Proposal: Add singleton function to Data.List module

[Oliver Charles]

+1. All alternatives to construct a list "anonymously" are confusing
and take time to parse. An explicit `Data.List.singleton x` is very
clear what's going on.

On Mon, Aug 12, 2019 at 5:14 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 I originally made this suggestion on GitLab, but I was told to make it here instead.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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