The Importance of Limiting Functions to One Thing

One of the reasons I love dynamically-typed languages is that they tend to allow a programmer to use old unmaintained code in ways the original author didn’t anticipate. This is especially important if you’re trying to interface with a library that the user will likely already have installed previously, so you’d rather not vendorize it (include it in your source directly).

Using a dynamically-typed language doesn’t automagically preclude you from this problem, however; there are still a number anti-patterns that will lead to curses called down upon your name some unspecified time in the future.

I recently came across some code in the following form:

<div id="foo"><?php echo thingies_table('foo') ?></div>
<div id="bar"><?php echo thingies_table('bar') ?></div>
<div id="baz"><?php echo thingies_table('baz') ?></div>

[snip]

function thingies_table(parameter) {
	[get a list of thingies from the database]
	[do some stuff to the list]
	
	return build_table(thingies);
}

Now, this isn’t nearly as bad as the more common version:

<div id="foo"><?php thingies_table('foo') ?></div>
<div id="bar"><?php thingies_table('bar') ?></div>
<div id="baz"><?php thingies_table('baz') ?></div>

[snip]

function thingies_table(parameter) {
	[get a list of thingies from the database]
	[do some stuff to the list]
	
	print build_table(thingies);
}

but it’s still not good.

You see, I wanted to combine the results of several thingies_tables into one, something like

<div id="foo-bar-baz">
	<?php
	$foo = thingies_table('foo');
	$bar = thingies_table('bar');
	$baz = thingies_table('baz');
	print build_table(array_merge($foo, $bar, $baz));
	?>
</div>

This, however, isn’t possible with build_table inside thingies_table, which is why it should be pulled out, either into another function or the <div> code itself.

Be considerate to your successor and limit each function to only doing one thing.