Strange FSharpLint warning

Question

I am trying to reimplement the List.distinct function:

let inline distinct list =
    let folder curr = function
        | [] -> [curr]
        | l -> if List.contains curr l then l else curr :: l
    List.foldBack folder list []
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I got a lint warning that says

List.foldBack f x [] might be able to be refactored into x

However that doesn't make much sense to me as this leads me to return the original list without performing the distinct logic.

Is this a bug of FSharpLint?

Answer 1

Apparently that's a wrong rule. Take a look at this excerpt from FSharpLint's default configuration:

List.fold f x [] ===> x
Array.fold f x [||] ===> x
List.foldBack f x [] ===> x
Array.foldBack f x [||] ===> x

It's the same for fold and foldback here, so it doesn't follow the differences in signatures between the two.

The order of initial state and collection arguments should be flipped, see the "mnemonic" approach used in foldback definition.