Semantic Coding?

Marc Weistroff 2010-06-08

Don’t worry. Semantic coding is not a new concept. Actually, you’re
already doing it each time you code. At least, I hope for you and your
teammates. The principle of semantic coding is simple and is a great
part of what we all name “code readability”.

Look at those 2 pieces of code:

<?php
class A
{
    public function A($a, $b)
    {
        return $a + $b;
    }

    public function B($a, $b)
    {
        return $a - $b;
    }

    public function C($a, $b)
    {
        return $a / $b;
    }

    public function D($a, $b)
    {
        return $a * $b;
    }
}

This is not semantic code.

<?php
class Calculator
{
    public function Add($a, $b)
    {
        return $a + $b;
    }

    public function Substract($a, $b)
    {
        return $a - $b;
    }

    public function Divide($a, $b)
    {
        return $a / $b;
    }

    public function Multiply($a, $b)
    {
        return $a * $b;
    }
}

This is semantic code.

The difference between the two is that the second example class and
method names carry a meaning. When reading the code, you don’t have to
remember what the ‘Add’ method does because it’s clearly indicated
within its name.

In the first example, you have to remember which method name does what.
In the second example, what the method does is its name. It’s a major
difference.

Speaking in cognitive terms. Writing code should be a high cognitive
effort in order for the reading process to be a low cognitive effort.

Don’t forget that code is often read 100x time more that it’s written!

Now let’s apply semantic coding principle to exceptions!

Too much sfException is bad for the health

The use of sfException is a commong thing among all symfony projects’
code I have the chance to read. While it’s harmless to use it from time
to time, I think that using nearly only sfException in your code is like
using $a, $b or $c as unique variable name.

What is sfException?

As the sfException phpDoc says: > sfException is the base class for all
symfony related exceptions and provides an additional method for
printing up a detailed view of an exception.

If you look at the code of
sfException you’ll see that sfException has a bunch of tool to wrap an exception
inside an sfException. That’s why when an exception of any kind is
thrown in symfony, you see a nice html output.

In a symfony application, every thrown exception that bubbles up is
eventually catched in the sfFrontWebController
class. Then this exception is wrapped into an sfException one and is
displayed to the user.

The catching mechanism

 <?php
 try
 {
     // application running…
     […]
 }
 catch (sfException $e)
 {
     $e->printStackTrace();
 }
 catch (Exception $e)
 {
     sfException::createFromException($e)->printStackTrace();
 }


The wrapping mechanism

<?php
static public function createFromException(Exception $e)
{
    $exception = new sfException(sprintf('Wrapped %s: %s', get_class($e), $e->getMessage()));
    $exception->setWrappedException($e);
    self::$lastException = $e;

    return $exception;
}

As you can see, sfException was created mainly to display a nice debug
trace and not to replace ALL exceptions! Don’t worry to throw other
exception php provides, there’s always be a nice debug trace! 😉

PHP provides a lot of different exceptions

PHP provides 2 predefined exceptions known as Exception and
ErrorException.

Exception is the base class from where all other exceptions inherit
from. ErrorException can be used when you want that PHP throws exception
instead of reporting errors. For more information about ErrorException,
you can read the dedicated
documentation
.

The Standard PHP Library (SPL) provides 13 more
exceptions
. These exceptions
are:
BadFunctionCallException,
BadMethodCallException,
DomainException,
InvalidArgumentException,
LengthException,
LogicException,
OutOfBoundsException,
OutOfRangeException,
OverflowException,
RangeException,
RuntimeException,
UnexpectedValueException.

Each of these exception have a name that provide information on what the
problem is which is pretty useful.

Now let’s use those exceptions and see how they improve the global
readability (and scanability) of your code.

Semantic exceptions

Let’s read those two pieces of code.

This one with the uncool sfException everywhere:

<?php
class Container
{
    protected
        $maxItemCount = 2,
        $container = array();

    public function addItem($item)
    {
        if (count($this->container) < $this->maxItemCount)
        {
            $this->container[] = $item;
        }
        else
        {
            throw new sfException('Cryptic long message saying container is full');
        }
    }

    public function sliceItem()
    {
        if (empty($this->container))
        {
            throw new sfException('Cryptic long message saying container is empty');
        }

        array_slice($this->container);
    }
}

And the cool one with semantic exceptions:

<?php 
class Container
{
    protected
        $maxItemCount = 2,
        $container = array();

    public function addItem($item)
    {
        if (count($container) < $this->maxItemCount)
        {
            $container[] = $item;
        }
        else
        {
            throw new OverflowException('Cryptic long message saying container is full');
        }
    }

    public function sliceItem()
    {
        if (empty($container))
        {
            throw new UnderflowException('Cryptic long message saying container is empty');
        }

        array_slice($container);
    }
}

In the second example, you don’t have to read the message to know what
it’s all about. The name is sufficient. Furthermore, you’ll get more
informations while scanning the code with correct exceptions name than
with sfException everywhere.

More significative code = More readable code = More scannable code =
Happy developer = Rainbows in kittens’ eyes.

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