#2528: nub not as reliable as nubBy
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Reporter: jdressel | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler | Version: 6.8.2
Severity: minor | Resolution: invalid
Keywords: | Testcase:
Architecture: Unknown | Os: Linux
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Changes (by JeremyShaw):
* status: new => closed
* resolution: => invalid
Comment:
Your (==) instance is broken. Specifically, for:
-- ((9+9)+2)
a = Branch (Branch (Leaf (Just 9)) Add (Leaf (Just 9))) Add (Leaf (Just
2))
-- (9+(2+9))
b = Branch (Leaf (Just 9)) Add (Branch (Leaf (Just 2)) Add (Leaf (Just
9)))
we see that:
*Main> a == b
True
*Main> b == a
False
*Main>
There may be other bugs as well, that is just the first instance I found.
nubBy (==) will break as well, you just have not seen it yet.
All instances of Eq must be transitive, reflexive, symmetric, and
antisymmetric. Haskell can't automatically check this for you -- so it is
left up the developer to do, either by formal proof, or by unit testing.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2528#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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