In the JavaScript std library, the `then` method o...

# javascript

[natpryce]

In the JavaScript std library, the then method of the Promise class supports mapping, but not flatmapping. Calling then with a function of type (T)->Promise<S> should return a Promise<S>, but instead it returns a Promise<Promise<S>>. This makes it impossible to use the Fetch API.

[konsoletyper]

Can you provide a code sample? I can't understand what's your problem

[konsoletyper]

Ah, I see. When a JavaScript function passed to then returns a promise it's not passed forward as is, but gets resolved first.

[konsoletyper]

I'm afraid it's impossible to represent in Kotlin type system.

[natpryce]

Even with overloading?

[konsoletyper]

Yes, even with overloading

[natpryce]

It looks like we need some annotations to control the compilation to JS , like the @JvmName annotation on the JVM platform

[natpryce]

So that Promise in Kotlin could have map and flatMap methods that are both translated to then in JS

[konsoletyper]

There's @JsName annotation

[konsoletyper]

It's possible to declare two functions with different names in Promise, say then and thenPromise, though it seems a bit ugly

[natpryce]

The two behaviours are map & flatMap - it would be nice if they had the same names as used elsewhere in the Kotlin libraries

[natpryce]

I tried using @JsName and was able to map map and flatMap to the then function.

[natpryce]

Hmmm… is there no way to add a snippet to a thread?

[natpryce]

Can you see this program on try.kotlinlang.org?

[natpryce]

https://try.kotlinlang.org/#/UserProjects/hi5gtn3e98h5ivrpve1b193j80/e7m31fl1smt0r00jd1ii7pe9nv

[natpryce]

I had some trouble with mapError and flatMapError, and so had to hack around the type system. I’ll ask about that on the #general channel