[Haskell-cafe] Implementing PacMan

Ryan Ingram ryani.spam at gmail.com
Wed Dec 17 03:13:32 EST 2008

In the last "episode" you talk about an entity's update being a function like:

> input_state -> (output_state, [event])

for some suitably defined types of input state, output state, and event.

Connecting together functions with types like this is exactly what
Reactive does well.  You have an event stream representing the
behavior of the user during the course of the game, and you use that
to construct other values representing the *behavior* of the game.
The key difference is that instead of the game's behavior being a set
of functions from input to output, it is functions from one behavior
to another, eventually culminating in something like

> -- "user input", "clock tick"; output a new display list at each clock tick
> game :: Event UserInput -> Event () -> Reactive [RenderCommand]

In fact, I think your analysis in the first message is somewhat right
on; this exercise is somewhat like "writing a novel without using the
letter 'e'".  The benefits you get from a pure language are not
necessarily "lack of effects", but rather, "explicitly mentioning
where effects happen".

That said, I'm curious where you are going with it.  I'd love to see
the game running & take a look at your code!

  -- ryan

On Tue, Dec 16, 2008 at 12:43 PM, Andrew Coppin
<andrewcoppin at btinternet.com> wrote:
> What do we think of this, folks?
> http://prog21.dadgum.com/23.html
> (And the rest in the series, obviously.)
> To me, it seems that this plan would *work*... but it wouldn't be very
> eligant. You'd have the code to respond to user input and move PacMan in one
> place, the code for collision detection in other place, the code that
> decides what to *do* about a collision somewhere else, the code for drawing
> the animations in yet another place... If you wanted to change one game
> element, you'd have to make changes scattered all over the place. The whole
> thing seems messy, non-modular and non-composible to my ears.
> Thoughts, people?
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