sub visit_preorder(&$) { my ($visitor, $node) = @_; my ($val, $l, $r) = @$node; $visitor->() for $val; &visit_preorder($visitor, $l) if defined $l; &visit_preorder($visitor, $r) if defined $r; } #### sub visit_preorder { my ($node) = @_; my ($val, $l, $r) = @$node; return ( sub { return \$val; }, sub { return visit_preorder($l) if defined $l; return; }, sub { return visit_preorder($r) if defined $r; return; }, ); } #### sub visit_preorder { my ($node) = @_; my ($val, $l, $r) = @$node; return ( \$val, defined($l) ? sub { return visit_preorder($l); } : (), defined($r) ? sub { return visit_preorder($r); } : (), ); } #### sub make_iter { my $f = shift @_; my @todo = $f->(@_); return sub { while (@todo) { my $todo = shift @todo; if (ref($todo) eq 'CODE') { unshift @todo, $todo->(); } elsif (ref($todo) eq 'ARRAY') { return @$todo; } else { return $$todo; } } return; }; } #### { # a # / \ # b e # / \ \ # c d f my $tree = [ 'a', [ 'b', [ 'c', undef, undef, ], [ 'd', undef, undef, ], ], [ 'e', undef, [ 'f', undef, undef, ], ], ]; my $iter = make_iter(\&visit_preorder, $tree); while (my ($name) = $iter->()) { print($name); } print("\n"); }