Type inference error when using .andThrough()

[msickpaler]

When using .andThrough() with a function that returns a Result, TypeScript reports a type mismatch, even though the function signature should be compatible.

part of code

ok({
  title: "title",
  description: "description",
})
  .andThen((s) => ok(s))
  .andThrough(assertPublishable) // <= error happen

current return type

Result<{
    title: string;
    description: string;
}, Error_A>

expected return type

Result<{
    title: string;
    description: string;
}, Error_A | Error_B>

Curiously, if I comment out .andThen((s) => ok(s)), the error disappears and the return type becomes exactly as expected.

typescript playground is here

https://www.typescriptlang.org/play/?#code/JYWwDg9gTgLgBAbzgUylANHCBrOBfOAMyghDgCIA7ZAN1RgAsSB3cgbgCgOBjAGwEMAzoLgBRNNAD6AQRQAPGMkoATEeJJREHOHDBRgNforhRk-ZREq8AnnEkxrYZAC4K6qdPZcd3S4JhQAK7cMNAAFCDIwvwA5i5w-vqUMQCUWjo6goFOUBFRgrHIKZwZcIzAggB09o7IcAC8bhJQMl46eBwdfEJqzZIAQvKKKr0a6br6hsam5pY2dg5OruTuLf1e2nC+lInBobmR0XGuicDJaQibmdmoeUdFJRnlVTVODU0aA234nVzb-nAevQAAqBABGvAqDH4ELqjTCgko-DAggYEBgrkuOhgwBgvHip2Sj2UUW4+jAOMsJwCZxinDwaXqAD5xsBCHAwgBCRHI1Hoyo4vFFcY6UwwQJQSgoNBhajMMR9aRhcioDSeFLFTYdHRsjncpEotEwSokwRk4AU4CWC5XEzIcWS6W5OUKz79ZWq6DrDWPbV2h1SnBhTV4TgcINYsq4-HLQX48joTam82WqkUZPkymUBOdFKbSr8FQAFQYSjCCMZLKDgg1+cLyhLJECMQYYSBsFBEKhMPxmqAA

[rivea0]

In the example you gave, the type inference for assertPublishable is this:

Err<never, Error_A> | Err<never, Error_B> | Ok<void, never>

Whereas andThrough() expects a function that returns Result<unknown, Error_A>.

You can add a Result type to it like this:

Result<void, Error_A | Error_B>

So the function looks like this:

const assertPublishable = (snapshot: {
  title: string;
  description: string;
}): Result<void, Error_A | Error_B> => {
  if (!snapshot.title) {
    return err(new Error_A("errorA"));
  }
  if (!snapshot.description) {
    return err(new Error_B("errorB"));
  }
  return ok();
};

Also importing Result at the top:

import { err, ok, Result } from "neverthrow";

Adding it gets rid of the error.

Also, andThen() might be unnecessary there? The function inside doesn't modify or do anything with s, which is just this:

{
  title: "title",
  description: "description",
}

So you take that value and wrap it as Ok again:

ok({
  title: "title",
  description: "description",
})
  .andThen((s) => ok(s))

I'm not sure about the goal there, but hope this helps.