[GHC] #3993: allow implicit parameter bindings in patternseee

[GHC]

#3993: allow implicit parameter bindings in patternseee
---------------------------------+------------------------------------------
    Reporter:  aavogt            |       Owner:              
        Type:  feature request   |      Status:  new         
    Priority:  normal            |   Component:  Compiler    
     Version:  6.12.1            |    Keywords:              
          Os:  Unknown/Multiple  |    Testcase:              
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown
---------------------------------+------------------------------------------
 It could work like such:

 {{{
 main = do
     ?conts <- getContents
     fun

 fun :: (?conts :: String) -> IO ()
 fun = ...
 }}}

 becoming something like

 {{{
 main = do
     conts <- getContents
     let ?conts = conts
     fun
 }}}

 Of course there are still questions regarding interactions with pattern
 guards and view patterns (whether those variables should be in scope for
 other guards, or for view patterns that follow).

 This interaction with view patterns (and pattern guards), should likely
 elicit a warning:

 {{{
    let ?x = 1 in
    let f ?x ((?x +) -> y) = y in
    f 0 0
 }}}

 As it might be expected that this expression should be 1, while I think it
 should evaluate to 0.

 This probably would lead to more syntax stealing, but it seems unlikely
 that there would be code that enables -XImplicitParams that does stuff
 like:

 {{{
 x ?y = ...
 }}}

 Expecting to define the function `?', rather than binding the implicit
 parameter ?y.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3993>
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