#4019: deriving Ord can produce incorrect and inefficient instances
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Reporter: rl | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.14.1
Component: Compiler | Version: 6.13
Keywords: | Difficulty:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: Incorrect result at runtime
Patch: 0 |
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Changes (by igloo):
* milestone: => 6.14.1
Comment:
This ticket seems to be mixing several issues. Based on the original
description, I thought this was the problem you were reporting:
This derived Ord instance:
{{{
data T = T Double deriving( Eq, Ord )
}}}
looks like this (after Haskellifying the generated code):
{{{
instance Ord T where
compare a b = cmp_eq a b
where cmp_eq (T a1) (T b1) = compare a1 b1 }
}}}
so it only lifts a minimum number of methods. But it ought to lift all of
them, rather than relying on the default methods being both correct and
efficient:
{{{
instance Ord T where
compare a b = cmp_eq a b
where cmp_eq (T a1) (T b1) = compare a1 b1 }
(<=) a b = cmp_eq a b
where cmp_eq (T a1) (T b1) = (<=) a1 b1 }
(<) a b = cmp_eq a b
where cmp_eq (T a1) (T b1) = (<) a1 b1 }
(>=) a b = cmp_eq a b
where cmp_eq (T a1) (T b1) = (>=) a1 b1 }
(>) a b = cmp_eq a b
where cmp_eq (T a1) (T b1) = (>) a1 b1 }
max a b = cmp_eq a b
where cmp_eq (T a1) (T b1) = max a1 b1 }
min a b = cmp_eq a b
where cmp_eq (T a1) (T b1) = min a1 b1 }
}}}
which makes sense to me and I imagine is easy to fix.
I suggest we leave this ticket to deal with that issue, and open separate
tickets for other issues.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4019#comment:4>
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