Using Extensions: Implicit Extensions 

$rules = ['name' => 'unique'];

$input = ['name' => null];

Validator::make($input, $rules)->passes(); // tru
Validator::extendImplicit('foo', function ($attribute, $value, $parameters, $validator) {
    return $value == 'foo';
});

Only implies that the attribute is required,whether it actually invalidates a missing or empty attribute is up to you.

Using Extensions: Implicit Extensions — Structure map

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Using Extensions: Implicit Extensions — Related pages:

Validation