#!/bin/perl

# bank accounts in perl
# there's another way of doing oop in perl - involving packages and 
# something called "bless".  I'm not using that - I'm using perl's 
# more fundamental capabilities to build oop from scratch, like in scheme


# in perl, "scalar" variables which include things like integers and
# pointers, are always prefixed with a $.

# in perl, "sub" means lambda.  Instead of specifying the formal parameters
# like in other languages, the parameters are always in an array, which
# can be accessed inside the function body as $_[0], $_[1], etc ...
# The entire array can be referred to as @_

# A perl function body always returns the last expression it evaluates.
# "my" in perl is equivalent to "let" in scheme.

# in perl, to apply a function that's returned from another function call,
# we can use one of two forms. Assume that $f, $g, and $h are all
# pointers to functions.  The following are equivalent

#  &{&$f($g)}($h)    and  $f->($g)->($h)
# both expressions are equivalent to ((f g) h) in scheme.  The ->
# operator is not the same as the -> in C/C++, but it's similar:
# it dereferences a pointer, which in the case of Perl, can be a function.

# Goto this link to learn more:
# http://iis1.cps.unizar.es/Oreilly/perl/advprog/ch04_01.htm


sub newaccount
{
   my $balance = $_[0];

   my $inquiry = sub { $balance };
   my $deposit = sub { $balance = $balance + $_[0]; };

   my $chargefee = sub { $balance -= 3; };
   my $withdraw = sub 
		  { $balance = $balance - $_[0]; &$chargefee(); };
  
   # return interface function:
   sub
   {
     my $method = $_[0]; # requested method
     if ($method eq withdraw) { return $withdraw; }
     if ($method eq deposit)  { return $deposit; }
     if ($method eq inquiry)  { return &$inquiry(); }
       else { die "error"; }
   }
}

my $myaccount = &newaccount(500);  # the & is actually optional here.
my $youraccount = &newaccount(800);

my $balance = 50;

$myaccount->(withdraw)->(30);  # ((myaccount 'withdraw) 30)

&{&$youraccount(deposit)}(70);   # alternate syntax

print "my balance is ", $myaccount->(inquiry), "\n";
print "your balance is ", $youraccount->(inquiry), "\n";