#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:
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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 | i<0 = x `shiftR` (-i)
| i>0 = 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>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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