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Partial function application.
Partial function application., (*1)
, (*2)
The recommended way to install react/partial is through composer., (*3)
{
"require": {
"react/partial": "~2.0"
}
}
Partial application (or partial function application) refers to the process of fixing a number of arguments to a function, producing another function of smaller arity. Given a function
f:(X x Y x Z) -> N, we might fix (or 'bind') the first argument, producing a function of typef:(Y x Z) -> N. Evaluation of this function might be represented asf partial(2, 3). Note that the result of partial function application in this case is a function that takes two arguments., (*4)
Basically, what this allows you to do is pre-fill arguments of a function, which is particularly useful if you don't have control over the function caller., (*5)
Let's say you have an async operation which takes a callback. How about a file download. The callback is called with a single argument: The contents of the file. Let's also say that you have a function that you want to be called once that file download completes. This function however needs to know an additional piece of information: the filename., (*6)
public function handleDownload($filename)
{
$this->downloadFile($filename, ...);
}
public function downloadFile($filename, $callback)
{
$contents = get the darn file asynchronously...
$callback($contents);
}
public function processDownloadResult($filename, $contents)
{
echo "The file $filename contained a shitload of stuff:\n";
echo $contents;
}
The conventional approach to this problem is to wrap everything in a closure like so:, (*7)
public function handleDownload($filename)
{
$this->downloadFile($filename, function ($contents) use ($filename) {
$this->processDownloadResult($filename, $contents);
});
}
This is not too bad, especially with PHP 5.4, but with 5.3 you need to do the
annoying $that = $this dance, and in general it's a lot of verbose
boilerplate that you don't really want to litter your code with., (*8)
This is where partial application can help. Since we want to pre-fill an
argument to the function that will be called, we just call bind, which will
insert it to the left of the arguments list. The return value of bind is a
new function which takes one $content argument., (*9)
use function React\Partial\bind;
public function handleDownload($filename)
{
$this->downloadFile($filename, bind([$this, 'processDownloadResult'], $filename));
}
Partialing is dependency injection for functions! How awesome is that?, (*10)
use function React\Partial\bind;
$add = function ($a, $b) {
return $a + $b;
};
$addOne = bind($add, 1);
echo sprintf("%d\n", $addOne(5));
// outputs 6
use function React\Partial\bind_right;
$div = function ($a, $b, $c) {
return $a / $b / $c;
};
$divMore = bind_right($div, 20, 10);
echo sprintf("%F\n", $divMore(100)); // 100 / 20 / 10
// outputs 0.5
It is possible to use the … function (there is an alias called
placeholder) to skip some arguments when partially applying., (*11)
This allows you to pre-define arguments on the right, and have the left ones bound at call time., (*12)
This example skips the first argument and sets the second and third arguments
to 0 and 1 respectively. The result is a function that returns the first
character of a string., (*13)
Note: Usually your IDE should help but accessing the "…"-character (HORIZONTAL ELLIPSIS, U+2026) differs on various platforms., (*14)
ALT + 0133
ALT + ; or ALT + .
AltGr + .
use function React\Partial\bind;
use function React\Partial\…;
$firstChar = bind('substr', …(), 0, 1);
$mapped = array_map($firstChar, array('foo', 'bar', 'baz'));
var_dump($mapped);
// outputs ['f', 'b', 'b']
To run the test suite, you need PHPUnit., (*15)
$ phpunit
MIT, see LICENSE., (*16)
Partial function application.
MIT
functional-programming partial
Partial function application.
MIT
functional-programming partial