GHC] #9898: Wanted: higher-order type-level programming (was: Couldn't match type `(Char, ())' with `()')

[GHC]

#9898: Wanted: higher-order type-level programming
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              Reporter:  erisco      |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.8.3
             Component:  Compiler    |         Keywords:
  (Type checker)                     |     Architecture:  x86_64 (amd64)
            Resolution:              |       Difficulty:  Unknown
      Operating System:  Windows     |       Blocked By:
       Type of failure:  GHC         |  Related Tickets:
  rejects valid program              |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Changes (by simonpj):

 * type:  bug => feature request


Comment:

 HEAD correctly says
 {{{
 T9898.hs:20:10:
     Type family ‘Equal’ should have 2 arguments, but has been given 1
     In the type signature for ‘test1’:
       test1 :: ApplyFilter ((Equal Int) Char) (Char,
                                                Filter (Equal Int) ())

 T9898.hs:27:10:
     Type family ‘Equal’ should have 2 arguments, but has been given 1
     In the type signature for ‘test3’:
       test3 :: Filter (Equal Int) (Char, ())

 T9898.hs:31:10:
     Type family ‘Equal’ should have 2 arguments, but has been given 1
     In the type signature for ‘test4’:
       test4 :: Filter (Equal Int) (Char, ())
 }}}
 You can't partially apply a type function, I'm afraid.  Yes, that limits
 higher-order programming.  Better solutions (that still support type
 inference) most welcome.

 I'll leave this open as a feature request for higher order type level
 programming.


 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9898#comment:1>
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