#implements something like "do" notation for the Identity monad.
#bound variables become subs in the main namespace
#add_var thunks the subs to get laziness with sharing

sub add_var { 
   my ($name, $fn) = @_;
   *{"main::$name"} = sub{ my $ret = $fn->(@_);  
                           *{"main::$name"}= sub{$ret};
                           return $ret
                         };
   return 1;
}

use strict;

sub mdo {
   while (scalar @_ > 1) {
      my $name = shift;
      my $fn = shift;
      add_var($name,$fn);
   }

   my $fn = shift;
   return $fn->();

}

my $f = sub { print "a: ", a(), " b: ", b(), "\n";};

mdo( a => sub{1}
   , b => sub{ a() + 5}
   , $f
   );

mdo( a => sub{2}
   , b => sub{ a() * 5}
   , $f
   );