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