On 25 April 2016 at 21:03, Rafał Cieślak <ravici...@gmail.com> wrote:
> Hi,
>
> I'm writing a turn-based game and one of the biggest functions is the one
> which handles player moves. It receives the current game state (a map) and
> the index of the cell that the player clicked on (an integer).
>
> It has to make a couple of decisions based on the game state. So far I
> just used -> and cond->. The code looks more or less like this, I removed
> a bunch of function calls for brevity.
>
> (-> game-state
> ; This stuff happens on each move.
> (reveal-cell cell-index)
> ; And this stuff happens only under certain conditions.
> (cond->
> (zero? (:surrounding-power cell))
> (reveal-safe-cells cell-index)))
>
> The code looks clear IMHO. But then I wanted to add another feature which
> requires me to make decisions based on the updated value of game-state
> (the one that is being threaded inside cond->).
>
> I thought about cond-> a bit and came to the conclusion that the reason
> you can't access the currently threaded value is to keep you from writing
> order-dependent statements. But in my case, I sometimes do want to write
> order-dependent code. For example, I want to give player exp for killing a
> monster and then if the exp is above certain threshold, I want to level up
> his character.
>
> So the problem is that I have to check the current value being threaded,
> but cond-> doesn't allow me to do that.
>
>
You don't need a macro for this sort of thing, and can get some way with
reduce. For example:
(defn apply-conditional-actions
[game-state & pred-actions]
(reduce (fn [game-state [pred action]]
(if (pred game-state)
(action game-state)
game-state))
game-state
(partition 2 pred-actions)))
Then call this function with the game-state and a list of pred/action pairs:
(apply-conditional-actions game-state
safe-cell? reveal-safe-cells
monster-killed? inc-exp-points)
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