I'm not sure this works. Consider this newContinuation (\k -> return (callContinuation k)) ... The partial application (callContinuation k) has no 's' in its type, and so can go anywhere. Simon | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of John | Meacham | Sent: 21 February 2006 12:40 | To: haskell-cafe@haskell.org | Subject: [Haskell-cafe] typesafe non-local returns in the IO monad | | 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⑈ | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe