Introducing Erlang

Chapter 9. Exceptions, Errors, and Debugging

“Let it crash” is a brilliant insight, but one whose application you probably want to control. While it’s possible to write code that constantly breaks and recovers, it can be easier to write and maintain code that explicitly handles failure where it happens. However you choose to deal with errors, you’ll definitely want to be able to track them down in your application.

Flavors of Errors

As you’ve already seen, some kinds of errors will keep Erlang from compiling your code, and the compiler will also give you warnings about potential issues, like variables that are declared but never used. Two other kinds of errors are common: runtime errors, which turn up when code is operating and can actually halt a function or process, and logic errors, which may not kill your program but can cause deeper headaches.

Logic errors are often the trickiest to diagnose, requiring careful thought and perhaps some time with the debugger, log files, or a test suite. Simple mathematical errors can take a lot of work to untangle. Sometimes issues are related to timing, when the sequence of operations isn’t what you expect. In severe cases, race conditions can create deadlocks and halting, but more mild cases can produce bad results and confusion.

Runtime errors can also be annoying, but they are much more manageable. In some ways you can see handling runtime errors as part of the logic of your program, though you don’t want to get carried away. In Erlang, unlike ...