How to correctly turn closure to promise in amphp?

Question

I'm learning amphp. I want to convert sync call to async call using event loop in amphp. My sample code use file_get_contents as sample blocking call.

Using sync call, it look like this:

$uris = [
    "https://google.com/",
    "https://github.com/",
    "https://stackoverflow.com/",
];

$results = [];
foreach ($uris as $uri) {
    var_dump("fetching $uri..");
    $results[$uri] = file_get_contents($uri);
    var_dump("done fetching $uri.");
}

foreach ($results as $uri => $result) {
    var_dump("uri : $uri");
    var_dump("result : " . strlen($result));
}

And the output:

string(30) "fetching https://google.com/.."
string(34) "done fetching https://google.com/."
string(30) "fetching https://github.com/.."
string(34) "done fetching https://github.com/."
string(37) "fetching https://stackoverflow.com/.."
string(41) "done fetching https://stackoverflow.com/."
string(25) "uri : https://google.com/"
string(14) "result : 48092"
string(25) "uri : https://github.com/"
string(14) "result : 65749"
string(32) "uri : https://stackoverflow.com/"
string(15) "result : 260394"

I know there's artax that would do the call in async. But, I want to learn how to correctly turn blocking code into async concurrent code (not parallel). I'm already success implementing it in parallel using amp.

I believe the correct output if I successfully implement it in async in amp, will be something like this:

string(30) "fetching https://google.com/.."
string(30) "fetching https://github.com/.."
string(37) "fetching https://stackoverflow.com/.."
string(34) "done fetching https://google.com/."
string(34) "done fetching https://github.com/."
string(41) "done fetching https://stackoverflow.com/."
string(25) "uri : https://google.com/"
string(14) "result : 48124"
string(25) "uri : https://github.com/"
string(14) "result : 65749"
string(32) "uri : https://stackoverflow.com/"
string(15) "result : 260107"

I've tried with this code:

<?php
require __DIR__ . '/vendor/autoload.php';

use Amp\Loop;
use function Amp\call;

Loop::run(function () {
    $uris = [
        "https://google.com/",
        "https://github.com/",
        "https://stackoverflow.com/",
    ];

    foreach ($uris as $uri) {
        $promises[$uri] = call(function () use ($uri) {
            var_dump("fetching $uri..");
            $result = file_get_contents($uri);
            var_dump("done fetching $uri.");
            yield $result;
        });
    }

    $responses = yield $promises;

    foreach ($responses as $uri => $result) {
        var_dump("uri : $uri");
        var_dump("result : " . strlen($result));
    }
});

Instead giving me my expected result, it give me this error:

string(30) "fetching https://google.com/.."
string(34) "done fetching https://google.com/."
string(30) "fetching https://github.com/.."
string(34) "done fetching https://github.com/."
string(37) "fetching https://stackoverflow.com/.."
string(41) "done fetching https://stackoverflow.com/."
PHP Fatal error:  Uncaught Amp\InvalidYieldError: Unexpected yield; Expected an instance of Amp\Promise or React\Promise\PromiseInterface or an array of such instances; string yielded at key 0 on line 20 

The result also seems run in sync, not async.

How should I correctly do it?

Answer 1

You need to use a non-blocking implementations of the functions you use. file_get_contents is blocking. You can find a non-blocking implementation for example in amphp/file. If you replace file_get_contents with Amp\File\get, it should work as expected.