As Simon explained to me recently, that is the problem with this type of lazy IO: the sequence of operations is undefined. There is no requirement that the read consume only the characters that have already been requested; in fact, that would likely be horribly inefficient. Plus, it is quite possible that the underlying operating system is consuming both characters before it feeds the first one to GHC. So, this type of IO should only be used when you don't care about the precise order of operations. On Monday 16 September 2002 18:06, David Sabel wrote:
The runtime system mostly has no concept of IO vs. non-IO evaluation. The IO monad is implemented as the following type:
newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #)
which means that an IO action is simply a function from the world state to a pair of a new world state and a value. In other words, it's a straightfoward state monad, except that we use some optimised representations to eliminate some of the overhead of passing the state around.
Cheers, Simon
Ok so far, but when is the IO action performed?
Look at this small programm:
module Main(main) where
main = echoTwice
echo = getChar >>= putChar
echoTwice = echo >> echo
while executing: the program reads first two chararcters and then it writes the characters, but in my opinion it would be right to read one character, then write one characater, then read, then write. What's the reason for it?
---------------------------------- David Sabel JWGU Frankfurt
_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
-- ----------------------------------- Seth Kurtzberg M. I. S. Corp. 1-480-661-1849