[erlang-questions] Exceptions vs Tagged Values

Samuel Rivas <>
Thu Nov 9 08:41:15 CET 2006


Robert Virding wrote:
> In many cases you need to differentiate this. If file:open bombs it 
> would be very nice to know if I couldn't open the file or if my 
> arguments were wrong. That is not the same thing and handling them as 
> such will not make users happy.

  I suppose that is why try does not catch errors unless you force it to
do so.
 
> There will be written an aweful lot of wrapper functions to help you 
> remember:
> 
> file_open(Name, Opts) ->
>      try file:crashing_open(Name, Opts) of
> 	Pid -> {ok,Pid}
>      catch
> 	error:not_found -> {error,not_found};
> 	error:permission -> {error,permission};
> 
> 	<all other reasons why a file could not be opened/created>
>      end.

  I do not get that point:

file_open_tagged_version(Name, Opts) ->
	try file_open_throwing_version(Name, Opts) of
		Pid -> {ok, Pid}
	catch
		Reason -> {error, Reason}
	end.

  You need not know, nor write, all the possible Reasons. Naturally,
there is the opposite version:

file_open_throwing_version(Name, Opts) ->
	case file_open_tagged_version(Name, Opts) of
		{ok, Pid} -> Pid;
		{error, Reason} -> throw(Reason)
	end.

  I do not see why throwing (or exiting, or erroring) is worse in terms
of documentation for error cases.

> BUT THEN AGAIN - it would simplify writing many of the libraries. For 
> example in io when scanning/parsing a file there would be no need to try 
> and recover on an error to keep going - just BANG exception and let the 
> some other process handle it. Epp detects an error - BANG exception. 
> Hell we could do it for the compiler as well. Why go to all the trouble 
> to keep track of errors and warnings and return something helpful? It 
> detects an error - BANG exception and let the user handle it. Seeing we 
> have removed the grey area between right and exception warnings have to 
> go - BANG exception. The compiler would return ModuleName | 
> {ModuleName,Bin} (untagged) depending on the arguments, and the 
> exceptions {'EXIT',wrong} and {'EXIT',unused_variable}.

  Here you are talking about applications, not about functions. In both
tagged and throwing approaches error handling is still needed somewhere
in the application.

  Regards
-- 
	Samuel



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