[perl #94350] definition of truthiness in the perldocs

On Sat Aug 06 09:29:22 2011, aristotle wrote:
> * l.mai@web.de <perlbug-followup@perl.org> [2011-07-08 07:15]:
> > So I was looking for the official definition of "truth" in the
> > perldocs.
> >
> > perldata says:
> >
> >     A scalar value is interpreted as TRUE in the Boolean sense
> >     if it is not the null string or the number 0 (or its string
> >     equivalent, "0").
> >
> > perlsyn says:
> >
> >     The number 0, the strings C<'0'> and C<''>, the empty list
> >     C<()>, and C<undef> are all false in a boolean context. All
> >     other values are true.
> >
> > I think both are wrong.
> >
> > perldata is missing undef in its list of false values
> 
> Agreed.
> 
> > and perlsyn claims the empty list is false, which doesn't even
> > make sense. There are no lists in scalar context. Booleans are
> > scalars. Therefore you can't have an "empty list" in boolean
> > context (C<()> simply evaluates to C<undef> but I don't know if
> > that's documented anywhere).
> 
> I never really understood the point of the “no lists in scalar
> context” orthodoxy. I don’t see how it helps in reasoning about
> 
>     $foo = ();
> 
> And likewise you can write `if (()) { this_never_runs }`.
> 
> I don’t see what mistakes are caused by the inaccurate mental
> model in “a list in scalar context evaluates to its last element
> or to undef if it’s empty” or what you stand to gain by going
> through the gymnastics required to re-interpret list-ish syntax
> as being special set of operators in scalar context, even if it’s
> true on a grammar level.
> 
> In any case, the fact that you can write `if (())` remains. And I
> argue that it needs to be called out as another way to express
> falsity. If you want to be fastidious, be my guest, but you are
> going to end up with contortions like “Note that expressions that
> look like empty lists yield undef in scalar context, so in the
> boolean sense they’ll also be false.”
> 
> But to omit it entirely because of the fact that it evaluates
> to undef would in my view be a disservice to anyone who does
> not read the document already primed with knowledge of that
> consequence. It’s a matter of how you approach documentation:
> are you trying to be exact and concise, or trying help the user
> achieve an aim?
> 
> > I propose the following changes:
> >
> > * perldata:
> > A scalar value is interpreted as TRUE in the Boolean sense if
> > it is not undef, the empty string, or the number 0 (or its
> > string equivalent, "0").
> 
> I don’t like the double negation in “if it is not undef”. I would
> reverse the sense just how perlsyn does: a scalar value is FALSE
> if it is undefined, 0, "0" or the empty string, and it is TRUE if
> it is anything else.
> 
> > * perlsyn:
> > The number 0, the strings C<'0'> and C<''>, and C<undef> are
> > all false in a boolean context. All other values are true.
> 
> As per above, I’d want some note about empty lists here.
> 
> Regards,

I’ve just made a change (32860eee) that mentions undefined in perldata
and removes the double negative.

I don’t think perldata needs to mention the empty list, because the
empty-list-as-undef is a side effect of *syntax*, and does not apply to
data.  (Hence perlsyn should keep it.)

• [perl #94350] definition of truthiness in the perldocs by Father Chrysostomos via RT
• [perl #94350] definition of truthiness in the perldocs by Father Chrysostomos via RT