Raising an Exception

Programming Elixir ≥ 1.6 — by Dave Thomas (219 / 230)

👈 Appendix 1 Exceptions: raise and try, catch and throw | TOC | catch, exit, and throw 👉

You can raise an exception using the raise function. At its simplest, you pass it a string and it generates an exception of type RuntimeError.

You can also pass the type of the exception, along with other optional fields. All exceptions implement at least the message field.

You can intercept exceptions using the try function. It takes a block of code to execute, and optional rescue, catch, and after clauses.

The rescue and catch clauses look a bit like the body of a case function — they take patterns and code to execute if the pattern matches. The subject of the pattern is the exception that was raised.

Here’s an example of exception handling in action. We define a module that has a public function, start. It calls a different helper function depending on the value of its parameter. With 0, it runs smoothly. With 1, 2, or 3, it causes the VM to raise an error, which we catch and report.

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