[GHC] #11641: Allow wildcards for parameters functionally determined (also type synonyms)

25 Feb 2016

      #11641: Allow wildcards for parameters functionally determined (also type synonyms)
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  lowest         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  PartialTypeSignatures              |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 class F a b | a -> b where
   foo :: a
   -- ...

 myFoo :: F a b => a
 myFoo = foo
 }}}

 Since ''b'' is not used and fully determined by ''a'' could the
 restriction on wildcards in constraints be lifted?

 {{{#!hs
 myFoo :: F a _ => a
 myFoo = foo
 }}}

 and eventually hiding it behind a type synonym:

 {{{#!hs
 type F' a = F a _

 myFoo' :: F' a => a
 myFoo' = foo
 }}}

 I raised this issue at ICFP 2014, I haven't looked into whether
 dominique's response to [https://haskell-
 phabricator.global.ssl.fastly.net/D168#4818 my comment] applies.

 ----

 I could achieve similar things with a type family but that work as I'd
 intended.

 ----

 Or CPP :)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11641
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