Proposal: keep Data.Map.foldWithKey

[Christian Maeder]

Am 16.12.2010 12:35, schrieb Christian Maeder:
> Am 15.12.2010 15:38, schrieb Johan Tibell:
>> Hi all,
>>
>> On Tue, Dec 14, 2010 at 1:28 PM, Christian Maeder
>> <Christian.Maeder at dfki.de> wrote:
>>> I've now created a proper library proposal to keep Data.Map.foldWithKey.
>>> Exchange arguments and make up your mind until Jan. 15th.
>>>
>>> http://hackage.haskell.org/trac/ghc/ticket/4842
>>
>> My vote is "no", there are already too many fold variants in Data.Map. We have
>>
>> {left,right} x {non-strict,strict} x {with key, without key} = 8
>> combinations together.
> 
> Currently only two of these 8 are visible and a third foldlWithKey' is
> commented out. Is there a demand for the strict variants (to be
> expressed by a proposal)? It would need some more documentation to
> choose the right version.

Sorry, I've looked into the old containers-0.3.0.0 version. So the
release notes are also not accurate. foldrWithKey' was added, too.

http://www.haskell.org/ghc/docs/7.0.1/html/users_guide/release-7-0-1.html

1.5.10.7. containers
    *
      Version number 0.4.0.0 (was 0.3.0.0)
    *
      Strictness is now more consistent, with containers being strict in
their elements even in singleton cases.
    *
      There is a new function insertLookupWithKey' in Data.Map.
    *
      The foldWithKey function in Data.Map has been deprecated in favour
of foldrWithKey.

Christian

> foldr' is also missing for list probably because foldr is not tail
> recursive. Control.Monad.foldM is also only a left version (and I do not
> propose to add foldrM and foldlM and deprecate foldM later).
> 
> For the "without key" variants I see no need.
> 
> Omitting an unused first argument can be easily done using "const".
> 
>   foldx = foldxWithKey  . const
> 
> (Maybe "foldxWithKey" should have been named just "foldx" in the first
> place.)
> 
>>
>> Adding back the "directionless" version would mean another 4 versions
>> of we want symmetry. This in a module that already has 150 functions.
> 
> My count of exported names is 97 (via ":browse Data.Map" inside ghci)
> 
> Cheers Christian
> 
>> We need to do something about the large amount of both API (i.e.
>> usability tax) and implementation (maintainer overhead) duplication in
>> the containers library. Throwing out functions that don't add much to
>> expressibility* only gets us so far, but I think it's a step in the
>> right direction.
>>
>> * I appreciate that being able to use a "directionless" fold lets the
>> author communicate his intent slightly better, but I think it's
>> outweighed by the API growth and code duplication.
>>
>> Johan