Difference between foreach and forelse in Laravel

问题:

What's the basic difference between Laravel's foreach and forelse? How does forelse work?

I know that foreach duplicates your given array and works on it while in forelse it checks whether it exists and loops on it if it exists but if not it uses your provided else condition.

回答1:

I believe the answer to your question is that, essentially, ForElse is a ForEach loop, but with extra handling for empty input(s).

From the Laravel 5 docs on Blade Templates, an example illustrating both loops with the same list of Users as Input:

@foreach ($users as $user)
    <p>This is user {{ $user->id }}</p>
@endforeach

@forelse ($users as $user)
    <li>{{ $user->name }}</li>
@empty
    <p>No users</p>
@endforelse

---

Now if we compare this to the source code for the Laravel Framework to see how the loops are compiled (all Blades constructs are compiled into HTML when being rendered by PHP on the web page):

   /**
     * Compile the for-each statements into valid PHP.
     *
     * @param  string  $expression
     * @return string
     */
    protected function compileForeach($expression)
    {
        preg_match('/\( *(.*) +as *(.*)\)$/is', $expression, $matches);

        $iteratee = trim($matches[1]);

        $iteration = trim($matches[2]);

        $initLoop = "\$__currentLoopData = {$iteratee}; \$__env->addLoop(\$__currentLoopData);";

        $iterateLoop = '$__env->incrementLoopIndices(); $loop = $__env->getLastLoop();';

        return "<?php {$initLoop} foreach(\$__currentLoopData as {$iteration}): {$iterateLoop} ?>";
    }


   /**
     * Compile the for-else statements into valid PHP.
     *
     * @param  string  $expression
     * @return string
     */
    protected function compileForelse($expression)
    {
        $empty = '$__empty_'.++$this->forElseCounter;

        preg_match('/\( *(.*) +as *(.*)\)$/is', $expression, $matches);

        $iteratee = trim($matches[1]);

        $iteration = trim($matches[2]);

        $initLoop = "\$__currentLoopData = {$iteratee}; \$__env->addLoop(\$__currentLoopData);";

        $iterateLoop = '$__env->incrementLoopIndices(); $loop = $__env->getLastLoop();';

        return "<?php {$empty} = true; {$initLoop} foreach(\$__currentLoopData as {$iteration}): {$iterateLoop} {$empty} = false; ?>";
    }

---

Essentially, the ForElse loop has a built-in code and compile check for an empty input, in which case it optionally outputs an alternate display template for the empty input.

I would imagine you can use a ForEach loop in every case that you might use a ForElse loop, adding that you include and empty checks that ForElse might catch for you.

---

Links for reference:

1. Blade Templates
2. Blade View Compile Functions

回答2:

The foreach loop as you described loops over each element in an array, the foresee loop in essence is exactly the same with one extra element as you already noticed, the else statement. The primary use for this to use when you have an empty array.

Thus looping over all elements and for example rendering a row of a table but putting an empty notice when you do not have any data.