type SumOperations = SumOperations

​

let inline getSum b = SumOperations &+ b // needs to be declared before the operators to avoid defaulting

​

type SumOperations with

    static member inline (&+) (h:'h, x  : int      ) =       x 

    static member inline (&+) (h:'h, xl : 'x   list) =       xl |> List .sumBy ((&+) h)

    static member inline (&+) (h:'h, xa : 'x   []  ) =       xa |> Array.sumBy ((&+) h)

    static member inline (&+) (h:'h, xy : 'x * 'y  ) = match xy with x, y   -> ((&+) h) x + ((&+) h) y

    static member inline (&+) (h:'h, xo : 'x option) = match xo with Some x -> ((&+) h) x |_-> 0

​

printfn "RESULTS:"

​

2                  |> getSum |> printfn "2 = %d" // = 2

[ 2 ; 1 ]          |> getSum |> printfn "[ 2 ; 1 ] = %d" // = 3

[[2; 3] ; [4; 5]]  |> getSum |> printfn "[[2; 3] ; [4; 5]] = %d" // = 14

[[[|2; 3|] ; [|4; 5|] ] ; [[|6; 7|] ; [|4; 5|] ]] |> getSum |> printfn "[[[|2; 3|] ; [|4; 5|] ] ; [[|6; 7|] ; [|4; 5|] ]] = %d" // = 36

(Some 2, [| 2 ; 1 |])   |> getSum |> printfn "Some (2, [| 2 ; 1 |]) = %d" // = 5