Curry: An Integrated Functional Logic Language

A.6 Concurrent Object-Oriented Programming
The following example shows a simple method to program in a concurrent object-oriented style in
Curry. For this purpose, an object is a process waiting for incoming messages. The local state is a
parameter of this process. Thus, a process is a function of type
State -> [Message] -> Success
In the subsequent example, we implement a bank account as an object waiting for messages of the
form Deposit i, Withdraw i, or Balance i. Thus, the bank account can be defined as follows:
data Message = Deposit Int | Withdraw Int | Balance Int
account :: Int -> [Message] -> Success
account eval rigid -- account is a consumer
account _ [] = success
account n (Deposit a : ms) = account (n+a) ms
account n (Withdraw a : ms) = account (n-a) ms
account n (Balance b : ms) = b=:=n & account n ms
-- Install an initial account with message stream s:
make_account s = account 0 s
A new account object is created by the constraint make_account s where s is a free variable.
When s is instantiated with messages, the account process starts processing these messages. The
following concurrent conjunction of constraints creates an account and sends messages:
make_account s & s=:=[Deposit 200, Deposit 50, Balance b]
After this goal is solved, the free variable b has been bound to 250 representing the balance after
the two deposits.
To show a client-server interaction, we define a client of a bank account who is finished if his
account is 50, buys things for 30 if his account is greater than 50, and works for an income of
70 if his account is less than 50. In order to get the client program independent of the account
processing, the client sends messages to his account. Therefore, the client is implemented as follows:
-- Send a message to an object identified by its message stream obj:
sendMsg :: [msg] -> msg -> [msg]
sendMsg msg obj | obj =:= msg:obj1 = obj1 where obj1 free
-- Client process of an bank account
client s | s1 =:= sendMsg (Balance b) s =
if b==50 then s1=:=[] -- stop
else if b>50 then client (sendMsg (Withdraw 30) s1) -- buy
else client (sendMsg (Deposit 70) s1) -- work
where s1,b free
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