[Haskell-cafe] nubBy changed?

Ross Paterson ross at soi.city.ac.uk
Sun Mar 1 19:56:48 EST 2009

On Sun, Mar 01, 2009 at 02:30:34PM -1000, Tim Newsham wrote:
> Did nubBy change recently?  In 6.8.2 I could generate primes as:
>    nubBy (\a b -> b `mod` a == 0) [2..]
> but in 6.10.1 I have to use
>    nubBy (\a b -> a `mod` b == 0) [2..]
> Is this change intentional?  If so, what is the reason?

Tue Sep  2 10:29:50 BST 2008  Simon Marlow <marlowsd at gmail.com>
  * #2528: reverse the order of args to (==) in nubBy to match nub
  This only makes a difference when the (==) definition is not
  reflexive, but strictly speaking it does violate the report definition
  of nubBy, so we should fix it.

See also: http://hackage.haskell.org/trac/ghc/ticket/2528

The Report imposes the precondition that the relation be an equivalence,
so your use was out of spec.

Still, perhaps it would have been better to fix #2528 by changing nub
instead of nubBy.  As it is the USE_REPORT_PRELUDE versions of nub and
nubBy differ from the faster versions.  (And the comment on elem_by is

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