Monday, June 19, 2017

Go To Statement Considered Harmful (revisited)

Hopefully you've heard of the famous letter by Edsger Dykstra, "Go To Statement Considered Harmful." This was one of the most influential letters ever written in Computer Science, published in Communications of the ACM in March, 1968. This letter effectively defined structured programming for generations of coders. It has been taken as gospel and taught in so many programming classes that most developers take it for granted. The goto statement is just too low-level for modern programming.

This has been taken so far that some languages simply didn't include goto in the language. Java is a prime example. Though in that case, they were hedging their bets, as they kept a reserved word for it, and they did include it in the bytecode.

The letter itself is rather dry and academic. You can find it online, and it's worth the read.  (I found it at under the original title; it was the ACM editor that selected the now-famous "harmful" title.)  It has good points. In general, the use of goto results in spaghetti code that is hard to debug and harder to maintain. Clean code breaks things into functions, loops, and other well-defined control structures, with little obvious need for direct jumps.

I first heard of this when my world of programming consisted of Atari BASIC. The idea of not using goto was rather shocking in that context, but then most BASIC implementations of the day lacked functions, while loops, and a number of other higher-level structures that even shell scripts have today. When I moved on to Pascal and C, there seemed to be little use for the old goto, and I readily accepted the idea that it was a bad idea.

But the idea of avoiding direct jumps has been treated as a religious dogma instead of as a general guideline, and this has been harmful itself. There are many reasons why using an explicit goto is a good idea. In fact, no matter how nicely structured your code is, it's impossible to avoid that pesky goto. How do you think all those nice loops are implemented? At the assembly level, it's all comparisons and jumps.

Now you can say that that's just a technical nitpick, and you're mostly correct. Dykstra even referenced it in his original letter. Just because the good structured statements your language provides are implemented with jumps doesn't mean you should use them directly. However, what it does do is give a good consistent reason for when you should use gotos in your code.

Use a goto only when you need to implement something that your language doesn't provide for.

Following this, I've written a series of examples where a goto statement provides some useful and interesting features for C programmers.

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