use strict;
use warnings;
use Stream;

my $increment = sub { $_[ 0 ] + 1 };
my $upfrom2   = Stream::iterate_function( $increment, 2 );
my $primes    = prime_filter( $upfrom2 );

my $n = shift || 1;
print nth_term( $primes, $n ), "\n";

exit 0;

sub nth_term {
  my ( $s, $n ) = @_;
  $s = Stream::tail $s while --$n > 0;
  return Stream::head $s;
}

# a closure factory
sub not_multiple_of {
  my $d = shift;
  return sub { $_[ 0 ] % $d };
}

sub prime_filter {
  my $s = shift;             # a stream
  my $p = Stream::head $s;   # a "simple scalar"
  my $t = Stream::tail $s;   # another stream
  my $not_multiple_of_p = not_multiple_of( $p );

  return
    [
      $p,
      sub {
        prime_filter( &Stream::filter( $not_multiple_of_p, $t ) );
      }
    ];
}

##</code><code>##

% perl sieve.pl 97
509
% perl sieve.pl 98
Deep recursion on subroutine "Stream::tail" at Stream.pm line 90.
Deep recursion on subroutine "Stream::tail" at Stream.pm line 90.
521

##</code><code>##

  node(head($s),
       promise { filter($f, tail($s)) });

##</code><code>##

  node( head( $s ),
        promise {
                  $s->[ 1 ] = $s->[ 1 ]->() if is_promise $s->[ 1 ];
                  filter( $f, $s->[ 1 ] )
                }
      );

##</code><code>##

--- Stream.pm.orig	Tue May  3 16:41:37 2005
+++ Stream.pm	Mon Aug 29 20:00:14 2005
@@ -11,13 +11,16 @@
 
 ## Chapter 6 section 2
 
+use strict;
+use warnings;
+
 package Stream;
-use base Exporter;
-@EXPORT_OK = qw(node head tail drop upto upfrom show promise
+use base 'Exporter';
+@Stream::EXPORT_OK = qw(node head tail drop upto upfrom show promise
                 filter transform merge list_to_stream cutsort
                 iterate_function cut_loops);
 
-%EXPORT_TAGS = ('all' => \@EXPORT_OK);
+%Stream::EXPORT_TAGS = ('all' => \@Stream::EXPORT_OK);
 
 sub node {
   my ($h, $t) = @_;
@@ -29,13 +32,13 @@
   $s->[0];
 }
 
-sub tail {
-  my ($s) = @_;
-  if (is_promise($s->[1])) {
-    return $s->[1]->();
-  }
-  $s->[1];
-}
+# sub tail {
+#   my ($s) = @_;
+#   if (is_promise($s->[1])) {
+#     return $s->[1]->();
+#   }
+#   $s->[1];
+# }
 
 sub is_promise {
   UNIVERSAL::isa($_[0], 'CODE');

##</code><code>##

  return
    Stream::node (
                   $p,
                   Stream::promise {
                     prime_filter( &Stream::filter( $not_multiple_of_p, $t ) );
                   }
                 );