trying to extend the result type.. unsuccesfully, in F#

Question

Right now, when I have a Result type, it seems like I need to do a match to find if it is ok, or not.

So, I am trying to simplify this to be able to use if, like that:

let a : Result<string, string> = ...
if a.IsOk then ....

Here is my attempt

[<AutoOpen>]
module ResultHelper =
    type Result with
        member this.IsOk =
            match this with
            | Ok _    -> true
            | Error _ -> false

but this will not compile.

Using:

    type Result<_, _> with

doesn't help either

How can I achieve this?

Note that `Result.map` does more or less exactly what you are trying to do here. — Guran, Nov 24 '20 at 14:45
@Guran I had no idea result.map was a thing, but it looks like it's exactly what I need since I need to chain operations. thanks! — Thomas, Nov 24 '20 at 19:28
you’re welcome. Map and Bind are really nice constructs. Just beware the M-word... — Guran, Nov 24 '20 at 19:33

score 3 · Accepted Answer · answered Nov 24 '20 at 01:58

The correct syntax is to give the type parameters names. As a bonus, this will allow you to refer to them in the code if you need to.

type Result<'a, 'b> with
    member this.IsOk =
        match this with
        | Ok _    -> true
        | Error _ -> false

P.S. For the future, please note that "will not compile" is not a good description of a problem. Please make sure to always include the full error message.

trying to extend the result type.. unsuccesfully, in F#

1 Answers1

Linked