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Postings from August 2012
On Wed, Aug 22, 2012 at 02:25:38PM +0200, demerphq wrote:
> On 22 August 2012 13:44, Damian Conway <damian@conway.org> wrote:
> > I've found the discussion in this thread (and the newline thread it
> > has spawned) extremely useful. Thank-you to everyone who has
> > contributed.
> >
> > I don't want to curtail that discussion, but I felt it might be useful
> > to summarize some of those ideas...or rather, the way those ideas have
> > adjusted my own thinking.
> >
> > So, in light of those discussions, here's my current preferred option...
> > ______________________________________________________________________
> >
> > Smartmatch (~~) table
> > =====================
> >
> > $a $b Meaning
> > ===== =================== ========================
> > Any undef ! defined $a
> > Any ~~-overloaded [1] $b->ol_method($a, 'rev')
> > ~~-ol Any [1] $a->ol_method($b)
> > Any CodeRef, &{}-ol $b->($a)
> > Any Regexp, qr-ol $a =~ $b
> > Any unambiguous Num [2] $a == $b
> > Any unambiguous Str [3] $a eq $b
> > Any Any die
> >
> >
> > [1] Includes junctions, whose overloading distributes the
> > smartmatch over their elements in their several ways.
> >
> > [2] If $b has an internal numeric representation but no
> > internal string representation (or if some putative
> > internal flag assures us it was originally a number).
> >
> > [3] If $b has an internal string representation but no
> > internal numeric representation (or if some theoretical
> > inner flag promises us it was initially a string).
> >
> >
> > The when construct
> > ==================
> >
> > Form Meaning
> > ================== ==================================
> > when (EXPR) {...} if ($_ ~~ (EXPR)) {...; break}
> > when {BLOCK} {...} if ($_ ~~ sub{BLOCK}) {...; break}
> >
> >
> > No exceptions.
> > No special cases.
> > ______________________________________________________________________
> >
> >
> > Note that this proposal still differs slightly from *all* of the various ideas
> > that were explicitly suggested.
> >
> > In particular, I'm now proposing to fix the number/string ambiguity by
> > making *any* ambiguous case fatal, rather than falling back on eq
> > when we're not sure. So we only do == when we're certain it's a
> > number, and only do eq when we're certain it's a string, and if we're
> > not certain, we always die.
> >
> > In practice, that would mostly only affect isolated cases (and those
> > cases could still be resolved easily, by explicit coercion or explicit
> > specification):
> >
> > $var = 1; # only a number inside
> > say $var; # now a number and a string inside
> >
> > given ($someval) {
> > when ( "$var" ) {...} # expr produces only a string, so always eq
> > when ( 0 + $var ) {...} # expr produces only a number, so always ==
> > when ( $var ) {...} # expr may produce either, so always fatal
> >
> > when {$_ == $var} {...} # always ==
> > when {$_ eq $var} {...} # always eq
> > }
> >
> >
> > if ($otherval ~~ 1 ) {...} # number -----> always ==
> > if ($otherval ~~ "1" ) {...} # string -----> always eq
> > if ($otherval ~~ $var) {...} # ambiguous --> always fatal
> >
> > if ($otherval == $var) {...} # d'oh
> > if ($otherval eq $var) {...} # ditto
> >
> >
> > The point being: you either specify precisely what you mean (with a
> > typed literal, or with an expression that produces an unambiguously
> > typed value, or with an explicit operator), or else you accept the
> > possibility of catastrophic failure in a very limited number of cases.
> >
> > This approach seems to cover most of the objections and concerns that
> > have been voiced so far, and seems both simple enough and powerful
> > enough to be worth considering.
>
> It seem to me that the problem here is that the flag you speak of is
> currently theoretical. IOW, any case where a string was numified, or a
> num stringified would end up being ambiguous, and I think this happens
> a lot more than you seem to suggest. For instance, simply _printing_
> a numeric var upgrades it from a SvIV or SvNV to a SvPVIV or SvPVNV.
Isn't this the sort of thing that the magicflags branch was supposed to
fix?
-doy
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