Re: Guile equivalent to JS promises?

[Thompson, David]

Hello!

On Sun, Dec 9, 2018 at 5:52 AM <address@hidden> wrote:
>
> Hi
>
> I'm trying to understand JS promises.
>
> Are promises relevant in Guile?
>
> According to https://www.promisejs.org/ they seem to be a tool to
> read/write JSON in a nonblocking way.
>
> Is this related to threading where the JS dev want multiple threads to
> read stuff asynchroniusly at the same time?

So, promises are basically just callback functions + error handling +
a way to compose them together in a chain.  I don't know of any
existing Guile library that implements this API, but it wouldn't take
much code to make a Scheme equivalent.  However, I am hesitant to
recommend promises for writing asynchronous programs for a variety of
reasons. [1]

Fortunately, Guile is pretty neat and provides a low-level feature
that allows for much nicer asynchronous programming models: delimited
continuations. I won't go into much detail about them here (see
call-with-prompt in the manual), but Andy Wingo's guile-fibers [2]
project is a really neat asynchronous programming library built on top
of delimited continuations.

And here's my favorite guile-user post of all time in which Andy drops
a 13 line coroutine implementation (this blew my mind many years ago):
https://lists.gnu.org/archive/html/guile-user/2011-02/msg00031.html

tl;dr - use a system based on delimited continuations or write your own!

Hope this helps!

- Dave

[1] 
http://wingolog.org/archives/2016/10/12/an-incomplete-history-of-language-facilities-for-concurrency
[2] https://github.com/wingo/fibers