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