S E N D
+   M O R E
-----------
  M O N E Y

##</code><code>##


sub bd { my ($ls, $f) = @_;
  [ map @{$f->($_)}, @$ls ]      # Yow
}

sub guard { $_[0] ? [undef] : [] }

sub remove {
  my ($b, $a) = @_;
  my %h = map { $_ => 1 } @$a;
  delete $h{$_} for @$b;
  return [ keys %h ];
}


my $digits = [0..9];
my $solutions =
  bd remove([0],        $digits) => sub { my ($s) = @_;
  bd remove([$s],       $digits) => sub { my ($e) = @_;
  bd remove([$s,$e],    $digits) => sub { my ($n) = @_;
  bd remove([$s,$e,$n], $digits) => sub { my ($d) = @_;
    my $send = "$s$e$n$d";

  bd remove([0,$s,$e,$n,$d],     $digits) => sub { my ($m) = @_;
  bd remove([$s,$e,$n,$d,$m],    $digits) => sub { my ($o) = @_;
  bd remove([$s,$e,$n,$d,$m,$o], $digits) => sub { my ($r) = @_;
    my $more = "$m$o$r$e";

  bd remove([$s,$e,$n,$d,$m,$o,$r], $digits) => sub { my ($y) = @_;
    my $money = "$m$o$n$e$y";
  bd guard($send + $more == $money) => sub { [[$send, $more, $money]] }}}}}}}}};

  for my $s (@$solutions) {
    print "@$s\n";
  }

##</code><code>##

use strict;
use warnings;
use Data::Dump qw/pp dd/;




=pod

https://blog.plover.com/prog/haskell/monad-search.html

https://blog.plover.com/prog/monad-search-2.html

=cut


# --- List comprehension

sub from (&$;$) {
    my ($c_block, undef, $c_tail) = @_;
    my $var = \$_[1];
    sub {
        for ( &$c_block ) {
            $$var = $_;
            $c_tail->()
        }
      }
}

sub when (&$){                          # guard
    my ($c_block, $c_tail) = @_;

    sub { $c_tail->() if &$c_block }
}


# --- rem() Helper function to return digits 0..9 except @_
my %digits;
@digits{0..9}=();

sub rem {                               # set difference
    my %h = %digits;
    delete @h{@_};
    keys %h;
}


my ($send,$s,$e,$n,$d);
my ($more,$m,$o,$r);
my ($money,$y);



my $do =
  # send
  from { rem 0 } $s =>
  from { rem $s } $e =>
  from { rem $s, $e } $n =>
  from { rem $s,$e,$n } $d =>

  # more
  from { rem 0,$s,$e,$n,$d } $m =>
  from { rem $s,$e,$n,$d,$m } $o =>
  from { rem $s,$e,$n,$d,$m,$o } $r =>

  # money
  from { rem $s,$e,$n,$d,$m,$o,$r } $y =>

  # guard
  when { "$s$e$n$d" + "$m$o$r$e" == "$m$o$n$e$y" }

  # output
  sub { pp [ "$s$e$n$d" , "$m$o$r$e" , "$m$o$n$e$y" ] }
  ;

&$do;

##</code><code>##

Compilation started at Wed Dec  2 19:11:14

C:/Perl_524/bin\perl.exe -w d:/tmp/pm/send_more_money_monad.pl 
[9567, 1085, 10652]

Compilation finished at Wed Dec  2 19:11:18