Rescript -> typescript -> js structural typing issue

#1

Hi,

I try rescript on a little PoC project and in order to have a chance to integrate it in typescript environments, I explore interoperability between the 2 languages.

Everything seemed to work smoothly when I bumped into an issue when handling rescript sum types on the ts side (in fact, on the transpiled js side).

I use ts-pattern in order to pattern-match on the output of a rescript function from typescript (but the exact same problem arises with a standard switch statement).

In my case it’s based on a Belt.Result.t<'a, 'e>.

the generated typescript (gen.ts) gives me a

type MyResult<a,e> = 
    { tag: "Ok"; value: a }
  | { tag: "Error"; value: e };

so I can pattern match on it (pure typescript side) with

match(result)
     .with({ tag: "Ok" }, () => ...)
     .with({ tag: "Error"}, () => ...)
     .exhaustive()

seemed great, typescript is happy… until I run the code : it breaks at runtime because it receives something like {"TAG":1,"_0":0}

Do you know how I can overcome this (without handling the result in rescript).

NB: I cross-posted as a comment on a similar issue on the gentype github : https://github.com/rescript-association/genType/issues/585, sorry for the noise if you saw it there too.

Best,
Matthieu

#2

I think I’m missing something : I did a minimal example to replicate and it works fine.

I’ll try to figure out the difference with the problematic code and update this thread in case someone else bumps into this.

#3

here’s a minimal example that reproduces the issue : https://github.com/err0r500/rescript-ts-issue-minimal

looks like it happens when a value of a rescript type (the type is exported with @genType) is passed to typescript.

EDIT : ok, I found a workaround by treating the ts code as IO in Haskell : once you’re in, you don’t leave :smiley:

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#4

From your edit it looks like you figured it out, but you have a problem here.

type t = Belt.Result.t<int, string> => unit
@module("./index") external idontwork: t = "idontwork"

The problem is that you’re telling the compiler that the external idontwork function takes a Belt.Result.t<int, string>, when really it doesn’t. It takes this thing:

export type resType = 
    { tag: "Ok"; value: number }
  | { tag: "Error"; value: string };

Which is not the way that ReScript variants are implemented.

#5

thanks @Ryan that’s the kind of issue I was suspecting.

So I guess I should definitely abandon my idea to go back & forth between rescript & typescript like : rescript with gentype → typescript + rescript with interop → rescript.

That makes sense, actually.
thanks again!

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