`this` null check in constructors conflicts with some environments (React Native)

[forki]

I'm porting my fable 1.x React Native App to Fable 2.x and I'm having problems with Array.groupBy .

    let mappedCityGroups = 
        requests
        |> Array.map (fun x ->
            match x with 
            | MeterReadingRequest x -> x.GetCity()
            | MeterChangeRequest x -> x.GetCity())


    printfn "mapped"

    let cityGroups = 
        requests
        |> Array.groupBy (fun x ->
            match x with 
            | MeterReadingRequest x -> x.GetCity()
            | MeterChangeRequest x -> x.GetCity())

the mapping part works, but during groupby it fails with "Cannot read property 'MoveNext' of undefined"

[forki]

ok somthing really weird is happening.

let ys = [| 1;2;3 |] |> Array.groupBy (fun x -> x = 2)

^^ even this fails.

[forki]

btw: why is fable-splitter generating minified code?

[forki]

ok I was able to repro this thing in SAFE-Nightwatch:

compiled to:

repro: SAFE-Stack/SAFE-Nightwatch#67 (run build.cmd debug)

[forki]

a bit of the stacktrace:

which points to:

[forki]

so it fails at:

en is null for some reason

[forki]

I found an "fix":

this change makes the groupBy work. I have no idea what's it doing tbh!

[Zaid-Ajaj]

I think array has to be converted to seq<_> first for this to work. Repl example:

let x = [| 1 .. 10 |]
let y = Array.groupBy (fun x -> x % 2 = 0) x

compiles to

import { groupBy, ofSeq } from "fable-library/Array";
import { rangeNumber } from "fable-library/Seq";
import { structuralHash } from "fable-library/Util";
export const x = ofSeq(rangeNumber(1, 1, 10), Int32Array);
export const y = groupBy(function (x$$1) {
  return x$$1 % 2 === 0;
}, x, Int32Array, {
  Equals($x$$1, $y$$2) {
    return $x$$1 === $y$$2;
  },

  GetHashCode: structuralHash
});

See the ofSeq being called on the array, for some reason this function is not called in your compiled code before invoking groupBy

[alfonsogarciacaro]

Maybe #1754 has fixed this too, I will push a new 2.2 beta version (remember you need to install them with "next" tag: npm i fable-compiler@next) so you can try it out.

[forki]

@alfonsogarciacaro please take a look at the commented code at https://user-images.githubusercontent.com/57396/53325627-0939e980-38e4-11e9-936e-a08fb796f965.png

It looks weird doesn't it?

[alfonsogarciacaro]

It does. Not sure if RN fails when you use this in the wrong place. If that's the issue we have a much bigger problem, because Fable 2 uses this pattern to distinguish if the constructor is being called directly or from a child type. But in that case I assume you should have seen the problem much earlier 🤔

[forki]

I don't think RN is special. but this is not null in my case and therefor it doesn't do the normal ctor

[forki]

this is a DedicatedWorkerGlobalScope. that can't be right!?

[forki]

repro can even be simpler:

let m = Map.empty 
let m2 = Map.add 1 2 m  // fails already

[forki]

m is undefined!