Re: no return value vs. nil return value(s)

[Lorenzo Donati <lorenzodonatibz@...>]

On 11/09/2011 19.33, Florian Weimer wrote:
> * Lorenzo Donati:
>
> > Maybe I'm missing something, but IMHO the manual is not clear about it.
>
> I just can repeat myself: based on my reading, the no-results case is
> covered by the manual, in the excerpts as I quoted.  It's not a formal
> document, but the formulation "no adjustment is made" pretty clearly
> indicates that the no-arguments case is intentionally observable.
> (It's also visible to the C API, so it's not just select("#"))t.
Sorry, but I don't agree. I know this may be nitpicking, but the 
sentence you cite refers to an *list of expressions* (emphasis mine):

"If an expression is used as the last (or the only) element of a *list 
of expressions*, then no adjustment is made (unless the call is enclosed 
in parentheses)."

In the Lua grammar a list of expressions is defined like this:

explist ::= {exp `,`} exp

so it cannot be empty and the return statement is defined correctly as:

return [explist]

that is, explist is optional; thus the statement:

return nil

has an explist, whereas:

return

has *no explist*, therefore the sentence you cite is not applicable to 
this latter case (there is no expression list to adjust or not to adjust 
whatsoever).

The meaning of a "bare" return statement, doesn't appear to be *clearly* 
defined (or I couldn't find/understand it). Besides the effect 
detectable by select (which is not part of the language), I cannot find 
anything that guarantees that

return

and

return nil

have different semantics. For example, Lua team could well change the 
implementation between bugfix releases (say 5.1.4 and 5.1.5) and make 
"return" behave as "return nil" and this won't go against anything 
written in the manual. "select" will return 1 instead of 0, but this 
again won't contradict the manual as it is".


Cheers.
-- Lorenzo