Proposal #2659: Add sortOn and friends to Data.List

[Bart Massey]

Twan van Laarhoven <twanvl <at> gmail.com> writes:
> Almost all uses of sortBy in user code use 'comparing',
> 'on' or a similar construction [1].

I certainly agree that there is room for improvement here.
But I hate to see the proliferation of functions, almost
all alike.  I guess I would support the proposal if that's
the way folks would like to go, though.

I would prefer:

  1) Get rid of any need for Data.Function by exporting
     'fix' and 'on' into the Prelude.  The rest of it is
     already in the Prelude anyway, and the implementations
     of 'fix' and 'on' are trivial.  (I'd say Data.Function
     is kind of a failed experiment at this point, but what
     do I know?)

  2) Move the sort functions into Data.Ord.  Obviously, they
     would have to be re-exported by Data.List.  It seems to
     me that 'sort' should have been there in the first
     place, and that 'sortBy" should be in the same place as
     'sort'.
  
  3) Incorporate the 'Down' (Is 'Descending' too long?) type
     in Data.Ord as suggested (but see below).

Now Data.Ord would be the one-stop shopping place for all
our sorting needs.

If all of this were done, I really wouldn't mind importing
Data.Ord to get the sorting functions and continuing to
write 'sortBy (comparing f)' or 'sortBy (p `on` f)' when I
wanted to do such a thing.  Even 'sortBy (comparing Down
`on` f)' isn't so bad.

Minor nits in the proposal:

  * You need to implement either (<=) or `comparing` for
    Down (preferably `comparing`), not just (<).  Otherwise
    sort nonterminates with a stack overflow.  This one took
    me a while to figure out.

    As far as I can tell you'll need to make Down an
    instance of Eq as well as Ord to get the latter instance
    to typecheck.

    These problems suggest that you haven't yet tried your
    proposal at all. :-) :-)

  * getDown might better be called fromDown?


    Bart Massey
    bart <at> cs.pdx.edu