I think this is the thing under the bonnet I was after though to be perfectly honest is slightly beyond me at the time of writing. Thank you all. Jon
Let's just have a look at the monad instance of IO which is defined in the files ghc-prim/GHC/Types.hs and base/GHC/Base.hs
newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))
instance Monad IO where ... (>>=) = bindIO ...
bindIO :: IO a -> (a -> IO b) -> IO b bindIO (IO m) k = IO $ \ s -> case m s of (# new_s, a #) -> unIO (k a)
If you can forget for a minute about all the # you will end up with this.
newtype IO a = IO (RealWorld -> (RealWorld, a))
bindIO (IO m) k = IO $ \ s -> case m s of (new_s, a) -> unIO (k a)
when the following part is evaluated:
case m s of (new_s, a) -> unIO (k a)
(m s) has to be evaluated first in order to ensure that the result matches the pattern (new_s, a) and is not bottom/some infinite calculation/an error.
This is why IO statements are evaluated in order.
Silvio _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe