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

[apfelmus]

Twan van Laarhoven wrote:

> -- Proposal--
> 
> Ticket:   #2659 (for sortOn and friends)
> Ticket:   #2660 (for Down newtype)
> Deadline: now+2 weeks = 2008-10-20
> 
> Questions:
>   1. should sortOn be added to Data.List?
>   2. should all other *On functions be added as well?
>   3. what name should these functions get?
>   4. should the sortOn' variations be added? What about the naming?
>   5. should Down be added to Data.Ord?

-1 concerning sortOn and friends

I think that

  sortBy (comparing f)

is a very good and modular name for the functionality intended, no need
to create another one. The only advantage of a special  sortOn  function
would be that you can document it, i.e. there's no haddock blurb for
sortBy (comparing f) .

Hm, though  sortOn  could be useful if it caches the values of  f  by
default, like the  sortOn'  proposed. But Jean-Philippe's code is a
beautiful solution for this situation.


+1 concerning Down,

but I don't like the name. In particular I don't like the "get" prefix
in "getDown". I'd simply use

  newtype Desc a = Desc a

without label or  fromDesc  if really necessary.


Regards,
apfelmus