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