Statically Resolved Type Parameters

Syntax#

s is an instance of ^a you want to accept at compile-time, which can be anything that implements the members you actually call using the syntax.
^a is similar to generics which would be 'a (or 'A or 'T for example) but these are compile-time resolved, and allow for anything that fits all the requested usages within the method. (no interfaces required)

Simple usage for anything that has a Length member

let inline getLength s = (^a: (member Length: _) s)
//usage:
getLength "Hello World" // or "Hello World" |> getLength
// returns 11

Class, Interface, Record usage

// Record
type Ribbon = {Length:int}
// Class
type Line(len:int) = 
    member x.Length = len
type IHaveALength =
    abstract Length:int

let inline getLength s = (^a: (member Length: _) s)
let ribbon = {Length=1}
let line = Line(3)
let someLengthImplementer =
    { new IHaveALength with
        member __.Length = 5}
printfn "Our ribbon length is %i" (getLength ribbon)
printfn "Our Line length is %i" (getLength line)
printfn "Our Object expression length is %i" (getLength someLengthImplementer)

Static member call

this will accept any type with a method named GetLength that takes nothing and returns an int:

((^a : (static member GetLength : int) ()))