Question

It’s hard to explain, here is what I want:

let internal (=%=) (W : int, thriller : Dev -> unit -> bool) =
    let id = get W
    if id <> -1 then
        workbase.[id] |> thriller

It doesn’t allow my thriller to return any type.

So I can do W =%= fun w -> foo() where foo() is unit

But how can I do W =%= fun w -> foo() where foo() is unit -> T' (some type) ?

thank you

just added final usage (because like it):

let CanRead     W dev v = W =%= fun w -> w.GetCanRead dev v
                        |> fun opt -> if opt.IsSome then opt.Value
                                      else false

after comment:

let CanRead     W dev v =   match W =%= fun w -> w.GetCanRead dev v with
                            | Some(t)   -> t
                            | None      -> false

Answer 1

An else branch is missing.

When you return unit in if/else expression, if... then... is a shortcut to if... then... else (). So you can always return some type T in a complete if/else expression, for example, an option type as follows:

let internal (=%=) (W : int, thriller : Dev -> unit -> bool) =
    let id = get W
    if id <> -1 then
        Some (workbase.[id] |> thriller)
    else None