[Haskell-cafe] Execution order in IO

[Jon Schneider]

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
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