Re^2: coderefs and (&) prototypes

The execution of the argument passed to grep is deferred until after grep is called, and it is called once for every following argument.

sub func {}
my $x; sub foo { ++$x }
$x = 0; func foo(), 1,2,3; print "$x\n";  # 1
$x = 0; grep foo(), 1,2,3; print "$x\n";  # 3
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grep and map don't have a prototype since this ability can't be prototyped.

You seem to think there's something special about &cb when passed to grep. It's not.

grep &cb,           ...  === grep { &cb           } ...
grep &cb(),         ...  === grep { &cb()         } ...
grep $cb->(),       ...  === grep { $cb->()       } ...
grep /.../,         ...  === grep { /.../         } ...
grep substr($_, 2), ...  === grep { substr($_, 2) } ...
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(minus the extra scope).

&cb means "call non-builtin cb without changing or localising @_", whether it's the first arg of grep or otherwise.

grep cannot accept a ‘calculated’ first argument. [...] Can anyone (like ikegami :-) ) clarify for me?

Using map since it's easier to visualize,

sub mymap (&@) { my ( $code, @list ) = @_; map &$code, @list }

sub get_cb { return sub { uc } }

print(( mymap  \&{ &get_cb }, qw( a b c ) ), "\n");
print(( map    &get_cb,       qw( a b c ) ), "\n");
print(( map    get_cb,        qw( a b c ) ), "\n");
print(( map    get_cb(),      qw( a b c ) ), "\n");
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ABC
CODE(0x1829a54)CODE(0x1829a54)CODE(0x1829a54)
CODE(0x1829a54)CODE(0x1829a54)CODE(0x1829a54)
CODE(0x1829a54)CODE(0x1829a54)CODE(0x1829a54)
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Just can't do it. The first arg of map and grep is never evaluated before map or grep is called.

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Re^3: coderefs and (&) prototypes by JadeNB (Chaplain) on Jul 28, 2009 at 17:59 UTC
Thanks very much for the clarification. I keep forgetting that the sub passed to `map` operates on `$_` rather than receiving its argument in `@_`, so I'm always writing something like `map &f, qw/1 2 3/` and wondering why I don't get `(f(1), f(2), f(3))`. For your second example, though, when I tested it, the code `print(( map &{ get_cb() }, qw/a b c/), "\n");` [download] printed `ABC`, just as `mymap` did. Am I still missing something?	[reply] [d/l] [select]
Re^4: coderefs and (&) prototypes by ikegami (Patriarch) on Jul 28, 2009 at 18:28 UTC
`sub mymap (&@) { my ( $code, @list ) = @_; map &$code, @list } my @cbs; sub reset_cbs { @cbs = ( sub { uc }, sub { lc } ); } sub get_cb { push @cbs, shift(@cbs); return $cbs[0]; } reset_cbs(); print(( mymap \&{ get_cb() }, qw( a b c d ) ), "\|"); print(( mymap \&{ get_cb() }, qw( a b c d ) ), "\n"); reset_cbs(); print(( map &{ get_cb() }, qw( a b c d ) ), "\|"); print(( map &{ get_cb() }, qw( a b c d ) ), "\n");` [download] `abcd\|ABCD aBcD\|aBcD` [download]	[reply] [d/l] [select]