# bank accounts using closures in ruby

# Ruby variables: lower case are local, upper-case are global.  So in order
# to have nested scope, we need to have nested lambda-expressions.

# Here's a basic boolean toggle:

def toggle()
  val = true   # initial value
  lambda{ val = !val; val }
end
t = toggle()
t[]  # returns false
t[]  # returns true


### bank accounts.  
## instead of a series of if-else statements or 'cond', I will use a 
## Ruby hash table to store the fields of the "object"

def newaccount(bal)  # accounts are created with initial balance
  # a "private" method to chare a 5% fee for withdraws.
  chargefee = lambda {|x| bal = bal - (x/20.0)}
  # return the following hash table:
  {
     "inquiry" => lambda { bal },
     "withdraw" => lambda {|x| if bal>=x then bal -= x end;  chargefee[x]},
     "deposit"  => lambda {|x| if x>0 then bal += x end},
     "compareTo" => lambda{|other| bal - other["inquiry"][]}
  }
end #newaccount

## Note that the "interface" returned by the newaccount function is a closure
## involving bal, the account balance.  Unlike in scheme, the interface takes
## the form of a hash table.  We could use a lambda like in scheme, but
## a hash table more effectively implements this kind of a "function".

Myaccount = newaccount(100)
Youraccount = newaccount(200)
Myaccount["withdraw"][30]     # myaccount.withdraw(30);
Youraccount["deposit"][50]
puts Myaccount["inquiry"][]
puts Myaccount["compareTo"][Youraccount]

## note to change the default value returned by a hash table, initialize with
## h = Hash.new("don't know")