[erlang-questions] Re: Concept of Side-effect

Fred Hebert (MononcQc) mononcqc@REDACTED
Fri Sep 18 15:44:16 CEST 2009

On Fri, Sep 18, 2009 at 9:03 AM, Ulf Wiger

> Jayson Vantuyl wrote:
>> Actually, I'm not entirely sure that an exception would be a side effect.
>>  It doesn't get "returned" like a normal value, but I don't see that it
>> changes state any more than a return value does.
> It straddles the fence in some ways. Functions can be pure for all
> legal input and raise exceptions for illegal input. This is fine,
> but there are also functions that raise exceptions even for legal
> input.
> Consider the following code:
> pmap(F, L) ->
>  Pids = [spawn_monitor(fun() ->
>                          exit({ok, F(X)})
>                        end) || X <- L],
>  [receive {'DOWN',Ref,_,_,R} -> unwrap(R) end
>    || {_,Ref} <- Pids].
> (A simplified version of slide 7 in this talk:
> http://www.erlang-factory.com/upload/presentations/56/UlfWiger_ErlangMulticoreEF.pdf
> )
> While it might seem contrived, the shell evaluator
> does roughly the same thing: evaluates the expression
> inside an exit() call in order to deliver the result and
> die in one and the same action.
> Such functions are only meaningful when considering their
> side-effects - the exceptions.
> BR,
> Ulf W
> --
> Ulf Wiger
> CTO, Erlang Training & Consulting Ltd
> http://www.erlang-consulting.com
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Wouldn't the ability to just drop in and either catch errors and exit signal
or trap them in a monitor mean exceptions and errors are not necessarily
side effects, no matter the purpose of the function?

Tthe caller's side doing:
case catch Mod:Fun(Args) of
     {ok, X} -> X;
     {'EXIT',Reason} -> ...

Pretty much ends up transforming an error into a returnable value. There is
no visible difference between the function called having an error or just
returning {'EXIT',Reason} as a normal value, no matter the intent of the

(I guess what I'm saying is a bit similar to what you are saying, in the

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