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Hello.

I recently wanted to know how the element 4 in [2,4] is ruled out by:

nubBy (\x y -> x `mod` y == 0) [2,4]

and discussing this on #haskell we discovered that the documentation or the code is buggy.

The numBy source states that we keep the call to eq with arguments in the same order as in the reference implementation, which, comparing the following two lines, is not true:

nubBy eq (x:xs)         =  x : nubBy eq (filter (\ y -> not (eq x y)) xs)

elem_by eq y (x:xs)     =  y `eq` x || elem_by eq y xs

Also this is easily proved by defining:

nubBy' eq [] = []
nubBy' eq (x:xs) = x : nubBy' eq (filter (\ y -> not (eq x y)) xs)

Then running:

nubBy (\x y -> x `mod` y == 0) [2,4]

which yields [2] because of eq y x and

nubBy' (\x y -> x `mod` y == 0) [2,4]

which yields [2,4] because of eq x y.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Prelude
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