自前 fold & foldr で合成関数

gistfile1.pl

#!/usr/bin/env perl -l
use strict;
use warnings;
use Data::Dumper;

sub fold {
    my $f     = shift;
    my $res   = shift;
    my @rest  = @_;

    ref $f ne 'CODE' and die qq(first argument should be 'code ref');

    while (my $first = shift @rest) {
        $res = $f->($res, $first);
    }
    $res;
}

sub foldr {
    my $f = shift;
    fold($f, reverse @_);
}


# case 1: eq reduce{ $a + $b }@nums;

my @nums = (1, 2, 3, 4);
print fold(sub { $_[0] + $_[1] } , @nums); # 10


# case 2: eq reduce{ [ @$a, @$b ] }@arrays;

my @arrays = ([ 1, 2 ], [ 3, 4 ], [ 5, 6 ]);
print Dumper fold(sub { [ @{$_[0]}, @{$_[1]} ] }, @arrays);
# [ 1, 2, 3, 4, 5, 6 ]



sub inc {
    my $n = shift;
    $n + 1;
}
sub double {
    my $n = shift;
    $n * 2;
}
sub square {
    my $n = shift;
    $n * $n;
}
sub fg {
    my($f, $g) = @_;
    sub { $f->($g->(@_)) };
}

# case 3

my $func = foldr(\&fg, \&inc, \&double, \&square);
print $func->(3); # 64; ((3+1) * 2) ^ 2

__END__

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

my $func = fg(\&inc, \&double, \&square);
print $func->(3);