Proposal: add `on` to the Prelude

[Elliot Cameron]

That's certainly true but I'm not sure what that has to do with being in
Prelude. If anything, I'd expect Prelude to be home to the *least* exciting
things because no one wants to bother talking about them.

On Tue, Sep 10, 2019, 10:20 PM David Feuer <david.feuer at gmail.com> wrote:

> I agree with your general conservatism, but I think some of these things
> are not like the others. I like `on` for the Prelude because, despite its
> size, it carries with it the idea of carrying a relationship between two
> values through a function. Notably, for *any* function f,
>
>     eq `on` f defines a decidable equivalence relation if eq does.
>     cmp `on` f defines a decidable partial order if cmp does.
>     d `on` f is a pseudometric if d is.
>
> What a beautiful and useful concept! I find it much harder to get excited
> about the idea of flipping function application.
>
> On Tue, Sep 10, 2019, 9:50 PM Elliot Cameron <eacameron at gmail.com> wrote:
>
>> Part of me would too...but I just don't know when this line of reasoning
>> stops. Do we add (>>>) and (<<<) from Category too? Then (&&&)? Of course
>> then we might want first, second... Oh and (<$) isn't exported but ($>) is?
>> Fix that too. Then come join, fix, void, when, unless, bool, fromMaybe.....
>> I've come to the conclusion that Prelude should err on the side of very
>> small because Haskell is just too diverse a place to expect everyone to
>> agree on all these little things. We should all be making project-specific
>> Preludes instead IMO.
>>
>> On Tue, Sep 10, 2019 at 9:41 PM chessai . <chessai1996 at gmail.com> wrote:
>>
>>> I would also prefer if (&) and `for` were exported from the Prelude.
>>>
>>> On Tue, Sep 10, 2019, 9:33 PM Elliot Cameron <eacameron at gmail.com>
>>> wrote:
>>>
>>>> Hilariously, I'm mild -1 on this. Haskell is an extremely import-heavy
>>>> language. Anyone who isn't willing to just write their own mini-Prelude
>>>> should be ready to import things like `on`. Why isn't `for` exported in
>>>> Prelude? What about `&`? Both of these are extremely useful and common,
>>>> even moreso than *on*! And their implementation is *even shorter*. It's a
>>>> slippery slope.
>>>>
>>>> On Tue, Sep 10, 2019 at 6:59 PM David Feuer <david.feuer at gmail.com>
>>>> wrote:
>>>>
>>>>> Indeed, there are a lot more conflicts than I'd have expected.
>>>>> Ignoring functions with the same type, Hoogle shows this name in the below
>>>>> packages. I have no sense of the overall significance of the specific
>>>>> packages or (equally importantly) of the `on` function within each.
>>>>>
>>>>> haskell-gi-base:
>>>>> on :: forall object info m . (GObject object, MonadIO m, SignalInfo
>>>>> info) => object -> SignalProxy object info -> HaskellCallbackType info -> m
>>>>> SignalHandlerId
>>>>>
>>>>> brick:
>>>>> on :: Color -> Color -> Attr
>>>>>
>>>>> esqueletto:
>>>>> on :: SqlExpr (Value Bool) -> SqlQuery ()
>>>>>
>>>>> relational-query (both):
>>>>> on :: MonadQuery m => Predicate Flat -> m ()
>>>>> on :: MonadQuery m => QueryA m (Predicate Flat) ()
>>>>>
>>>>> threepenny-gui:
>>>>> on :: (element -> Event a) -> element -> (a -> UI void) -> UI ()
>>>>>
>>>>> miso:
>>>>> on :: MisoString -> Decoder r -> (r -> action) -> Attribute action
>>>>>
>>>>> wild-bind:
>>>>> on :: i -> v -> Binder i v ()
>>>>>
>>>>> massiv-io:
>>>>> on :: Pixel X Bit
>>>>>
>>>>> selda-postgresql:
>>>>> on :: Text -> Text -> PGConnectInfo
>>>>>
>>>>>
>>>>> On Tue, Sep 10, 2019, 6:42 PM Ryan Trinkle <ryan.trinkle at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> One note: this does conflict with some other libraries, for instance
>>>>>> GTK2HS
>>>>>>
>>>>>> On Tue, Sep 10, 2019 at 6:26 PM chessai . <chessai1996 at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>> +1
>>>>>>>
>>>>>>> On Tue, Sep 10, 2019, 5:53 PM David Feuer <david.feuer at gmail.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>>> Every time I reach for Data.Function.on, I feel like a total dolt
>>>>>>>> for having to import a module to get a function whose implementation is
>>>>>>>> barely longer than the import. And it's a really good function too! Can we
>>>>>>>> please add it to the Prelude?
>>>>>>>>
>>>>>>>>   on :: (b -> b -> c) -> (a -> b) -> a -> a -> c
>>>>>>>>   (.*.) `on` f = \x y -> f x .*. f y
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