I am building some equations in F#, and when working on my polynomial class I found some odd behavior using List.mapi

Basically, each polynomial has an array, so 3*x^2 + 5*x + 6 would be [|6, 5, 3|] in the array, so, when adding polynomials, if one array is longer than the other, then I just need to append the extra elements to the result, and that is where I ran into a problem.

Later I want to generalize it to not always use a float, but that will be after I get more working.

So, the problem is that I expected List.mapi to return a List not individual elements, but, in order to put the lists together I had to put [] around my use of mapi, and I am curious why that is the case.

This is more complicated than I expected, I thought I should be able to just tell it to make a new List starting at a certain index, but I can't find any function for that.

type Polynomial() =
    let mutable coefficients:float [] = Array.empty
    member self.Coefficients with get() = coefficients
    static member (+) (v1:Polynomial, v2:Polynomial) =
        let ret = List.map2(fun c p -> c + p) (List.ofArray v1.Coefficients) (List.ofArray v2.Coefficients)
        let a = List.mapi(fun i x -> x)
        match v1.Coefficients.Length - v2.Coefficients.Length with
            | x when x < 0 ->
                ret :: [((List.ofArray v1.Coefficients) |> a)]
            | x when x > 0 ->
                ret :: [((List.ofArray v2.Coefficients) |> a)]
            | _ -> [ret]