https://twitter.com/__josejuan__/status/445564129521434624

gistfile1.fs

// https://twitter.com/__josejuan__/status/445564129521434624

// dummy definitions:
type Variable() = class end
type Expression() = class end
type Term() = class end

// overloads:
type E =
    static member Sum(x: Variable list) = 0
    static member Sum(x: Expression list) = 1
    static member Sum(x: Term list) = 2

// example:
let _ =
    let v = E.Sum [Variable()]
    let e = E.Sum [Expression()]
    let t = E.Sum [Term()]
    ()

// alternatively, using inline overloads:
type SumClass = SumClass with
    static member Sum(x: Variable list) = 0
    static member Sum(x: Expression list) = 1
    static member Sum(x: Term list) = 2

let inline sum x =
    let inline isum (a: ^a, b: ^b) = ((^a or ^b) : (static member Sum: ^b -> int) b)
    isum (SumClass, x)

// example:
let _ =
    let v = sum [Variable()]
    let e = sum [Expression()]
    let t = sum [Term()]
    ()