[Haskell-cafe] Execution order in IO

[silvio]

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