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Postings from October 2015
On Mon, Oct 12, 2015 at 11:22:03AM +0100, Dave Mitchell wrote:
> On Wed, Oct 07, 2015 at 03:57:26PM -0400, Eric Brine wrote:
> > On Wed, Oct 7, 2015 at 4:24 AM, Dave Mitchell <davem@iabyn.com> wrote:
> >
> > > 2. Make when's behaviour entirely dependent on the *compile-time* class
> > > of of its arg. In particular, make 'when (FOO)' have exactly the following
> > > behaviours:
> > >
> > > FOO condition evaluates to
> > > -------------- ----------------------
> > > literal undef !defined($_)
> > > numeric const $_ == FOO
> > > literal pattern $_ ~= FOO
> > > all else $_ eq FOO
> > >
> > > 3. when { FOO } has condition FOO.
> > >
> > > For example:
> > >
> > > use constant ME => "davem";
> > > when (ANS) # equivalent to when { $_ == 42 }
> > >
> >
> > According to your table, shouldn't that be "when { $_ eq ANS }"? ANS could
> > have any value, so when (ANS)" is no different than "when ($x)". There's no
> > "compile-time class" available to study.
>
> No, its specifically if FOO is a compile-time constant. At compile time
> there is no difference between
>
> when(42)
>
> and
>
> use constant FOO;
> when(FOO);
>
> They have both been compiled to a single OP_CONST with a constant SV
> attached that can be examined at compile-time for numness. Of course,
>
> sub FOO {}
> when (FOO)
>
> would be treated as ($_ eq FOO) still.
While sympathetic, and perhaps easier on the implementation side, I
think deciding what to do depending how an expression is evaluated at
compile time is a really bad idea:
- Most Perl programmers will not be familiar enough with the perl
internals to know what is actually happening at compile time.
Nor should they.
- It creates an action at a distance, as your latter example shows.
Programs changing their behaviour in subtle ways if one replaces
use constant FOO => "...";
with
sub FOO {...}
(or in the other direction) doesn't seem like a good idea to me.
It's a maintenance nightmare.
- It makes it harder in the future to change what's done at compile
time, as it may break the meaning of when().
Abigail
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