F#, why can't I access the "Item" member

Question

In F#, why can't I access the "Item" member on the Array here:

let last (arr:System.Array) =
    let leng = arr.Length
    arr.[leng-1]   // Error: Field, constructor or member "Item" is not defined.

Answer 1

Can you try this?

let last (arr:_[]) =
       let leng = arr.Length
       arr.[leng-1]