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Postings from December 2011
On Sun Apr 03 16:22:34 2005, perl5-porters@ton.iguana.be wrote:
> In article <20050403195113.GH96525@plum._lirble.org>,
> Nicholas Clark <nick@ccl4.org> writes:
> > On Thu, Mar 17, 2005 at 11:08:00AM -0000, Bram wrote:
> >
> >> Is this a bug or not? or it the @normal_hash{qx/a b c/}++ an
'accidental
> >> feature' which has not been copied to tie?
> >
> >
> > On Tue, Mar 29, 2005 at 05:30:27PM -0600, David Nicol wrote:
> >
> >> post-increment appears to increment the last element of hash slices
> >> but not against
> >> arrays in general. Is this by design or laxity?
> >
> > I assume that being able to increment a hash (or array) slice is a bug,
> > given that it's a syntax error on a plain array or hash:
> >
> For golfers this is a well known fact :-)
>
> As noted, it works on array slices too:
>
> perl -wle '@a=(1..4); @a[1..2]++; print for @a'
> 1
> 2
> 4
> 4
>
> It's also sort of consistent. A slice is is documented to be a list of
> values, and in fact the docs are quite explicit about @a[1..2] being
> like ($a[1], $a[2]), though the usage of "list" is quite ambiguous
> in that part of perldoc perldata. But that's *really* the way it behaves.
>
> e.g.:
>
> perl -wle '@a=1..4; print for \@a[1..2]'
>
> is not some array reference, but:
>
> SCALAR(0x817350c)
> SCALAR(0x8173518)
>
> just like you'd get from \($a[1], $a[2])
>
> And in scalar context such a list of values evaluates to the last value:
>
> perl -wle '@a=1..4; print for scalar @a[0..2]'
> 3
>
> So the result is actually as expected and I wouldn't call it a bug.
>
> PS:
> At least until you see that this doesn't work:
>
> perl -wle '@a=1..4; ($a[1], $a[2])++'
> Useless use of array element in void context at -e line 1.
> Too many arguments for postincrement (++) at -e line 1, near ")++"
> Execution of -e aborted due to compilation errors.
>
> Maybe THAT'S a bug :-)
The ‘too many arguments’ thing is based on the number of operands at a
syntactic level.
If an expression could (or usually would) return a list, but is still
one single term, it gets evaluated in scalar context by operators that
propagate scalar context.
A hash or array slice in scalar context evaluates to the last element of
the slice.
$ perl -le' @_ = a..z; print scalar @_[0..5];'
f
Hash and array slices are valid lvalues.
So I think this is not a bug, and that the behaviour is as expected.
Changing it would mean adding special cases.
--
Father Chrysostomos
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