I'm working on a turn-based game and I'm looking for a good way to manage
states. In the game each turn is composed of multiple phases. I started by
using atoms for the phases field (this is a sequence of functions) in a
record and realized that it wouldn't be ideal to keep track of states in
the case where I'd need to keep a snapshot of every phases. Here's the
original code I had:
(defrecord Game [phases]
(next-phase [this]
(stop-timer)
(swap! phases #(conj (vec (rest %)) (first %)))
(log :info "change phase to %s" (key (first @phases)))
(start-phase this))
I then started to think that this would be a good opportunity to use a
state monad. I've tried to reimplement the above code using the algo.monads
library but the result was less than satisfactory (probably due to my own
shortcoming), here's the monadic version:
(defrecord Game [phases]
(next-phase [this]
(->
((domonad state-m
[_ (fn [s] (stop-timer) [s s])
_ (update-state
(fn [s]
(update-in s [:phases]
#(conj (vec (rest %)) (first %)))))
_ (fn [s]
(log :info "change phase to %s" (key (first (:phases s)))) [s s])]
nil)
state)
second
start-phase))
As my code probably doesn't need the full power of the state monad, I tried
to write a lighter-weight version using the following macro:
(defmacro >> [& state-and-forms]
(reduce #(list (if ('#{fn fn*} (first %2))
%2
`(fn [s#] ~%2 s#)) %)
state-and-forms))
Which let me write:
(next-phase [state]
(>> state
(stop-timer)
(fn [s] (update-in s [:phases] #(conj (vec (rest %)) (first %))))
#(do (log :info "change phase to %s" (key (first (:phases %)))) %)
#(start-phase %)))
With some more helper macro this version looks promising. In the end I
wonder if there's some Clojure feature I'm overlooking or if I should
rethink the whole solution? Is there a better way to accomplish this?
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