Field assignments are reordered in record creations

Romain Lenglet <>
Thu May 25 04:39:55 CEST 2006


David, thanks a lot fot this great explanation.

I expected the Erlang compiler to behave like a C compiler, and 
to execute subexpressions with possible side-effects in the same 
order as they are written...

>  wrote:
> > So what is ,?  Can I not depend on an expression prior to a
> > , being evaluated before something after the ,? What about ;
> > in guard statements? Is it the same idea, simply not called
> > a sequence point? Or is the idea not existent?

Yes, it is. In:
start() ->
    format("foo"),
    format("bar").
the two function calls are executed in the order in which they 
are written, because "," in that case separates two expressions 
and evaluates them in order.

Cf. section 6.5 in the ref cited by David Hopwood 
(http://citeseer.ist.psu.edu/513634.html):

"The order in which the subexpressions of an expression are 
evaluated is not defined, with one exception:
In a body, the expressions are evaluated strictly from left to  
right."

So, using "," to separate expressions seems to be the only way to 
have deterministic behaviour in Erlang.

-- 
Romain LENGLET



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