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Blue

An actor that remembers

An actor is a small, independent part of a program, with its own state, that you talk to by sending it messages. In Blue, actors are also durable: their state is kept, and outlives the program.

Save this as account.blue:

type AccountMsg =
  | Deposit Int
  | Balance (Reply Int)


actor Account (id: String) receives AccountMsg
  state balance = 0


  on Deposit amount =
    become { balance = balance + amount }


  on Balance replyTo =
    send replyTo balance


fn main () =
  let acct = Account "acc-42"
  print "balance at start: {ask acct Balance}"
  send acct (Deposit 100)
  print "balance now: {ask acct Balance}"
balance at start: 0
balance now: 100

Reading it from the top:

  • AccountMsg lists the messages an account understands. Balance carries a Reply Int: whoever asks gets an Int back.
  • actor Account (id: String) declares a kind of actor. Each id is a separate account: Account "acc-42" and Account "acc-7" have their own balances.
  • state balance = 0 is the state a new account starts with. on clauses say what to do with each message, and become gives the actor its new state.
  • send delivers a message and carries on; ask sends one and waits for the reply.

Run it twice

Section titled “Run it twice”
Terminal window
blue run account.blue
blue run account.blue

The second run starts where the first left off:

balance at start: 100
balance now: 200

There is no save call and no database code. blue run keeps the program’s actors in account.db, beside the file, and each message is committed as a whole: the new state, the messages it sent, and a journal entry, together or not at all. Stop the program at any moment, even with kill -9, and nothing is half-done. To start again from nothing, delete account.db.

Look inside

Section titled “Look inside”

The REPL can open the same store and show an actor’s state and every message it has handled:

$ blue repl --store account.db
blue> :load account.blue
loaded account.blue: 2 names (Account, main); run it with main ()
blue> :state Account "acc-42"
  balance = 200
after 6 messages
blue> :journal Account "acc-42"
     1  Balance (<reply>)
     2  Deposit 100
     3  Balance (<reply>)
     4  Balance (<reply>)
     5  Deposit 100
     6  Balance (<reply>)

The journal is what makes the rest of Blue possible: blue replay account.blue runs every actor’s history again and checks it comes out the same, and the simulator uses it to test what happens when things fail.

Next: watch it in a browser.