Proposal: add `on` to the Prelude

[Elliot Cameron]

You might be on2 something.

But then you rename it in Data.Function too and break backward compat??

On Thu, Sep 12, 2019 at 10:55 AM David Feuer <david.feuer at gmail.com> wrote:

> For whatever it's worth, the unidiomatic but arguably more logical name
> `on2` has no hits on Hoogle.
>
> On Tue, Sep 10, 2019, 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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>>>>>
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