Although I am sure I am not the first to discover this, I thought it was a neat application of the runST trick. Safe non-local returns for the IO monad. What I mean by safe is that it is impossible to return to a context that no longer exists. the api is simple:

module System.IO.Continuation(IOCont(),newContinuation,callContinuation) where

data IOCont s a = ....

newContinuation :: (forall s . IOCont s a -> IO b) -> (a -> IO b) -> IO b newContinuation act cc = ...

callContinuation :: IOCont s a -> a -> IO b callContinuation cont x = ...

newContinuation runs its first argument passing a fresh jumppoint into it, if it is jumped to then the argument is passed to the second argument of newContinuation. the universal quantification means that the continuation is unable to escape the action so you know its context is still available on the stack. in jhc these are just implemented as straight FFI calls to setjmp(2) and longjmp(2) and an IORef. I feel using the term 'continuation' is something of a misnomer as true continuations would allow jumping between multiple stacks I would think but am unsure what a good name for this is then. Is there a fully typesafe interface for 'true' IO continuations for some definition of 'true'? hmm.... John -- John Meacham - ⑆repetae.net⑆john⑈