Re: simple anonymous subroutine and variable interpolation timing question

[ikegami]

There's no interpolation involved. The sub contains a lookup of a variable's content, so the variable needs to remain unchange if you want the output of the lookup to be constant.

You can do this by copying the value of $str into into a variable that doesn't change, and using that variable instead.

sub make_closure {
   my ($str) = @_;
   return sub { $str };
}

my $str = 'hi';
my $sub = make_closure($str);

say $sub->();   # 'hi'
$str = 'hello';
say $sub->();   # 'hi'

You could actually interpolate the current value into code, but producing code is trickier.

my $str = 'hi';
my $sub = eval qq{ sub { "\Q$str\E" } } or die $@;

say $sub->();   # 'hi'
$str = 'hello';
say $sub->();   # 'hi'

[tj_thompson]

I was considering the closure with a local variable, but I was wondering if there was some sneaky way the experts did it. I don't like the looks of the eval method, so I think I'll stick with the first.

Thanks as always Ikegami!

[ikegami]

You don't have to create a separate sub. Alternatives:

my $sub = sub { my ($str) = @_; sub { $str } }->($str);

my $sub = sub { my $str = $str; sub { $str } }->();

my $sub = do { my $str = $str; sub { $str } };

my $str_copy = $str;
my $sub = sub { $str_copy };

In practice, it's usually a non-issue since you either have a variable that doesn't change to begin with

for my $str (...) {
   ... sub { ... $str ... } ...
}

or you want to factor out the code to a sub anyway.

sub make_list_iterator { 
   my @list = @_;
   return sub {
      return @list ? shift(@list) : ();
   };
}

my $iter = make_list_iterator(@list);

[tj_thompson]

errr, meant lexical variable, not local.