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

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