[GHC] #7913: Argument order not preserved by nubBy

[GHC]

#7913: Argument order not preserved by nubBy
-------------------------+--------------------------------------------------
Reporter:  paullik       |          Owner:                  
    Type:  bug           |         Status:  new             
Priority:  normal        |      Component:  Prelude         
 Version:  7.6.3         |       Keywords:  nubBy           
      Os:  Linux         |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown  |      Blockedby:                  
Blocking:                |        Related:                  
-------------------------+--------------------------------------------------
 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 [http://hackage.haskell.org/packages/archive/base/latest/doc/html/src
 /Data-List.html#nubBy 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'''.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7913>
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