In this lesson, we will write a door that can act as a bank account with the ability to withdraw, deposit, and check the account's balance.

:-  %say
|=  *
:-  %noun
=<  =~  new-account
      (deposit 100)
      (deposit 100)
      (withdraw 50)
      balance
    ==
|%
++  new-account
  |_  balance=@ud
  ++  deposit
    |=  amount=@ud
    +>.$(balance (add balance amount))
  ++  withdraw
    |=  amount=@ud
    +>.$(balance (sub balance amount))
  --
--

We start with the three lines we have in every %say generator:

:-  %say
|=  *
:-  %noun

In the above code chunk, we're creating a cell. The head of this cell is %say. The tail is a gate (|= *) that produces another cell (:- %noun) with a head of the mark of a the kind of data we are going to produce, a noun; the tail of the second cell is the rest of the program.

=<  =~  new-account
      (deposit 100)
      (deposit 100)
      (withdraw 50)
      balance
    ==

In this code above, we're going to compose two runes using =<, which has inverted arguments. We use this rune to keep the heaviest twig to the bottom of the code.

=~ is a rune that composes multiple expressions. We take new-account and use that as the subject for the call to deposit. deposit and withdraw both produce a new version of the door that's used in subsequent calls, which is why we are able to chain them in this fashion. The final reference is to balance, which is the account balance contained in the core that we examine below.

|%
++  new-account
  |_  balance=@ud
  ++  deposit
    |=  amount=@ud
    +>.$(balance (add balance amount))
  ++  withdraw
    |=  amount=@ud
    +>.$(balance (sub balance amount))
  --
--

We've chosen here to wrap our door in its own core to emulate the style of programming that is used when creating libraries. new-account is the name of our door. Recall that a door is a core with one or more arms that has a sample. Here, our door has a sample of one @ud with the face balance and two arms deposit and withdraw.

Each of these arms produces a gate which takes an @ud argument. Each of these gates has a similar bit of code inside:

+>.$(balance (add balance amount))

+> is wing syntax. This particular wing construction looks for the tail of the tail (the third element) in $, the subject of the gate we are in. The withdraw and deposit arms create gates with the entire new-account door as the context in their cores' [battery sample context], in the "tail of the tail" slot. We change balance to be the result of adding balance and amount and produce the door as the result. withdraw functions the same way only doing subtraction instead of addition.

It's important to notice that the sample, balance, is stored as part of the door rather than existing outside of it.