[pro] (values) for for-effect functions

[Peter Seibel]

My taste tells me that's an over-clever idiom and should not be used.
If it's not clear that a function is for-effect without (values)
you've already lost.

-Peter

On Thu, Dec 2, 2010 at 3:00 PM, Daniel Weinreb <dlw at itasoftware.com> wrote:
> In case these terms are too old for anyone to know
> them, we used to use the phrase "for effect" to
> mean a function that was called for the sake
> of its side-effects, versus "for value" when it was
> called for the sake of its returned value. (Actually
> I'm not sure I remember what we called a function
> with both properties.)
>
> In some code I have seen, the author of the code
> has written (values) at the end of an implied-progn
> body.  The purpose, pretty clearly, is to signal
> the reader of the code that the function was
> purely for-effect, i.e. that the returned value
> does not mean anything.
>
> This is mostly stylistic.  In most cases, returning
> "nil" means the same thing as returning no values,
> i.e. (values).  The only situation where it makes
> any difference, as far as I know, is when
> multiple-value-list is being called, and that's
> used very, very rarely.
>
> The question is, is it good style?  That is, ought
> we consider it something to be recommended?
>
> The pro is that it presents clear information to
> the reader of the code.  Merely not putting
> anything at the end of the implied progn means
> that the value of the last function called is
> the returned value, and the reader of the
> code might think that was on purpose.
>
> On the other hand, having a (values) at the end
> does not actually prove that the function
> does not have any meaningful value, since a
> "return" within the function could return
> a value (or many values).
>
>  From a purely emotional/historical point of
> view, I am not used to seeing these (values)
> forms, and they seem somewhat ugly and
> verbose.  But that's just based on my own
> experience and should not carry much weight.
>
> I'd like to write in our programming standards
> either that they should be used, or that they
> should not be used.  I think it's suboptimal
> for them to be used in some places and
> not others, since the inconsistency could lead
> a reader of the code to draw wrong conclusions.
>
> What do people think?
>
> -- Dan
>
>
> _______________________________________________
> pro mailing list
> pro at common-lisp.net
> http://common-lisp.net/cgi-bin/mailman/listinfo/pro
>



-- 
Peter Seibel
http://www.codequarterly.com/