[ ghc-Bugs-435234 ] nubBy flips args to nubbing function

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Bugs item #435234, was opened at 2001-06-21 14:52
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Category: None
Group: 5.00
>Status: Closed
>Resolution: Remind
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: nubBy flips args to nubbing function

Initial Comment:
The nubBy function in the List library seems to flip
the order of arguments to the nubbing function from
that specified in the prelude:

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

I.e., it seems to use (eq y x) instead of (eq x y). 
This doesn't matter of course for symmetric nubbing
functions, but I just tried to use it with the
following function:

subsetEq :: (Eq a) => [a] -> [a] -> Bool
subsetEq l1 l2 = all (`elem` l2) l1

> nubBy subsetEq [[1],[1,2],[2,3],[2]] 
[[1],[1,2],[2,3]]

> nubBy (flip subsetEq) [[1],[1,2],[2,3],[2]]
[[1],[2,3],[2]]


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>Comment By: Simon Peyton Jones (simonpj)
Date: 2001-06-25 07:57

Message:
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Readily fixed.  Thanks.

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