How about this:
type ActionLog v = Writer [IO v]
myTell :: v -> ActionLog v () myTell a = tell [sleep 1 >> return a]
foo :: ActionLog Int () foo = mapM_ myTell [1..10]
main = sequence_ results where (_, vals) = runWriter foo results = map (>>= print) vals
-- ryan
On Sun, May 3, 2009 at 2:17 PM, Magnus Therning
I've been playing around with (WriterT [Int] IO), trying to get the log out and map `print` over it... and do it lazily. However, I'm not really happy with what I have so far, since I've had to resort to `unsafePerformIO`. Any hints are welcome.
What I have so far is:
foo = let _tell i = do a <- return $ unsafePerformIO $ sleep 1 tell [a + 1 `seq` i] in do mapM_ _tell [1..10]
main = do (_, ~res) <- runWriterT foo mapM_ print res
Without the `seq` the call to sleep will simply be skipped (is there an easier way to force evaluation there?). Without `unsafePerformIO` all the sleeping is done up front, and all numbers are print at once at the end.
The goal is of course to use code along the same shape to do something more useful, and then `unsafePerformIO` will really be unsafe...
/M
-- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe
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