I want to echo everything Matt Parsons is saying. I also want to publicly thank him for participating in this discussion and for making solid arguments. Thanks, Matt!

By way of explanation, I was motivated to propose this addition because a member of my team wondered aloud if there was a function to do this. I told them that I didn’t know of one, but I typically used `pure` in point-free expressions and `[x]` when the value had already been named. We searched around and couldn’t find anything:

- https://www.stackage.org/lts-14.0/hoogle?q=a+-%3E+%5Ba%5D
- https://hoogle.haskell.org/?hoogle=a%20-%3E%20%5Ba%5D

We got to talking about other ways to express the same thing. (Everything we came up with has been seen in this thread already.) That got me wondering why there *wasn’t* a specific, monomorphic function for this. The omission seemed especially strange to my team member because they were familiar with Elm, which has this function. 

All that to say: This proposal is motivated by real confusion coming from a junior developer using Haskell at work. Consider how easy it is to discover and do something, rather than if it’s technically possible. 

On Aug 13, 2019, at 7:20 PM, Matt <parsonsmatt@gmail.com> wrote:

`(\x -> [x])` has the same polymorphism problem in the context of `OverloadedLists` as `pure`.

`(:[])` is also unsatisfactory. To parse it properly, you need to:

- Know that `:` is only allowed as an operator to prefix data constructors,
- Know that `[]` are not legal operator characters,
- Infer that you're intended to insert a space between the `:` and `[]` to get `(: [])`
- Recognize it as an operator section being used prefix as a normal function

I'm neither an expert nor a lifelong Haskeller, but I have been writing it in some professional capacity for four years now, and the operator section trips me up. I can't imagine less experienced folks find it easy or intuitive. You can't even search for it on hoogle: https://hoogle.haskell.org/?hoogle=(%3A%5B%5D)

Is `(:[])` a core idiom? I never see it in work code and I've never seen it in Hackage. To check, I grepped my software directory which has all my Haskell code, and got 122 matches of (:[]) over 1,768,231 loc (as given by wc -l **/*.hs).

Matt Parsons


On Tue, Aug 13, 2019 at 4:56 PM Herbert Valerio Riedel <hvriedel@gmail.com> wrote:
> The point is to create a monomorphic variant [...]

But we already have at least two monomorphic variants to express this
with Haskell's concise native syntax and vocabulary which has by
design a preferential treatment of lists (it was considered even more
important than type-sigs so that we got the `:` cons operator for
lists and the `::` for type-sig annotations) -- so let's not try to
fight Haskell's core idioms by hiding them behind some trivial
additional redundant synonyms! I still fail to see the actual
*technical* problem being solved by the original proposal asking to
add yet another, wordy way to construct single-item-lists.
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