DCI money transfer example implemented using Java 8 Lambda functions

[Rickard Öberg]

Hi guys,

I've been playing around with the functional support in Java 8, and
wrote up an attempt at expressing the DCI money transfer example using
functions. All methods are parameter-less and produce functions, and all
state is immutable (accounts and context state).

The code can be found here:
https://github.com/rickardoberg/dcifun/tree/master/src

For laziness sake, here is the test client code that invokes it:
Account from = new Account(100);
Account to = new Account(0);
MoneyTransfer moneyTransfer = new MoneyTransfer(from, to);

moneyTransfer = MoneyTransfer.bind(). // Context binding function
apply(moneyTransfer). // Bind to state
apply(new MoneyTransfer.Transfer(50));
// Resulting money transfer has new state. Old state is untouched

assertThat(moneyTransfer.getFrom().getBalance(), equalTo(50));
assertThat(moneyTransfer.getTo().getBalance(), equalTo(50));
---

bind() looks like this:
public static Function<MoneyTransfer,
Function<Transfer,MoneyTransfer>> bind()
{
return moneyTransfer -> transfer -> MoneySource.transfer().
apply(moneyTransfer.getFrom(),
MoneySink.transferTo().apply(moneyTransfer.getTo())).
apply(transfer);
}

The MoneySource role within the MoneyTransfer context looks like this:
private static class MoneySource
{
static BiFunction<Account, Function<Transfer, Account>,
Function<Transfer, MoneyTransfer>> transfer()
{
return (account, moneySink) -> transfer ->
new MoneyTransfer(withdraw().
apply(account).
apply(transfer.getAmount()),
moneySink.apply(transfer));
}
}
---

And so on... check GitHub for details.

This works with Java 8, Lambda edition. Run as-is or load up in IntelliJ
12 as I did.

The resulting amount of code is ridiculously small. Probably takes some
time to get used to reading all the fun-fun code, but other than that it
was pretty straightforward. I'm not a functional programmer per se, but
wrote this example up in about half an hour.

regards, Rickard

[Ant Kutschera]

A bit boilerplatey...