Traits-SRTP.fsx

module T =
    open System

    [<Trait>]
    type Show<'a> = 
        abstract member show: 'a -> string
    
    [<Witness>] 
    type ShowInt =
        interface Show<int> with
            member __.show x = x.ToString()

    [<Witness>] 
    type ShowFloat =
        interface Show<float> with
            member __.show x = x.ToString()
    
    let inline show'< ^w, ^a when ^w : (member show : ^a -> string)> (w: 'w, a: 'a) =
        (^w: (member show : 'a -> string) Unchecked.defaultof<'w>, a)
    let inline show x = show'(Unchecked.defaultof<_>, x)
    
    //error FS0332: Could not resolve the ambiguity inherent in the use of the operator 'show' at or near this program point. Consider using type annotations to resolve the ambiguity.
    //error FS0071: Type constraint mismatch when applying the default type 'obj' for a type inference variable. The type 'obj' does not support the operator 'show' Consider adding further type constraints

    show 1 |> printfn "%s"
    show 1.0 |> printfn "%s"