Luke Palmer lrpalmer at gmail.com
Tue Mar 18 08:51:18 EDT 2008

```On Tue, Mar 18, 2008 at 12:24 PM, iliali16 <iliali16 at gmail.com> 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.

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
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)

:-)

Luke
```