└── README.md


/README.md:
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 1 | # Asynchronous "do expressions"
 2 | 
 3 | ## Overview
 4 | 
 5 | Building on
 6 | [async functions](https://github.com/tc39/ecmascript-asyncawait)
 7 | and the not-formally-proposed 
 8 | ["do expressions"](http://wiki.ecmascript.org/doku.php?id=strawman:do_expressions),
 9 | "async do expressions" let you write asynchronous blocks in an
10 | expression context, returning the completion value (a Promise).
11 | 
12 | This is useful when functions expect an asynchronous result (a Promise)
13 | as input, and the steps to produce that result can be expressed in line
14 | with the function call.
15 | 
16 | Example:
17 | ```js
18 | fetchEvent.respondWith(async {
19 |   var response = await fetch('https://example.com/resource');
20 |   var body = await response.text();
21 |   'BEGIN---\n' + body + '\n---END\n';
22 | });
23 | ```
24 | 
25 | This is similar to an "immediately invoked async function expression", i.e.:
26 | 
27 | ```js
28 | fetchEvent.respondWith((async () => {
29 |   var response = await fetch('https://example.com/resource');
30 |   var body = await response.text();
31 |   return 'BEGIN---\n' + body + '\n---END\n';
32 | })());
33 | ```
34 | 
35 | Or more verbosely:
36 | 
37 | ```js
38 | async function f() {
39 |   var response = await fetch('https://example.com/resource');
40 |   var body = await response.text();
41 |   return 'BEGIN---\n' + body + '\n---END\n';
42 | };
43 | let p = f();
44 | fetchEvent.respondWith(p);
45 | ```
46 | 
47 | ## Syntax
48 | 
49 | ```
50 | PrimaryExpression :
51 |   ...
52 |   AsyncBlockExpression
53 |   
54 | AsyncBlockExpression :
55 |   "async" [no LineTerminator here] Block
56 |   
57 | ExpressionStatement :
58 |   [lookahead ∉ {{, function, class, let [, async}] Expression;
59 | ```
60 | 
61 | ## Semantics
62 | 
63 | TODO - static and runtime semantics
64 | 
65 | * `var` hoisting behavior needs definition
66 | * `break`, `continue` and `return` need definition
67 | 
68 | ## References
69 | 
70 | * [async functions](https://github.com/tc39/ecmascript-asyncawait)
71 | * [do expressions strawman](http://wiki.ecmascript.org/doku.php?id=strawman:do_expressions)
72 | * TC39 meeting notes: [Do Expression](https://github.com/rwaldron/tc39-notes/blob/master/es6/2014-01/jan-29.md#do-expression)
73 | * es-discuss thread: [monadic extension to do-notation](https://esdiscuss.org/topic/monadic-extension-to-do-notation)
74 | 
75 | 


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