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Lambda expressions for Twig and filters that make use of them

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# Twig Lambda
> Lambda expressions for Twig and filters that make use of them

----------------------------------------------------------------

<a name="examples"></a>
## Quick examples

Listing names of all authors ordered by age:
```twig
{% for author in articles|map(=> _.author)|unique_by('===')|sort_by(=> _.age) %}
    * {{ author.name }}, {{ author.age }}
{% endfor %}
```

Counting elements starting from specified letter:
```twig
{% for key, count in ['foo', 'bar', 'foobar']|countBy(=> _|first|capitalize) %}
    * {{ count }} elements start from {{ key }}.
{% endfor %}
```

----------------------------------------------------------------

<a name="install"></a>
## Installation

**Install via Composer:**
```bash
composer require dpolac/twig-lambda
```

**Add the extension to Twig:**
```php
$twig->addExtension(new \DPolac\TwigLambda\LambdaExtension());
```

**... or if you use Symfony**, add the following to your `services.yml` config file:

```yaml
services:
    # ...
    dpolac.twig_lambda.extension:
        class: DPolac\TwigLambda\LambdaExtension
        tags: [ { name: twig.extension } ]
```

----------------------------------------------------------------

## Usage

<a name="lambda"></a>
### Lambda expression
To create lambda expression prepend any valid Twig expression
with `=>` operator. Inside of the lambda expression you can use
any variable from the outside. There are also two special
variables available:
  * `_` (single underscore) - first argument,
  * `__` (double underscore) - array of arguments counted
    from zero.

```
=> _.name
=> _ * 2
=> _|first
=> 'foobar'
=> _ is even
=> __[0] + __[1]
```

To create lambda expression with list of arguments, add it
before `=>` operator. Separate multiple arguments with
semicolons. You can use brackets for readability.

```
x => x + 1
(book) => book.author
arg1; arg2 => arg1 ~ arg2
(a; b; c) => a + b - c
```

Note that if you use list of arguments, `_` variable is not
longer available.

----------------------------------------------------------------

Below is a list of available filters and tests. All works
with arrays and any Traversable object and preserve it keys.
All lambdas are called with two arguments: element and key.

----------------------------------------------------------------

<a name="map"></a>
### |map
**Alias:** `|select`<br>
**Signature:** `array|map(lambda)`

Applies a given function to each element and returns
array of results in the same order.

```twig
{% for i in  [1, 2, 3, 4]|map(=> _ * 2) %}
    {{ i }} {# prints '2 4 6 8' #}
{% endfor %}
```

----------------------------------------------------------------

<a name="filter"></a>
### |filter
**Alias:** `|where`<br>
**Signature:** `array|filter(lambda)`

Returns array of elements that passes a test specified by lambda.

```twig
{% for i in [1, 2, 3, 4, 5, 6]|filter(=> _ is even) %}
    {{ i }} {# prints '2 4 6' #}
{% endfor %}
```

----------------------------------------------------------------

<a name="unique_by"></a>
### |unique_by
**Signature:** `array|unique_by(lambda|'==='|'==')`

Returns array of unique elements. Uniqueness is checked with
passed lambda. PHP operators == or === will be used if
string '==' or '===' is passed instead of lambda.

Lambda should have two arguments - items to check. Keys of
elements are also passed as third and fourth argument.

```twig
    {% for i in [1, 2, 2, 3, 1]|unique_by((i1;i2) => i1 == i2) %}
        {{ i }} {# prints '1 2 3' #}
    {% endfor %}
```
equivalent
```twig
    {% for i in [1, 2, 2, 3, 1]|unique_by('==') %}
        {{ i }} {# prints '1 2 3' #}
    {% endfor %}
```

----------------------------------------------------------------

<a name="group_by"></a>
### |group_by
**Signature:** `array|group_by(lambda)`

Sorts an array into groups by the result of lambda.

```twig
{% for key, group in ['foo', 'bar', 'foobar', 'barbar']|group_by(=> _|first|capitalize) %}
    = {{ key }}
    {% for i in group %}
        * {{ i }}
    {% endfor %}
{% endfor %}
```
will produce
```
    = F
        * foo
        * foobar
    = B
        * bar
        * barbar
```

----------------------------------------------------------------

<a name="sort_by"></a>
### |sort_by
**Signature:** `array|sort_by(lambda[, direction = 'ASC'])`

Sorts array by values returned by lambda.
Direction can be 'ASC' or 'DESC'.

```twig
{% for i in ['bar', 'fo', 'foobar', 'foob']|sort_by(=> _|length, 'DESC') %}
    {{ i }} {# prints 'foobar foob bar fo' #}
{% endfor %}
```

----------------------------------------------------------------

<a name="count_by"></a>
### |count_by
**Signature:** `array|count_by(lambda)`

Sorts an array into groups and returns a count for the number of
objects in each group.

If lambda returns true, false or null, it will be converted to
string 'true', 'false' or 'null'. Float will be converted to
integer.

```twig
{% for key, count in ['foo', 'bar', 'foobar']|count_by(=> _|first|capitalize) %}
    * {{ count }} elements start from {{ key }}.
{% endfor %}
```
will produce
```
    * 2 elements start from F
    * 1 elements start from B
```

----------------------------------------------------------------

<a name="any"></a>
### is any
**Signature:** `array is any(lambda)`

Returns true if lambda returns true for any element from
an array.

**Returns false if array is empty.**

```twig
{{ [1, 2, 3] is any(=> _ is even) ? "There is even element in the array." }}
{# prints 'There is even element in the array.' #}
```

----------------------------------------------------------------

<a name="every"></a>
### is every
**Signature:** `array is every(lambda)`

Returns true if lambda returns true for every element from
an array.

**Returns true if array is empty.**

```twig
{{ [1, 2, 3] is every(=> _ > 0) ? "All elements in the array are positive." }}
{# prints 'All elements in the array are positive.' #}
```

----------------------------------------------------------------

<a name="call"></a>
### call()
**Signature:** `call(lambda [, arguments:array])`

Calls lambda and returns its result. You can provide array
of arguments.

This function is provided to allow creating twig macros taking
lambda as an argument.

```twig
{{ call(=> _ * 2, [10]) }}
{# prints '20' #}
{{ call(=> _.foo, [{foo: 12}]) }}
{# prints '12' #}
```

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