Re: [GHC] #6028: warning for cyclic unimplemented defaults

2012-10-09 Thread GHC 
#6028: warning for cyclic unimplemented defaults
---+
  Reporter:  lerkok|  Owner:  
  Type:  feature request   | Status:  closed  
  Priority:  normal|  Milestone:  
 Component:  Compiler  |Version:  7.4.1   
Resolution:  wontfix   |   Keywords:  
Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown  | Difficulty:  Unknown 
  Testcase:|  Blockedby:  
  Blocking:|Related:  
---+

Comment(by lerkok):

 It is indeed possible to create scenarios where the cycle check would be
 fooled, like the one you showed above. But I don't think that's the
 common case. I believe it remains a fact that a simply cycle check will
 catch a large number of practical instances of this problem. This is
 similar to the pattern-match completeness warning: While it may not be
 accurate 100%, it's so darn useful. And it's likely to be right more often
 than you'd think. In the 1% of cases where it's wrong, it's just a warning
 that the user can explicitly turn off via
 `--noWarnUnimplementedCyclicMethod` or some such.

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/6028#comment:4
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Re: [GHC] #6028: warning for cyclic unimplemented defaults

2012-10-07 Thread GHC 
#6028: warning for cyclic unimplemented defaults
---+
  Reporter:  lerkok|  Owner:  
  Type:  feature request   | Status:  closed  
  Priority:  normal|  Milestone:  
 Component:  Compiler  |Version:  7.4.1   
Resolution:  wontfix   |   Keywords:  
Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown  | Difficulty:  Unknown 
  Testcase:|  Blockedby:  
  Blocking:|Related:  
---+
Changes (by igloo):

  * status:  new = closed
  * difficulty:  = Unknown
  * resolution:  = wontfix


Comment:

 It's not quite that simple. The default implementations are:
 {{{
 negate x= 0 - x
 x - y   = x + negate y
 }}}
 so actually (-) is defined in terms of (+), not negate. For example, if we
 have:
 {{{
 data X = C
 deriving Show

 instance Num X where
 _ + _ = C
 }}}
 then -3 :: X evaluates to C.

 It also wouldn't work for shift:
 {{{
 x `shift`   i | i0   = x `shiftR` (-i)
   | i0   = x `shiftL` i
   | otherwise = x

 x `shiftL`  i = x `shift`  i
 x `shiftR`  i = x `shift`  (-i)
 }}}

 I'm not convinced that there are many instances where this would actually
 help, so I'm closing the ticket.

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/6028#comment:3
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Re: [GHC] #6028: warning for cyclic unimplemented defaults

2012-09-04 Thread GHC 
#6028: warning for cyclic unimplemented defaults
--+-
 Reporter:  lerkok|  Owner:  
 Type:  feature request   | Status:  new 
 Priority:  normal|  Component:  Compiler
  Version:  7.4.1 |   Keywords:  
   Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown  |   Testcase:  
Blockedby:|   Blocking:  
  Related:|  
--+-

Comment(by lerkok):

 Recent relevant discussion in stack overflow:
 http://stackoverflow.com/questions/12270239/can-ghc-warn-if-class-
 instance-is-a-loop

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/6028#comment:2
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Re: [GHC] #6028: warning for cyclic unimplemented defaults

2012-04-21 Thread GHC 
#6028: warning for cyclic unimplemented defaults
--+-
 Reporter:  lerkok|  Owner:  
 Type:  feature request   | Status:  new 
 Priority:  normal|  Component:  Compiler
  Version:  7.4.1 |   Keywords:  
   Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown  |   Testcase:  
Blockedby:|   Blocking:  
  Related:|  
--+-

Comment(by lerkok):

 Along similar lines; it'd be really awesome if ghc computed that
 information, and printed it with :i, something like:

 ghci :i Num
 minimal complete definition: +, *, abs, signum, fromInteger, and one of
 negate, -
 

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/6028#comment:1
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler

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