Proposal: add 'equating' function to Data.List

[Greg Weber]

On Sun, Jul 20, 2014 at 5:46 PM, John Lato <jwlato at gmail.com> wrote:

> The utility of the proposed "equating" extends far beyond simply using it
> with groupBy.  Just last week I used "on (==)" as part of a complicated
> boolean expression.
>
> That said, I just want to be clear that I'm understanding Greg properly.
>  Are you advocating that one should generally create a newtype+Eq instance
> rather than using "on (==)"?  What is the benefit of this approach?  The
> only one I can think of is that it makes the standard libraries smaller,
> which seems like a rather small gain considering that you've changed a
> 7-character expression into multi-line boilerplate for everyone, hampering
> readability in the process.  Is there something I'm missing here?
>

Sorry I didn't do a good job explaining. groupOn takes a function that is a
projection to an equality instance (Eq b => (a -> b)).
So there is no need for a newtype, and this interface has suited 100% of my
needs, and every usage I have ever seen in Haskell code.

My presumption is that the only reason this interface is not 100%
satisfactory and we have this weird interface based on Bool is because you
may want to compare something that does not have an Eq instance. One could
still handle that case by wrapping the projected value in a newtype and
writing an Eq instance for that newtype.


> (Currently +0 on the proposal)
>
> John L.
>
>
> On Mon, Jul 21, 2014 at 8:27 AM, Greg Weber <greg at gregweber.info> wrote:
>
>> I am -1 on things that encourage converting Eq to Bool rather than just
>> using Eq directly.
>>
>> Is there a use case for group that is not satisfied by groupOn with a
>> newtype with an Eq instance?
>> Granted, a newtype may be heavy-weight, but I feel that the current group
>> should be an escape hatch in the rare case that groupOn does not suffice,
>> not something we codify via Fairbairn threshold because that is the only
>> API that exists today. I can create a separate proposal for adding groupOn,
>> etc.
>>
>>
>> On Fri, Jul 18, 2014 at 12:57 PM, Greg Weber <greg at gregweber.info> wrote:
>>
>>> I think the `By` functions that expect a Bool are all cumbersome because
>>> they are too flexible. 100% of the time I personally use these functions I
>>> want to use Ord or Eq.
>>> What I would like to see is a function groupOn next to groupBy.
>>>
>>> groupOn :: Eq b => (a -> b) -> [a] -> [[a]]
>>>
>>> Then equating is no longer needed, and one just writes: groupOn snd
>>> I believe this style also gives better opportunity for optimization
>>> (Scwartzian transform).
>>>
>>> Of course, this function is still problematic because it operates only
>>> on lists and does not group over the entire list, but those are separate
>>> issues.
>>> All of this is solved in mono-traversable right now by the groupAllOn
>>> function [1]
>>>
>>> [1]
>>> http://hackage.haskell.org/package/mono-traversable-0.6.0.4/docs/Data-Sequences.html
>>>
>>>
>>> On Fri, Jul 18, 2014 at 12:26 PM, Frerich Raabe <raabe at froglogic.com>
>>> wrote:
>>>
>>>> Hi,
>>>>
>>>> A common use case for 'on' (from Data.Function) is to use it with
>>>> 'compare', e.g. 'compare `on` snd'. In fact, this pattern is so common that
>>>> there's a convenient 'comparing' function which provides a shortcut for
>>>> this use case such that one can write
>>>>
>>>>   sortBy (comparing snd)
>>>>
>>>> instead of
>>>>
>>>>   sortBy (compare `on` snd)
>>>>
>>>> I think another common use case is to use 'on' together with (==) as in
>>>>
>>>>   groupBy ((==) `on` snd)
>>>>
>>>> In a similiar vein as with 'comparing', I think it would be nice if
>>>> there was a function which encapsulates this use case, like
>>>>
>>>>   equating :: Eq b => (a -> b) -> a -> a -> Bool
>>>>   equating = on (==)
>>>>
>>>> such that one can write
>>>>
>>>>   groupBy (equating snd)
>>>>
>>>> In fact, groupBy is just one of many *By functions taking an a -> a ->
>>>> Bool -- many of which are Data.List, e.g. groupBy, nubBy, deleteBy,
>>>> intersectBy, unionBy. Hence, it seems plausible to define 'equating' in
>>>> Data.List. This is the same reasoning as why 'comparing' is in Data.Ord:
>>>> because the module exposes a lot of *By functions taking an a -> a ->
>>>> Ordering.
>>>>
>>>> - Frerich
>>>>
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>>>>
>>>
>>>
>>
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