[erlang-questions] Erlang style question - input handling

Richard A. O'Keefe <>
Thu Sep 28 06:47:17 CEST 2006


Nohl Attila Rajmund <> wrote:
	I have a couple of functions which follow this pattern:
	f(X, Y) ->
	     % collect some data
	     % do something with the collected data
	     .
	
	In C/C++/Java I'd insert a code like this
	between the collecting part and the "do something" part:
	
	if (!check1(data1)) return ERROR_CODE1;
	if (!check2(data2)) return ERROR_CODE2;
	if (!check3(data3)) return ERROR_CODE3;
	
	and then the rest of the function would be not touched. However, there
	is no return in Erlang so I can't do this.

As a matter of fact, there IS something very similar, and someone who
knows C++ and Java should be familiar with it:

    f(X, Y) ->
	try
	    ... collect some data ...
	    check1(data1),
	    check2(data2),
	    check3(data3),
	    ... do something with the collected data
	catch
	    check1 -> ERROR_CODE1;
	    check2 -> ERROR_CODE2;
	    check3 -> ERROR_CODE3
	end.

    check1(Data) ->
	... if all is well, answer true ...
	... if something is wrong, throw(check1) ...

    similar for check2, check3.

By the way, I have very serious doubts about the interface of this
function.  Why should the caller have to work so hard to figure out
whether they got a good result or a bad one?  The natural thing to do
in C++ or Java is *NOT* to return an error code but to throw an
error term, so the expected structure for f/2 would be

	f(X, Y) ->
	     ... collect some data ...
	     check1(data1),
	     check2(data2),
	     check3(data3),
	     ... do something with the collected data ...

and the way the caller finds out about a problem is by using
a try/catch.
	



More information about the erlang-questions mailing list