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Re: key/value hash and index/valuse array slices syntax
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From:
Ruslan Zakirov
Date:
June 12, 2013 10:38
Subject:
Re: key/value hash and index/valuse array slices syntax
Message ID:
CAMOxC8sB+Yk+JNqGjSMKmSr3PHXp06tT-kSWWNY0YYZExdaeYg@mail.gmail.com
On Tue, Jun 11, 2013 at 9:21 PM, Aristotle Pagaltzis <pagaltzis@gmx.de>wrote:
> * Ruslan Zakirov <ruz@bestpractical.com> [2013-06-11 14:05]:
> > On Mon, Jun 10, 2013 at 2:27 PM, demerphq <demerphq@gmail.com> wrote:
> > > Also what happens if I do:
> > >
> > > my $sub_hash= %hash{thing};
> > >
> > > does $sub_hash end up with the value 2 if "thing" existed, and with
> > > the value 0 if it didn't?
> >
> > It end ups with value, not 2 or 0 and produce a warning
> > (%hash{'thing'} is better written as $hash{'thing'}). As aslice and
> > hslice, kvhslice returns list, so last element is assigned to
> > $sub_hash which is value. I hate this "list" thing, but I'm for
> > consistency.
>
> Then immediately the question becomes, what does the following do?
>
> my %h = ( foo => 1 );
> my $e = %h{ "foo", "bar" };
>
> Does it assign undef, or 1? (I surmise from later mails of yours that it
> will assign undef – correct?)
>
Correct, undef. Depends on decision regarding whether existing or requested
elements are returned. As many people leaning towards returning requested
elements then undef.
> * Leon Timmermans <fawaka@gmail.com> [2013-06-11 12:55]:
> > One question I do have: what should it do with nonexistent keys? E.g.
> >
> > my %first = (a => 1, b => 2);
> > my %second = %first{qw/a c/};
> >
> > Should %second now contain «c => undef», not no c at all? I think I'm
> > currently leaning towards the latter, it seems like the more useful
> > behavior to me.
>
> Reluctant though I am, I feel that the right thing to do would be to
> return `c => undef` from the slice. For one thing, it makes the
> behaviour more regular and predictable. For another, it aligns with
>
> my @second = @first{qw/a c/};
>
> and also
>
> my @first = (1) x 3;
> my @second = @first[0,9];
>
> As well, it is not terribly difficult to get what you want by doing
>
> my %second = %first{ grep { exists $first{$_} } qw/a c/ };
>
> which is easy to tuck away in a utility function in such as Hash::Util
> as well. (The downside is that if your list of keys to copy is gigantic
> then doing it this way instead of natively will cost measurably in terms
> of both cycles and memory.)
>
> More importantly, given hash slices that *omit* non-existent keys, it is
> much harder to get the opposite behaviour. I cannot even think of any
> sensible way to do it as an expression within the same statement, only
> if I accept multiple statements, by doing such as
>
> my %second = %first{qw/a c/};
> @first{qw/a c/} = @first{qw/a c/};
>
> * Ruslan Zakirov <ruz@bestpractical.com> [2013-06-11 14:15]:
> > If only we had op modifiers for such ops like we have for s/// and
> > m///.
>
> I think there are in Perl 6…
>
>
> > Interesting thought is to make the following work like you want:
> >
> > my %second = exists %first{qw/a c/};
>
> My first gut reaction is that I don’t like this at all. Overloading
> existing things with new semantics without any systematic model, just
> hacking together ad-hoc syntactic sugar, rarely makes a solid design. It
> has a tendency to create strange corner cases not discovered for years,
> that then turn out to have no clear resolution.
>
> The approach of a keyword that modifies the slice operation does make
> sense though. It’s how `exists` works too – which OT1H feels like an
> integral part of the language’s character, though OTOH it is the lone
> and sole bit of precedent we have on this count.
>
> I would just rather mint a new keyword for this, or even a Hash::Util
> function. (And I think that the parser hooks we have now even allow
> writing such a function entirely outside core – no?)
>
> But when I then go to think about what to call this keyword/function,
> reusing `exists` seems like not necessarily a bad idea.
>
> So my feelings on this are unclear.
>
>
> Regards,
> --
> Aristotle Pagaltzis // <http://plasmasturm.org/>
>
--
Best regards, Ruslan.
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