#2528: nub not as reliable as nubBy
-------------------------------+--------------------------------------------
Reporter: jdressel | Owner:
Type: bug | Status: reopened
Priority: normal | Milestone:
Component: libraries/base | Version: 6.8.2
Severity: minor | Resolution:
Keywords: | Testcase:
Architecture: Unknown | Os: Linux
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Changes (by guest):
* status: closed => reopened
* component: Compiler => libraries/base
* resolution: invalid =>
Comment:
(From Adrian Hey)
Well there still seems to be a problem, considering the what the library
docs say about nubBy..
"The 'nubBy' function behaves just like 'nub', except it uses a user-
supplied equality predicate instead of the overloaded '==' function."
..and nub was indeed originally defined as,..
{{{
nub=nubBy compare
}}}
..as is the local nub in the example.
So right or wrong, shouldn't we see the same result in each case? Here's
the source of the problem I think:
{{{
elem_by :: (a -> a -> Bool) -> a -> [a] -> Bool
elem_by _ _ [] = False
elem_by eq y (x:xs) = x `eq` y || elem_by eq y xs
}}}
vs.
{{{
elem _ [] = False
elem x (y:ys) = x==y || elem x ys
}}}
Also, where does the H98 report say all instances of Eq must be
transitive, reflexive, symmetric, and antisymmetric? It just says "The Eq
class provides equality (==)..", whatever that might mean :-)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2528#comment:2>
GHC <http://www.haskell.org/ghc/>
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