use strict;
use warnings;
use feature qw/say/;
use Data::Dump qw/pp/;


sub gen_listiter {
  my @init=@_;
  my @list=@init;
  
  sub {
    if (@list) {
      return shift @list
    } else {
      @list=@init;
      return ()
    }
  }
}

sub get_array {
  my $iter=shift;
  my @res;
  while ( my ($res)= $iter->() ) {
    push @res,$res;
  }
  return @res;
}

  
my %operations =(
                 '+' => sub { defined $_[1] ? $_[0] + $_[1]->[0] : $_[0] },
                );


sub X (&;$) {
  my ($cr,$tail_itr)=@_;
#  my $op = $operations{$tail_itr} if defined $tail_itr;
#  undef $tail_itr if $op;
  

  $tail_itr //= gen_listiter([]);

  
  my $head_itr = gen_listiter($cr->());

  

  my $state='INIT';
  my $head;
  my $tail;
  my $op //= sub { [ $_[0],@{$_[1]} ] };
  
    
  my $cross_itr =
    sub {
      goto $state;
    INIT:
      $state="RESUME";
      while ( ($head) = $head_itr->() ) {
        while ( ($tail) = $tail_itr->() ) {
          return $op->($head,$tail);
        RESUME:
        }
      }
      $state='INIT';
      return;
    };

  if (wantarray) {
    return get_array($cross_itr);
  } else {
    return $cross_itr;
  }
  
}



pp X {"a".."d"};
pp X {"a".."b"} X {1..2} X {'x','y'};

my $iter = X {"a","b"} X {1..2} X {'x','y'};

while ( my ($aref)= $iter->() ) {
  pp $aref;
}