Proposal: Add singleton function to Data.List module

[Matthew Pickering]

Clear +1 from me. I have defined this function many times.

The addition of a one line function does not prevent anyone from using
`pure` in their own code if they wish to obfuscate matters.

Cheers,

Matt

On Tue, Aug 13, 2019 at 6:35 AM Elliot Cameron <eacameron at gmail.com> wrote:
>
> Seq is applicative and also has singleton. Some people like to avoid polymorphism when the use case is clearly monomorphic (c.f. discussion about map vs fmap).
>
> I'd like to point out that Pointed is sort of the abstraction we really want for this... But I still want a monomorphic function that doesn't delve into Kmett dependencies.
>
> On Mon, Aug 12, 2019 at 11:59 PM George Wilson <george at wils.online> wrote:
>>
>> Weak +1, I'd want it in NonEmpty too if it's going into List.
>>
>> On Tue, 13 Aug 2019 at 13:37, Joseph C. Sible <josephcsible at gmail.com> wrote:
>> >
>> > -1. AFAICT, "Polymorphic, works for any `Functor`" is an upside of
>> > just using `pure`, not a downside. (Please correct me if I'm wrong,
>> > and there is some disadvantage that I don't see.) Also, it looks like
>> > for everything that has a `singleton`, it's something that isn't an
>> > applicative functor, so they have them instead of `pure`, not in
>> > addition to it.
>> >
>> > Joseph C. Sible
>> >
>> > On Mon, Aug 12, 2019 at 12:14 PM Taylor Fausak <taylor at fausak.me> wrote:
>> > >
>> > > I originally made this suggestion on GitLab, but I was told to make it here instead.
>> > >
>> > > https://gitlab.haskell.org/ghc/ghc/issues/17042
>> > >
>> > > ---
>> > >
>> > > # Add list singleton function
>> > >
>> > > ## Motivation
>> > >
>> > > Sometimes it is convenient to have a function to wrap an element in a list. There are many ways to do this already, but none of them are as clear as a separate monomorphic function.
>> > >
>> > > - `pure`: Polymorphic, works for any `Functor`.
>> > > - `pure @[]`: Noisy, requires `-XTypeApplications`.
>> > > - `(: [])`: Subjectively ugly.
>> > > - `(\x -> [x])`: Syntactically noisy.
>> > >
>> > > This Twitter thread includes some additional commentary: https://twitter.com/taylorfausak/status/1159264862247280640
>> > >
>> > > ## Proposal
>> > >
>> > > I would like to add a `singleton` function to `Data.List` that mirrors the `singleton` function for other containers: https://www.stackage.org/lts-14.0/hoogle?q=singleton
>> > >
>> > > ``` hs
>> > > singleton :: a -> [a]
>> > > singleton x = [x]
>> > > ```
>> > >
>> > > Other Haskell-like languages include this function:
>> > >
>> > > - PureScript: https://pursuit.purescript.org/packages/purescript-lists/5.4.0/docs/Data.List#v:singleton
>> > > - Elm: https://package.elm-lang.org/packages/elm/core/latest/List#singleton
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