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