Re: [GHC] #2742: The - in ViewPatterns binds more weakly than infix data constructors.

[GHC]

#2742: The - in ViewPatterns binds more weakly than infix data constructors.
-+--
Reporter:  guest |Owner:  
Type:  feature request   |   Status:  new 
Priority:  normal|Milestone:  _|_ 
   Component:  Compiler  |  Version:  6.10.1  
Severity:  normal|   Resolution:  
Keywords:|   Difficulty:  Unknown 
Testcase:|   Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |  
-+--
Changes (by simonpj):

  * difficulty:  = Unknown
  * type:  bug = feature request
  * milestone:  = _|_

Comment:

 I agree with you.  Here's why it's tricky.  Haskell allows a pattern like
 this
 {{{
   case x of
  a:as - rhs
 }}}
 If view patterns bind more tightly than infix ops, then presumably this
 would be ok
 {{{
   case x of
  a : f as - bs - rhs
 }}}
 and now we get an awkward ambiguity between the two uses of `-`.

 The change I made to the parser was this:
  * Remove this production from `texp`:
 {{{
 | fexp '-' exp   { LL $ EViewPat $1 $3 }
 }}}
  * Add the same production to `exp10`.

 That added a new shift/reduce conflict, described above, which gets
 resolved the wrong way.

 If someone wants to investigate alternatives, that'd be great, so I'll
 leave this open as a feature request.  It's a pure parser question so it
 does not require deep GHC knowledge.

 Simon

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/2742#comment:2
GHC http://www.haskell.org/ghc/
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[GHC] #2742: The - in ViewPatterns binds more weakly than infix data constructors.

[GHC]

#2742: The - in ViewPatterns binds more weakly than infix data constructors.
-+--
Reporter:  guest |   Owner:  
Type:  bug   |  Status:  new 
Priority:  normal|   Component:  Compiler
 Version:  6.10.1|Severity:  normal  
Keywords:|Testcase:  
Architecture:  Unknown/Multiple  |  Os:  Unknown/Multiple
-+--
 The following code, essentially taken from the ViewPatterns page on the
 wiki doesn't seem to parse correctly.

 {{{
 mymap f [] = []
 mymap f (x : mymap f - xs) = f x : xs
 }}}

 However, this does:

 {{{
 mymap f [] = []
 mymap f (x : (mymap f - xs)) = f x : xs
 }}}

 (though it triggers bug #2395 about overlapping patterns)

 It would seem nicer to make the view pattern arrow bind ''tighter'' than
 any infix data constructors.

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/2742
GHC http://www.haskell.org/ghc/
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Re: [GHC] #2742: The - in ViewPatterns binds more weakly than infix data constructors.

[GHC]

#2742: The - in ViewPatterns binds more weakly than infix data constructors.
-+--
Reporter:  guest |Owner:  
Type:  bug   |   Status:  new 
Priority:  normal|Milestone:  
   Component:  Compiler  |  Version:  6.10.1  
Severity:  normal|   Resolution:  
Keywords:| Testcase:  
Architecture:  Unknown/Multiple  |   Os:  Unknown/Multiple
-+--
Changes (by guest):

 * cc: [EMAIL PROTECTED] (added)

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/2742#comment:1
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler___
Glasgow-haskell-bugs mailing list
Glasgow-haskell-bugs@haskell.org
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs