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F# - Custom exception with support for formatted strings

I have been coding F# for a while now, and I've managed to stay away from throwing custom exception so chances are I'm doing something wrong now as I'm looking into throwing one, but I thought I would share it, as it also shows how you can accomplish similar constructs as failwithf to allow formatted messages.

Lets start with the simple part, defining the exception

type ProtocolVersion =
    | V1 = 1

exception ProtocolException of ProtocolVersion * string

Raising this exception would be as simple as:

raise (ProtocolException (ProtocolVersion.V1, "Unknown marker found for Encoding."))

And if we want to inject some values in the string:

raise (ProtocolException (ProtocolVersion.V1, sprintf "%i reads '%s'." 42 "fourty two"))

What I want i something like failwith and failwithf:

raise (protocol1Exception "Unknown marker found for Encoding.")


raise (protocol1Exceptionf "%i reads '%s'." 42 "fourty two")

One working solution

I'm honestly not sure if this is THE solution, but it's at least a solution, but feel free to educate me of improvements. As F# is Open Source we can have a look at how e.g. failwithf has been implemented: https://github.com/fsharp/fsharp/blob/cb6cb5c410f537c81cf26825657ef3bb29a7e952/src/fsharp/FSharp.Core/printf.fs#L1645

And looking at this we can put together a working solution like this:

let protocol1Exception message =
    ProtocolException (ProtocolVersion.V1, message)

let protocol1Exceptionf format =
    Printf.ksprintf protocol1Exception format

Trying it out:

let willThrow() =
    raise (protocol1Exception "Unknown marker found for Encoding.")

let willThrow'() =
    raise (protocol1Exceptionf "%i reads '%s'." 42 "fourty two")

    | ProtocolException (v,m) -> printfn "Exception message: %s; ProtocolVersion: '%A';" m v

    | ProtocolException (v,m) -> printfn "Exception message: %s; ProtocolVersion: '%A'" m v


1: Exception message: Unknown marker found for Encoding.; ProtocolVersion: 'V1';
2: Exception message: 42 reads 'fourty two'.; ProtocolVersion: 'V1'

That's it for this time. Again, feel free to educate me about improvements. Not only about the implementation in itself but also things like: "you should really not throw, but use e.g Result and ROP instead".



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