[5.7] Add whenEmpty + variants to Collection

[Propaganistas]

I often find myself in need of executing actions with a collection if it is / is not empty. At the moment this is possible in several ways. For example:

• Assign the collection to a variable, then reuse the variable in when()

  $collection = collect()->filter(...);
  
  $collection->when($collection->isEmpty(), function($c) {
      return $c->push('item');
  });

• Pipe the collection and use if or when() in there:

  collect()->filter(...)->pipe(function($collection) {
      return $collection->when($collection->isEmpty(), function($c) {
          return $c->push('item');
      });
  });

The first example seems to defeat the purpose of a collection (creating a chain of calls on a collection instance), the second example just seems to be one hell of nested closures to me.

I always add the whenEmpty() method (and variants) to my applications as macros. This allows me to fluently do the following and keep chaining:

collect()->filter(...)->whenEmpty(function($c) {
    return $c->push('item');
});

I thought it could prove useful to suggest these shortcuts for implementation. If not I'lll happily continue defining these as macros.

[hichxm]

I too often find myself doing macros to execute a callback under any condition. I think it's a good start to add a series of features of this type to the collection.

[staudenmeir]

Isn't unlessNotEmpty() just the same as whenEmpty()? And unlessEmpty() the same as whenNotEmpty()?

[Propaganistas]

Isn't unlessNotEmpty() just the same as whenEmpty()? And unlessEmpty() the same as whenNotEmpty()?

It surely is. It seemed "incomplete" to me only suggesting the whenX() variants while the Collection class offers both when() and unless().