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

[GHC]

#11641: Allow wildcards for parameters functionally determined (also type synonyms)
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        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  PartialTypeSignatures
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by Iceland_jack:

@@ -33,2 +33,1 @@
- I could achieve similar things with a type family but that work as I'd
- intended.
+ I could achieve similar things with a type family but not quite.

New description:

 {{{#!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 not quite.

 ----

 Or CPP :)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11641#comment:1>
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