Remove getting needless list length

[auduchinok]

Removes some needless list traversals.

[dsyme]

I think isSingleton is more consistent (List.isEmpty, List.singleton) but no big deal

[KevinRansom]

I disagree with @dsyme here singleton has a particular meaning in programming. isSingleItem is a bit more specific here.

[vzarytovskii]

I disagree with @dsyme here singleton has a particular meaning in programming. isSingleItem is a bit more specific here.

It also means "list with a single element" in many functional languages, besides, we already use it in lists and arrays List.singleton, Array.singleton.

[smoothdeveloper]

@KevinRansom, the contention is that isSingleton is almost always going to be List.isSingleton.

I think the reasoning of making it the pendant of singleton is good, in case we like to surface the function in FSharp.Core.

[auduchinok]

I've been actually choosing between isSingleton and isSingleItem names and decided to go with the latter. I'm OK with changing it to isSingleton, thanks!

[KevinRansom]

sounds good.

[smoothdeveloper]

@duckmatt / @knocte / @jgardella, wondering if this wouldn't be worth a rule in FSharpLint?

[knocte]

wondering if this wouldn't be worth a rule in FSharpLint?

Good idea, although it would be much easier to make the violators of the rule respect it if there was an isSingleton function in FSharp's List, shouldn't we add that function first?

[smoothdeveloper]

@dsyme, a bit tangential, I remember some code reviews where we you preferred I change singleton calls into [value] literals, in the compiler code.

Do you feel = [_] could one day, be a legit operator application, and thus, a valid alternative to isSingleton?

It would be cool to have such bindings turn into boolean expression, if there is no ambiguity introduced in the language and such suggestion would makes sense.

[dsyme]

you preferred I change singleton calls into [value] literals, in the compiler code.

Yes, [expr] is generally preferred.

Do you feel = [_] could one day, be a legit operator application, and thus, a valid alternative to isSingleton?

The general construct would be "test if something matches a pattern and return a bool". F# doesn't have this construct mainly out of simplicity, Probably expr match? pat or something would be the natural way of writing it. But TBH I think F# is simpler without these numerous variations on pattern matching syntax cuteness, and it leads to less code churn between different ways of writing the same thing

[smoothdeveloper]

Thanks for the feedback, I think this, and fsprojects/FSharpLint#543 would push us towards bringing the function in FSharp.Core, so FSharpLint can propose a quick fix to the issue (and embark the diagnostic about wrong code which I was concerned about).