Re: [perl #129208] map sometimes does and sometime does not copyvals

Father Chrysostomos wrote:
>I would expect these two to be equivalent:
>
>$ ./perl -Ilib -le '$x = 4; sub {\@_; for (map {$_[0]} 1) { $_=3 } }->($x); print $x'
>4
>$ ./perl -Ilib -le '$x = 4; sub {\@_; for (map {shift} 1) { $_=3 } }->($x); print $x'
>3

Not sure why you'd expect them to be equivalent.  You're basically asking
for $_[0] and shift to be equivalent in whether they yield an lvalue.
Which they're not:

$ perl -lwe '$x = 4; sub { $_[0] = 3; }->($x); print $x'
3
$ perl -lwe '$x = 4; sub { shift = 3; }->($x); print $x'
Can't modify shift in scalar assignment at -e line 1, near "3;"
Execution of -e aborted due to compilation errors.

However, these two methods of using $_[0]/shift disagree on which one
yields an lvalue.  Furthermore, shift yields something lvalue enough to
get an equivalence out of it:

$ perl -lwe '$x = 4; sub { ${\shift} = 3; }->($x); print $x'
3
$ perl -le '$x = 4; sub {\@_; for (map {${\shift}} 1) { $_=3 } }->($x); print $x'
4

Then there's a third obvious way of using the maybe-lvalue that yields
different results again:

$ perl -lwe '$x = 4; sub { for($_[0]) { $_ = 3; } }->($x); print $x'
3
$ perl -lwe '$x = 4; sub { for(shift) { $_ = 3; } }->($x); print $x'
3
$ perl -lwe '$x = 4; sub { for(${\shift}) { $_ = 3; } }->($x); print $x'
3

So it's consistently inconsistent.

-zefram

• [perl #129208] map sometimes does and sometime does not copy vals by Father Chrysostomos
• Re: [perl #129208] map sometimes does and sometime does not copyvals by Zefram
• [perl #129208] map sometimes does and sometime does not copy vals by Father Chrysostomos via RT