Basic question about Iterator code

adamsj has asked for the wisdom of the Perl Monks concerning the following question:

I've been working through examples in MJD's Higher Order Perl and hit a snag. This code from Chapter 4 works fine:

# This software is Copyright 2005 by Elsevier Inc.  You may use it
# under the terms of the license at http://perl.plover.com/hop/LICENSE
+.txt .
###
### upto
###

## Chapter 4 section 2.1

sub upto {
  my ($m, $n) = @_;
  return sub {
    return $m <= $n ? $m++ : undef;
  };
}
my $it = upto(3,5);
my $value->it();
print "$value\n";
[download]

However, when I sweeten the function as suggested in the text:

sub upto {
  my ($m, $n) = @_;
  return Iterator {
    return $m <= $n ? $m++ : undef;
  };
}
sub Iterator (&) { return $_[0]; }
[download]

I get a failure: Undefined subroutine &main::3 called

They laughed at Joan of Arc, but she went right ahead and built it. --Gracie Allen

Comment on Basic question about Iterator code Select or Download Code

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Re: Basic question about Iterator code by choroba (Cardinal) on May 12, 2025 at 14:18 UTC
You probably meant `my $value = $it->();` [download] instead of `my $value->it();` [download] in the first sample. As for the error: in order to use the block syntax with a `(&)-`prototyped subroutine, you need to declare the subroutine before you use it. Move the last line to the top of the second sample and everything should work. `#!/usr/bin/perl use warnings; use strict; use feature qw{ say }; sub Iterator(&) { $_[0] } sub upto { my ($m, $n) = @_; return Iterator { $m <= $n ? $m++ : undef } } my $it = upto(3,5); while (my $value = $it->()) { say $value; }` [download] `map{substr$_->[0],$_->[1]\|\|0,1}[\\|\|{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^ARGV,3]`	[reply] [d/l] [select]
Re: Basic question about Iterator code by ikegami (Patriarch) on May 12, 2025 at 23:15 UTC
As a side note, if you want to support returning false values (such as zero, but particularly `undef`) easily, return an empty list when the iterator is exhausted. `sub upto { my ( $m, $n ) = @_; return sub { return if $m > $n; return $m++; }; }` [download] or `sub upto { my ( $m, $n ) = @_; return sub { $m <= $n ? $m++ : () }; }` [download] Then, use a list assignment in scalar/boolean context. `my $iter = upto( -2, 2 ); while ( my ( $ele ) = $iter->() ) { say $ele; }` [download] `-2 -1 0 1 2` [download] You could have used `while ( defined( my $i = $iter->() )`. But you won't be able to use that for every iterators, such as one that returns the elements of an array: `sub list { my @a = @_; return sub { @a ? shift( @a ) : () }; } my $iter = list( 1, 0, undef, "a" ); # This doesn't work: # while ( defined( my $ele = $iter->() ) ) { # say $ele // "[undef]"; # } while ( my ( $ele ) = $iter->() ) ) { say $ele // "[undef]"; }` [download] `1 0 [undef] a` [download] Since you sometimes need to use the empty-list "trick", you might as well always use it for consistency. Also, it's shorter than using `defined` :)	[reply] [d/l] [select]