Exceptions: try and catch

throw, error, exit

Three kinds of exceptions exist. throw carries any value, error carries an error term, and exit carries an exit reason between processes.

The runtime also raises errors itself: a bad match, a bad argument, or a missing key all arrive as errors.

1> throw(my_error).

** exception throw: my_error

2> error(bad_arg).

** exception error: bad_arg

3> exit(reason).

** exception exit: reason

try ... catch

The try expression catches exceptions by class. Each catch line names the class, binds the reason, and runs its body when a matching exception arrives.

Catch only the classes you can handle, and let the rest escape to the supervisor.

try

dangerous()

catch

error:Why -> {error, Why};

throw:What -> {thrown, What}

end.

Why catch

Expected failures travel as {error, Reason} tuples, not exceptions. Reserve try for the rare places where a crash is genuinely part of the flow, such as parsing untrusted input.

A caught exception inside a hot loop is slower than a plain match, so the ok tuple stays the default.

safe_divide(A, B) ->

try A / B

catch error:badarith -> {error, not_a_number}

end.

The after clause

The after clause runs whether the try body succeeds, fails, or throws. Use it to release a file, a socket, or an ETS table handle.

The after body cannot change the result of the try, only clean up around it.

try

work(Handle)

after

close(Handle)

end.

Next: Modules, exports and imports

Article author: Arthur Isaev