Re: coderefs and (&) prototypes

What confuses me is that, in Prototypes, it is suggested that a function prototyped (&@) behaves like grep. However, whereas one calls

grep &$code, @list
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to find all the elements $_ of @list for which $code->($_) is true, for a hypothetical mygrep (&@), one would call

mygrep \&$code, @list
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to achieve the same goal.

ikegami pointed out yet another subtlety, which is that grep cannot accept a ‘calculated’ first argument. Thus,

sub mygrep (&@) { my ( $code, @list ) = @_; return grep &$code, @list 
+}
sub gt { my ( $test ) = @_; return sub { $_ > $test } }
grep &{ gt 1 }, qw/1 2 3 1 4/; => ()
mygrep \&{ gt 1 }, qw/1 2 3 1 4/; => ( 2, 3, 4 )
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(While I'm at it, ikegami also pointed out that my first example could be written as grep $code->(), @list, which I find completely bizarre.)

UPDATE: Actually, having just run the code, it seems to me that the grep and mygrep invocations actually return the same thing (namely, (2, 3, 4)), so I must have misunderstood. Can anyone (like ikegami :-) ) clarify for me?

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Re^2: coderefs and (&) prototypes by ikegami (Patriarch) on Jul 28, 2009 at 01:42 UTC
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` [download] `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) } ...` [download] (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");` [download] `ABC CODE(0x1829a54)CODE(0x1829a54)CODE(0x1829a54) CODE(0x1829a54)CODE(0x1829a54)CODE(0x1829a54) CODE(0x1829a54)CODE(0x1829a54)CODE(0x1829a54)` [download] Just can't do it. The first arg of `map` and `grep` is never evaluated before `map` or `grep` is called.	[reply] [d/l] [select]
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]
Re^2: coderefs and (&) prototypes by LanX (Saint) on Jul 27, 2009 at 23:42 UTC
What confuses me is that, in Prototypes, it is suggested that a function prototyped (&@) behaves like grep. The answer is simple! IMHO perlsub is wrong at this place and you proved it! 8) Congratulations we found the second error in this perldoc chapter within two weeks! Should be noted that CORE::map has no prototype to be copied, it has hidden magic behavior: `print undef==prototype "CORE::map" # prints 1` [download] IMHO if there was a possibility to express the behavior with prototypes, perl would use it internally! Cheers Rolf	[reply] [d/l]