Using Cont to acquire values from the future and the past

Question

I\'m writing a brainfuck interpreter in Haskell, and I came up with what I believe to be a very interesting description of a program:

data Program m = Instructio         


        

Answer 1

Being terribly lazy with this answer since I'm not comfortable with Cont, but is MonadFix perhaps what you're looking for? State is an instance, though not Cont, and it lets you do things that look like (using "recursive do" notation):

{-# LANGUAGE DoRec #-}
parseInst str = do
    rec ctl <- parseInstructionsLinkingTo ctl str

This was the solution I discovered for my actors library: we want a spawn operation that returns the spawned actor's mailbox, but then how can we launch mutually-communicating actors? Or an actor with access to its own mailbox?

With a suitable MonadFix instance we can do:

fork3 = do
    rec mb1 <- spawn $ actorSpamming mb2 mb3
        mb2 <- spawn $ actorSpamming mb1 mb2
        mb3 <- spawn $ actorSpamming mb2 mb3
    send "go" mb1

Hope above gives you ideas.