While section 13.3
describes synchronous calls from Prolog to JavaScript, we also need
asynchronous calling to implement sleep/1,
wait for user input, downloading documents from the web, etc.
Asynchronous calling is achieved by yielding from the Prolog
virtual machine. This can only be done when Prolog is told to expect
that the VM may yield. This is implemented by Prolog.forEach()
as described in section
13.2.
- [det]await(+Promise,
-Result)
- Yield the Prolog VM, returning control back to JavaScript. When this is
called from Prolog invoked using
Prolog.forEach(),
execution of
await/2
completes when the Promise resolves and Result is
unified with the value passed to the Promise.then()
method. As an exception to the normal conversion rules, if the result is
a single
String, it is returned as a Prolog string rather than an
atom. When the Promise is rejected await/2
throws an exception. Note that await/2
allows, for example, downloading a URL from Prolog:
?- FP := fetch("test.pl"), await(FP, Response),
TP := Response.text(), await(TP, T).
FP = <js_Promise>(4),
Response = <js_Response>(5),
TP = <js_Promise>(6),
T = "% :- debug(js) ...".
Calls to await/2
may be asynchronously aborted by calling
Prolog.abort() if Promise implements .abort().
See section 13.3.2
for implementing such a promise.
- [semidet]is_async
- True when we can call await/2
in the current state. This implies Prolog has been called from
JavaScript code that is prepared to deal with Prolog yielding and Prolog
is not inside a callback from C (WASM).
