Handling Errors

Coast attempts to make clojure exceptions a little nicer by offering two functions: raise and rescue

In this guide, we learn how clojure exceptions are raised, how to write logic around them and finally creating your own custom exceptions.


Exceptions are great since they halt the program at a certain stage and make sure everything is correct before proceeding.

Exceptions, especially in clojure, are usually just treated as insane, indecipherable walls of text that tell devs that something went wrong, go dive in and find it.

By default, Coast handles all exceptions for you and displays them in a nice format during development. However, you are free to handle exceptions however you want.

Handling Errors

Errors can be handled by catching all of them or specifying a name

Gotta Catch 'Em All

Here's how raise works with one argument

(raise {:message "This is an error with a message key"})

That raises a clojure.lang.ExceptionInfo exception with ex-data: {:message "This is an error with a message key"}

You can rescue from this instead of using try and catch like this:

(let [[_ error] (rescue
                  (raise {:message "This is an error"}))])

The error variable in the above example now contains {:message "This is an error"}

So rescue is a macro, which wraps the body in try and catch and catches any ExceptionInfo that comes from raise

Named Errors

You can rescue individual errors as well

  (raise {:message "Error!" :custom true})
  (raise {:message "Error!"})

In the above example, the first error will be caught, the second one will not.

Custom Errors

raise can also change the "Error has occurred" message as well like this:

(raise "This is a custom error title" {})