On Tue, 2010-09-14 at 11:27 +0100, Neil Brown wrote:
On 13/09/10 17:25, Maciej Piechotka wrote:
import Control.Exception import Foreign import Prelude hiding (catch)
data StrictMonad a = StrictMonad a deriving Show
instance Monad StrictMonad where return x = unsafePerformIO $ do (return $! x) `catch` \(SomeException _) -> return x return $! StrictMonad x StrictMonad v>>= f = f v
It seems to be valid IMHO Functor and Monad (I haven't prove it) as long as functions terminates.
I'm not sure if I'm allowed to use unsafePerformIO in my counter-example, but you used it so why not ;-) The first monad law says: "return a >>= k = k a"
let k = const (StrictMonad ()) a = unsafePerformIO launchMissiles
In "k a" no missiles will be launched, in "return a >>= k", they will be launched.
I guess we enter a grey area - I did use unsafePerformIO but without side-effects.
You can construct a similar example against "m >>= return = m".
Assuming StrictMonad (constructor) is hidden - I don't think so.
Although, if you changed your definition of bind to:
StrictMonad v >>= f = return v >>= f >>= return
Then as long as "return x >>= return = return x" (which it does for you) then you automatically satisfy the first two monad laws! Which is an interesting way of solving the problem -- haven't checked the third law though.
My error.
Thanks,
Neil.
Regards