Proposal: Add singleton function to Data.List module

[Elliot Cameron]

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 I originally made this suggestion on GitLab, but I was told to make it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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