“No-else-after-return” considered harmful

At the time of writing this, the Mozilla Coding Style guidelines have this recommendation under “General C/C++ Practices”:

But as a blanket guideline I think it’s misguided because it promotes an impure style of programming.  And by “impure” here I mean the opposite of “pure” in this sense, i.e. “free from side-effects and other such error-prone junk”.  Let’s consider a simple example:  the max() function.  Here’s a way to write it that is quite sensible, in my opinion:

(I’m not exaggerating, see this comment and this comment from Mozilla’s VP of Engineering and CTO respectively, about an example very similar to this.)

Urk.  That’s horrible.  Any time you’re using an ‘if’ without a corresponding ‘else’, you’re relying on an impure feature — you must be doing something with a side-effect in the ‘then’ branch, or some kind of impure control flow.  Mozilla’s coding guidelines insist that we write max(), a pure function, in an impure way.  Also, it’s just harder to read because the two return statements have different indentation levels despite the fact that they are closely related.

In general, statements suck, expressions rule.  Even in a language like C/C++ you can do something approaching functional programming by avoiding statements where possible.  In a tiny example like max() the difference may seem trivial, but in bigger examples the difference becomes more important.  Consider the following code from Nanojit’s register allocator which picks a register by looking at the free registers, the registers that are allowed to be used, and possibly the preferred registers.  You don’t have to understand it closely, but it helps to know that RegisterMask is just a bitfield with one bit per register.

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