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catch exprs catch expression evaluate exprs and return its value on no exception. you can use try expression or catch expression try expression is strightforward, if you ignore the awkward erlang grammer that is. it's too verbose and ugly but i think alternatives are worse. for exceptions of class throw, that is the code called throw term , term is returned. the simplest code that assume no error will be like this. but i want to deal the the error. well, it's not bad. walkthrough of hello world http server with erlang, cowboy, and rebar3 instead of returning the hello world, we're going to return the sum of two query parameter a, b. my guess is to keep the backward compatibility. use try expression's of section to pattern match the error value. another type of error is although the query paramters are present, it has a string that cannot be parsed as an integer. let's quickly borrow a code from the hello world http server i explained in the previous article. well, at least, i understand the error handling of erlang.{/INSERTKEYS}{/PARAGRAPH} if you don't care about exception, you need to do nothing. after consdering the various code, i come up with this code. we can do condional branching with either case expression or function pattern match. {PARAGRAPH}{INSERTKEYS}so hopefully, the higher framework catch these exceptions and restart the process. here is the verbose code. whichever you use, it became quite verbose and boilar-plate. suppose, the users forget the query parameters. but if you care, that is, you want to run some code on the condition of exception, things get verbose. the erlang has three class of exception: error, exit, and throw. the catch expression. we have to use the patter match for the conditional execution. but i feel some oddities on erlang's exception and that is the concept of class. it can be done by case expression or function. but what if the caller want to deal with the error? try expression can have of section and it will be evaluated only on no exception in exprs. erlang has exception like many other langauges. putting it all togather, it's very verbose to handle errors in erlang.