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