Re: [Haskell-cafe] Small question about something easy

On 18 mar 2008, at 13.51, Luke Palmer wrote:

On Tue, Mar 18, 2008 at 12:24 PM, iliali16 wrote:

...

Now the problem comes here: play (p1 :>: p2) state |play p1 state == (i1,state1) && play p2 state1 == (i2,state2) = (i1+++i2,state2)

I know that if I manage to do that function the one above with this sign :>: do not need to be impelmented since this one will cater for all the cases. Can you please help me?

You just need a nice simple let or where clause:

play (p1 :>: p2) state = (i1 +++ i2, state2) where (i1,state1) = play p1 state (i2,state2) = play p2 state1

Or equivalently:

play (p1 :>: p2) state = let (i1, state1) = play p1 state (i2, state2) = play p2 state1 in (i1 +++ i2, state2)

And there's nothing lazily recursive about these, just the information usage is a little more complex. But it could be implemented perfectly naturally in scheme, for example.

For further exploration: the pattern here where the state is threaded through different computations, is captured by the module Control.Monad.State. So if "play" returned an object of a State monad, such as:

play :: Logo -> State TurtleState Image

Then this case could be implemented as:

play (p1 :>: p2) = do i1 <- play p1 i2 <- play p2 return (i1 +++ i2)

Pretty, ain't it? A little too pretty if you ask me. Let's make it uglier and shorter still:

play (p1 :>: p2) = liftM2 (+++) (play p1) (play p2)

Or use Applicative directly: play (p1 :>: p2) = (+++) <$> play p1 <*> play p2