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### Re^2: Question about recursively generated iterators

by ikegami (Patriarch)
 on Sep 21, 2007 at 16:48 UTC ( #640397=note: print w/replies, xml ) Need Help??

This method can be used when a function has a loop, since loops can be implemented using recursion. This is great because it facilitates making iterators from functions where the call to the visitor is not at the end and from functions with multiple calls to the visitor.

For example, let's create a fibonacci generator. A simple implementation is:

```sub fibonacci {
my (\$visitor) = @_;
my (\$i, \$j) = (0, 1);
\$visitor->() for \$i;
for (;;) {
\$visitor->() for \$j;
(\$i, \$j) = (\$j, \$i+\$j);
}
}

Replace the loop with recursion.

```sub fibonacci {
my (\$visitor) = @_;
my (\$i, \$j) = (0, 1);
\$visitor->() for \$i;
_fibonacci(\$i, \$j);
}

sub _fibonacci {
my (\$visitor, \$i, \$j) = @_;
\$visitor->() for \$j;
_fibonacci(\$j, \$i+\$j);
}

Convert for make_iter.

```sub fibonacci {
my (\$i, \$j) = (0, 1);
return ( \\$i, sub { _fibonacci(\$i, \$j) } );
}

sub _fibonacci {
my (\$i, \$j) = @_;
return ( \\$j, sub { _fibonacci(\$j, \$i+\$j) } );
}

Try it out

```{
my \$iter = make_iter(\&fibonacci);
for (0..15) {
my (\$n) = \$iter->()
or last;
print("\$n ");
}
print("\n");
}
```0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610

Replies are listed 'Best First'.
Re^3: Question about recursively generated iterators
by perlfan (Vicar) on Sep 21, 2007 at 19:32 UTC
Very much appreciated, ikegami! This is very helpful.

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