#!/usr/bin/perl

package Tie::Ensure;
use strict;


sub TIESCALAR {
  my ($class, $var, $code) = @_;
  bless {
    VAL => $$var,
    CODE => $code,
  }, $class;
}


sub FETCH { $_[0]{VAL} }


sub STORE {
  my ($self, $val) = @_;
  local $_ = $self->{VAL} = $val;
  die "[Tie::Ensure]\n" unless $self->{CODE}->();
}


package main;
use strict;


sub ensure (&@) {
  my ($cref, $obj, $loop) = @_;
  tied($$obj)->{CODE} = $cref;
  eval { { $loop->(); redo } };
  die $@ if $@ ne "[Tie::Ensure]\n";
  untie $$obj;
}


sub using (\$) {
  tie ${$_[0]}, 'Tie::Ensure', $_[0];
  return $_[0];
}


sub looping (&) { $_[0] }


my $foo = 10;

ensure { $_ < 15 } using($foo), looping {
  print "before: $foo\n";
  $foo++;
  print "after: $foo\n";
};