#2496: Invalid Eq/Ord instances in Data.Version
-------------------------+--------------------------------------------------
Reporter: guest | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries/base
Version: 6.8.3 | Severity: normal
Keywords: | Testcase:
Architecture: Multiple | Os: Multiple
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(From Adrian Hey)
In Data.Version we have:
{{{
data Version =
Version {versionBranch :: [Int]
,versionTags :: [String]
}
instance Eq Version where
v1 == v2 = versionBranch v1 == versionBranch v2
&& sort (versionTags v1) == sort (versionTags v2)
-- tags may be in any order
instance Ord Version where
v1 `compare` v2 = versionBranch v1 `compare` versionBranch v2
}}}
The "laws" for valid Eq/Ord instances were argued about recently but the
H98 report seems reasonably clear that (==) is supposed to test for
equality and the compare method is supposed to define a total ordering.
There is also an implied but not explicitly stated law that:
{{{
(x == y = True) <-> (x `compare` y = EQ)
}}}
and also this I guess..
{{{
(x == y = False) <-> ~(x `compare` y = EQ)
}}}
This law is implied by the Eq constraint on the Ord class (which seems to
serve no purpose otherwise).
See also:
[http://www.haskell.org/pipermail/haskell-prime/2008-March/002330.html]
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2496>
GHC <http://www.haskell.org/ghc/>
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