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Core implementation of List.contains #20847

Answered by LeandroCustodio2002

davidgrenier asked this question in Q&A

Core implementation of List.contains #20847

davidgrenier

Oct 22, 2026

· 1 comment

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davidgrenier
Oct 22, 2026

I'm wondering if it's something that's been overlooked or a type optimization I'm not aware of. As I see it currently, the core implementation of List.contains is:

[<CompiledName("Contains")>]
let inline contains value source =
    let equalityCheck listElement =
        value = listElement

    let rec contains e xs1 =
        match xs1 with
        | [] -> false
        | h1 :: t1 -> equalityCheck h1 || contains e t1

    contains value source

Notice that equalityCheck closes over the element being looked for. However the recursive function definition also carries that value around (argument e) but doesn't actually use it.

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Answered by LeandroCustodio2002

Oct 22, 2026

Good catch, that e parameter isn't doing anything. equalityCheck already closes over value, so passing it down the recursion is redundant. It looks like a leftover from an earlier version rather than a deliberate optimization.

It doesn't break anything, and the cost is basically one extra argument per call, so you won't notice it in practice. It's still dead code though, so it could just be:

let rec contains xs1 =
match xs1 with
| [] -> false
| h1 :: t1 -> equalityCheck h1 || contains t1

contains source

Probably worth opening a small PR for it, cleanups like this usually get accepted pretty quickly.

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