### Comment on

 Need Help??
Here is a more general solution to the problem. In this solution my iterators all indicate that they are done by returning an empty list, and when called next will restart. I didn't use i_grep, but I included it to show how you would do it.

Note that despite the length, the code is more straightforward than the original recursive code. And if this was part of a longer program, you would be reusing the bulk of this code.

```use strict;

my \$iter = i_map(
sub {print "@_\n"},
comb_iter(
list_iter(1..2), list_iter('a'..'c'), list_iter(3..5)
)
);

1 while \$iter->();

###################################################################
# The program proper ends here.  These are utility functions that #
# you could reuse                                                 #
###################################################################

# Takes a list of iterators that are "restartable"
# Returns a restartable iterator that iterates over all combinations
# of outputs of the input iterators, creating a flat list of combinati
+ons
# of the inputs.  (The output only makes sense in array context.)
sub comb_iter {
if (0 == @_) {
return sub {()}; # Stupid case needed for generality.
}
elsif (1 == @_) {
return shift;
}
else {
my \$outer_iter = shift;
my \$inner_iter = comb_iter(@_);
my @last_outer;
return sub {
if (@last_outer) {
my @ret = \$inner_iter->();
if (@ret) {
return (@last_outer, @ret);
}
else {
@last_outer = \$outer_iter->();
if (@last_outer) {
return (@last_outer, \$inner_iter->());
}
else {
return ();
}
}
}
else {
@last_outer = \$outer_iter->();
return (@last_outer, \$inner_iter->());
}
};
}
}

# Takes a function and an iterator, returns an iterator that uses that
# function to filter the output.
sub i_grep {
my (\$filter, \$iter) = @_;
my @last_ret = qw(just an initialization value);
sub {
while (@last_ret) {
@last_ret = \$iter->();
if (\$filter->(@last_ret)) {
return wantarray ? @last_ret : \$last_ret[0];
}
}
return ();
};
}

# Takes a function and an iterator, returns an iterator that applies t
+hat
# function to the returns of the iterator.
sub i_map {
my (\$filter, \$iter) = @_;
sub {
my @ret = \$iter->();
return @ret ? \$filter->(@ret) : ();
};
}

# Takes a list and turns it into an iterator over that list
sub list_iter {
my @vals = @_;
my \$i = 0;
sub {
if (\$i < @vals) {
return \$vals[\$i++];
}
else {
\$i = 0;
return ();
}
};
}
Note that the specific problem in the original question can now be solved as the author wanted using i_grep, or you can produce more efficient iterator as follows:
```my \$genome_iter = i_map(
sub {join '', @_},
join_iter(
map {
comb_iter(
map {
list_iter(qw(a c g n t));
} 1..\$_
)
} 2..3
)
);

while (my \$string = \$genome_iter->()) {
print "\$string\n";
}

# Takes a list of iterators, and returns an iterator that iterates
# over each in turn
sub join_iter {
my @iter = @_;
my \$i = 0;
return sub {
while (\$i < @iter) {
my @ret = \$iter[\$i]->();
if (@ret) {
return wantarray ? @ret : \$ret[0];
}
else {
\$i++;
}
}
\$i = 0;
return ();
};
}
Alternately if you want to turn the output into a list you can just create an easy method:
```# Takes an iterator and returns a list of results
sub iter2list {
my \$iter = shift;
my @out;
while (my @ret = \$iter->()) {
push @out, @ret;
}
return @out;
}
Note that most of the length here is because I am having to build my iterator interface from scratch. That is a lot of work! And some of the code looks more complex because what we are used to seeing in a few nested loops our minds balk at when you see it as a similar number of nested calls.

In reply to Laziness in a more consistent way by tilly
in thread Let's get lazy by guillaume

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