#!/usr/bin/perl -w
# summation examples from SICP done in Perl

use strict;
use warnings;

my $result = &sum_integers(1, 3);
print "sum_integers(1, 3) equals $result\n";
$result = &cube(3);
print "cube(3) equals $result\n";
$result = &sum_cubes(1, 3);
print "sum_cubes(1, 3) equals $result\n";
$result = &sum_cubes2(1, 3);
print "sum_cubes2(1, 3) equals $result\n";

sub sum_integers {
    my ($a, $b) = @_;
    if ($a > $b) { return(0); }
    else { return( $a + &sum_integers(($a + 1), $b) ); }
}

sub cube {
    my ($a) = @_;
    return( $a * $a * $a );
}

sub sum_cubes {
    my ($a, $b) = @_;
    if ($a > $b) { return(0); }
    else { return( &cube($a) + &sum_cubes(($a + 1), $b) ); }
}

sub sum {
    my ($term, $a, $next, $b) = @_;
    if ($a > $b) { return(0); }
    else { return( $term->($a) + &sum( $term, ($next->($a)), $next, $b) ); }
}

sub inc {
    my ($n) = @_;
    return( $n + 1 );
}

sub sum_cubes2 {
    my ($a, $b) = @_;
    return( &sum(\&cube, $a, \&inc, $b) );
}