sequence isn't lazy (not in the IO monad at least); it will try to run to completion, returning an infinite list of (as yet unevaluated, due
I should have learned that lesson already.. This is the second time I could have needed a lazy IO monad version.. Does something like this already exist? ============= LazyIO test ============================================ module Main where import Control.Monad import System.IO.Unsafe import Random data LazyIO a = LazyIO (IO a) -- conversion unLazy :: LazyIO a -> IO a unLazy (LazyIO a) = a -- my lazy monad instance Monad LazyIO where return a = LazyIO (return a) (LazyIO m) >>= k = LazyIO $ unsafeInterleaveIO $ m >>= unLazy . k main = do print "LazyIO test" putStrLn "this should work : (LazyIO version)" randoms <- unLazy . sequence . cycle $ [ LazyIO (randomIO :: IO Int) ] print $ take 5 randoms putStrLn "this should hang : (IO version)" randoms <- sequence . cycle $ [ randomIO :: IO Int ] print $ take 5 randoms ============= LazyIO test ============================================ compare this (adding unLazy and LazyIO) to reimplementing sequence, mapM, ...
unsafeInterleaveSequence :: [IO a] -> IO [a] unsafeInterleaveSequence [] = return [] unsafeInterleaveSequence (x:xs) = unsafeInterleaveIO $ liftM2 (:) x (unsafeInterleaveSequence xs)
I really start to love haskell :) Marc