do { ... } while (0);

I’m currently buried in over 20” of snow, so I might as well post something.

I wrote a piece of PHP code a while back to grab some data from a cache; if it couldn’t find what it needed, it should try a couple of other scenarios before giving up. The code looked something like the following:

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$category = 'someCategory';
$cacheHandler->setCategory($category);
$data = $cacheHandler->getData();
if (!count($data)) {
  $cacheHandler->setCategory($category + '-someAlternative');
  $data = $cacheHandler->getData();
}
if (!count($data)) {
  $cacheHandler->setCategory($category + '-someOtherAlternative');
  $data = $cacheHandler->getData();
}
if (!count($data)) {
  $data = null;
}

Yuck. An obvious solution would have been to refactor this logic into a method and return the data if it’s found, avoiding the extra checks. If this were JavaScript, a function expression would have done the job:

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var result = (() => {
  let category = 'someCategory';
  cacheHandler.setCategory(category);
  if (data = cacheHandler.getData()) return data;
  category = 'someCategory2';
  cacheHandler.setCategory(category);
  if (data = cacheHandler.getData()) return data;
  // etc...
})();

But this was PHP. I’m guessing at the time I didn’t want to bother making a new method for a one-off operation, so I left it as is. Maybe. There are many times I look at my old code and wonder what the hell I was thinking.

Months later, a veteran on my team refactored the file. When I checked the code again, I saw this block in its place:

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do {
  $category = 'someCategory';
  $cacheHandler->setCategory($category);
  if (!$data = $cacheHandler->getData()) break;
  $cacheHandler->setCategory($category + '-someAlternative');
  if (!$data = $cacheHandler->getData()) break;
  $cacheHandler->setCategory($category + '-someOtherAlternative');
  if (!$data = $cacheHandler->getData()) break;
} while (0);

The do-while loop will only execute once and creates a separate block that can be broken out of at any time. It was a pretty neat trick for avoiding another method that I hadn’t seen before.