# λ Functional

Siler is bundled with the `Siler\Functional` namespace. It brings some function declarations that aids the work with another **first-class** and **high-order** PHP functions!

## `identity()`

Returns a Closure that returns its given arguments.

```php
use Siler\Functional as λ;

array_map(λ\identity(), [1, 2, 3]);
// [1, 2, 3]
```

{% hint style="info" %}
Doesn't seem useful at first, but working with the functional paradigm, you'll find the reason shortly.
{% endhint %}

## `always($value)`

Almost like `identity()`, but it always returns the given value.

```php
use Siler\Functional as λ;

array_map(λ\always('foo'), range(1, 3));
// [foo, foo, foo]
```

## `if_else(callable $cond) -> $then -> $else`

A functional if/then/else.

```php
use Siler\Functional as λ;

$pred = λ\if_else(λ\equal('foo'))(λ\always('is foo'))(λ\always('isnt foo'));

echo $pred('foo'); // is foo
echo $pred('bar'); // isnt foo
```

## `partial(callable $callable, ...$partial)`

Partial application refers to the process of fixing a number of arguments to a function, producing another function of smaller [arity](https://en.wikipedia.org/wiki/Arity). Given a function![{\displaystyle f\colon (X\times Y\times Z)\to N}](https://wikimedia.org/api/rest_v1/media/math/render/svg/5c7acf81877307746cd88e2785967d9a2f287107), we might fix (or 'bind') the first argument, producing a function of type ![{\displaystyle {\text{partial}}(f)\colon (Y\times Z)\to N}](https://wikimedia.org/api/rest_v1/media/math/render/svg/d45fcfd39c660c562ebd3da8158dbfd8f673836e).\
<https://en.wikipedia.org/wiki/Partial_application>

Nothing like a good example:

```php
use Siler\Functional as λ;

$add = function ($a, $b) {
    return $a + $b;
};

$add2 = λ\partial($add, 2);

echo $add2(3); // 5
```

Works with any `callable`:

```php
use Siler\Functional as λ;

$explodeCommas = λ\partial('explode', ',');
print_r($explodeCommas('foo,bar,baz'));

/**
 * Array
 * (
 *  [0] => foo
 *  [1] => bar
 *  [2] => baz
 * )
 */
```

## `match(array $matches)`

A pattern-match attempt. Truthy Closure evaluations on the left calls and short-circuits evaluations on the right.

```php
use Siler\Functional as λ;

$nameOf = λ\match([
    [λ\equal(1), λ\always('one')],
    [λ\equal(2), λ\always('two')],
    [λ\equal(3), λ\always('three')],
]);

echo $nameOf(1); // one
echo $nameOf(2); // two
echo $nameOf(3); // three
```

{% hint style="info" %}
There are a lot more of them. A good place it check it out are [the tests](https://github.com/leocavalcante/siler/blob/master/tests/Unit/Functional/FunctionalTest.php).
{% endhint %}