Question

Say I have some function fn1() in Erlang which returns {ok, Result} if the function was executed successfully and {error, "ErrorReason"} if there was an error.

Now in another function fn2() I call fn1() and I need to check the result of fn1 and proceed only if it is {ok, Result}.

I figured, I can do this using either case or try catch. But Efficiency is my main concern and I’d like to know which of the two methods below is more efficient:

try-catch Method

fn2() ->
   try
      {ok, Result} = fn1(),
      %Do something with Result
      ok
   catch
      throw:Term -> Term;
      exit:Reason -> {exit, Reason};
      error:Reason -> {error,{Reason,erlang:get_stacktrace()}}
   end.

case Method

fn2() ->
   Res = fn1(),
   case Res of
      {ok, Result} -> 
         %Do something with Result
         ok;
      {error, Reason} ->
         Reason
   end.

Answer 1

The case method will be more efficient, as it simply pattern matches, and does not involve building a call stack and stuff.

In both examples you are about to handle the “error” locally, so there is no point in the try catch.What you might see sometimes is something like:

fn2() ->
  {ok, Result} = fn1(),
  %Do stuff with Result
  ok.

Here the intention is that you make fn2() throw a badmatch, if fn1() did not return ok. You let someone else “above” handle the problem. E.g. this might kill your process, and make your supervisor create a new one.