#! perl -l

my $f = sub { "f($_[0])" };
my $g = sub { "g($_[0])" };

print $f->("x");
# f(x)
print $g->("x");
# g(x)

##</code><code>##

sub compose2 {
    my ($f, $g) = @_;
    sub { $f->( $g->(@_) ) }
}

##</code><code>##

my $h = compose2( $f, $g );
print $h->("x");
# f(g(x))

##</code><code>##

sub foldl {
    my $f = shift;
    my $z = shift;
    $z = $f->($z, $_) for @_;
    $z;
}

sub compose {
    foldl( \&compose2, @_ )
}

##</code><code>##

print compose( $f,$f,$f,$f,$f,$f,$g )->("x");
# f(f(f(f(f(f(g(x)))))))

my $add1 = sub { $_[0] + 1 };
my $add2 = compose( $add1, $add1 );
my $add4 = compose( $add2, $add2 );

print $add2->(0);
# 2

print $add4->(0);
# 4

print compose( $add1, $add1, $add1 )->(0);
# 3

print compose( ($add1) x 4 )->(0);
# 4