Futures in Haskell

问题:

Does Haskell have an equivalent of Alice's ability to bind a variable to a future?

val a = spawn foo;

where foo is some function.

I know Haskell supports channels and threads; I'm hoping for syntax as natural as Alice's to bind a value to a future and spawn a thread to calculate it without having to deal with the details.

回答1:

You can use par from Control.Parallel as in

a `par` f a b c
where
  a = foo

This is a hint to the runtime that a could be evaluated in another thread.

回答2:

Funny, I was just reading a new post by Simon Marlow: Parallel programming in Haskell with explicit futures. Apparently he and others have been working on some new parallel programming abstractions that are intended to be more natural and explicit than the par and pseq APIs.

回答3:

Not in the standard library, but

http://ghcmutterings.wordpress.com/2010/08/20/parallel-programming-in-haskell-with-explicit-futures/

data Future a = Future a

fork :: Eval a -> Eval (Future a)
fork a = do a' <- rpar (runEval a); return (Future a')

join :: Future a -> Eval a
join (Future a) = a `pseq` return a