Improve active pattern errors

[MangelMaxime]

Description

Considering the next code:

type Test =
    | Msg1
    | Msg2

let testToString msg =
    match msg with
    | Msg1 -> "msg 1"

We generate this code:

import { setType } from "fable-core/Symbol";
import _Symbol from "fable-core/Symbol";
import { comparePrimitives } from "fable-core/Util";
export class Test {
  constructor(tag) {
    this.tag = tag | 0;
  }

  [_Symbol.reflection]() {
    return {
      type: "Test.Test",
      interfaces: ["FSharpUnion", "System.IEquatable", "System.IComparable"],
      cases: [["Msg1"], ["Msg2"]]
    };
  }

  Equals(other) {
    return this.tag === other.tag;
  }

  CompareTo(other) {
    return comparePrimitives(this.tag, other.tag);
  }

}
setType("Test.Test", Test);
export function testToString(msg) {
  if (msg.tag === 0) {
    return "msg 1";
  } else {
    throw new Error("test.fs", 7, 10);
  }
}

Proposition

I would like to add a custom Error type to Fable: MatchError (or something like that) and generate something like:

class MatchError extends Error {
  constructor(failingCase, ...params) {
    // Pass remaining arguments (including vendor specific ones) to parent constructor
    super(...params);

    // Maintains proper stack trace for where our error was thrown (only available on V8)
    if (Error.captureStackTrace) {
      Error.captureStackTrace(this, CustomError);
    }

    // Custom debugging information
    this.failingCase = failingCase;
  }
}

    throw new MatchError(msg, "test.fs", 7, 10);

Why ?

The idea behind this proposition is to make Elmish errors understandable.

Today we have something like:

Tomorrow we could have:

Unable to process message:
Msg2

From file: 
Test.fs

Also, if this is a nested DU we get direct access to the whole tree:

type Test =
    | Msg1 of string
    | Msg2 of Test

printfn "%A" (Msg2 (Msg1 "deep_string"))
// Log: Msg2 (Msg1 "deep_string"))

This can be really useful in Elmish application where every message is a nested DU

[alfonsogarciacaro]

Interesting, I usually just check the warnings that tell me about non-exhaustive pattern matching but it should definitely be possible to have a more informative message for pattern matching errors. We just need to find a way to identify this kind of exceptions.

[MangelMaxime]

I do that in general but when you have several non implement case it's not helping :)

Also, when adding a subcomponent this can help identify which parent didn't update it correctly etc.

Is it fable writting: throw new Error("test.fs", 7, 10); or something coming from the compiler AST ?

[alfonsogarciacaro]

It's from the F# compiler AST. I guess it's using a particular kind of exception (which we may use to identify it) but Fable is translating it as a general exception at the moment.

[alfonsogarciacaro]

Fixed by #1611, thanks @Zaid-Ajaj!