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Comment by phzbOx | Hacker News Reader

Note that I haven't tested ICS so it's only based on my understanding of the examples/docs.

"Is the following right? Syntactically, "defer json" is an argument to the function "$.getJSON". So you could write it like this: await $.getJSON( url, defer(json) )"

Yes, exactly.

"'m guessing it's some convenient compiler magic that ICS does"

Yes, exactly. When you say "defer(x)", it basically returns a callback like this:

  function(result) {
    *x = result;
    Code to terminate the await block*
 }

"One last thing: the description talks about "deferrals" begin "fulfilled", but it doesn't say how they are created. Now, I think they are created by calling defer."

Yeah, defer returns a callback and when you call it, it "fulfills" the await, i.e. you tell await that you're done with that async block.

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Thanks a second time, I'm getting there! I realized one can look at the transformed javascript output, to see just what it does. The setting of the variable in the defer becomes:

  $.getJSON(url, __iced_deferrals.defer({
      assign_fn: (function() {
        return function() {
          return json = arguments[0];
        };
      })(),
      lineno: 6
    }));

Presumably, when the function that __iced_deferrals.defer returns is called by the code in .getJSON that delivers the result, it will set arguments[0] to that result, and then call the assign_fn above. This will set json to that result.

Also, I realize that it doesn't set json to the returned value of $.getJSON, as $.getJSON doesn't return anything - it uses a callback to deliver its result. It seems that ICS always works this way - which I guess implies that the use-case it's for always uses callbacks...

BTW: I'm not sure why the above has a function which is then called immediately. Maybe a coffeescript thing, for separating namespaces? I assume the lineno: 6 is for debugging - I recall some debate against this. Good to see it got in.

About the function which is called automatically, it's a common pattern in javascript. It's used to create a new namespace/modules as block are defined per function instead of {}.

And, you are right that we're not assigning the return value of $.getJSON. Basically, once $.getJSON has finished with the request, it will call a function with the result. This result is set to the variable passed to defer. It's a bit of a brain teaser but that's the whole point of the await/defer thingy. Instead of using a callback to retrieve the result as an argument, you just write a variable after defer and you receive it.

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