I am working with SICP and exercise 2.29-b gave me the opportunity to have fun with the Continuation Passing Style while traversing mobiles and branches.
To make the story short, each mobile has left and right branch, which are composed by a length and either a numeric weight or another mobile. The question asks to find the total weight given a mobile.
After the first mutually recursive solution, quite simple, I tried and successfully implemented a cps' one:
(defn total-weight-cps [mobile]
(letfn
[(branch-weight-cps
[branch kont]
(let [structure (branch-structure branch)]
(if (mobile? (branch-structure branch))
(do (println "then " structure) (kont (traverse-mobile-cps structure identity)))
(do (println "else " structure) (kont structure)))))
(traverse-mobile-cps
[mobile kont]
(branch-weight-cps (left-branch mobile)
(fn [left-weight]
(branch-weight-cps (right-branch mobile)
(fn [right-weight] (kont (+ left-weight right-weight)))))))]
(traverse-mobile-cps mobile identity)))
At this point, I have tried to apply the trampoline in order to preserve my stack. But it blows with the following exception:
java.lang.ClassCastException: sicp_clojure.2_1_exercises_2_24_2_32$total_weight_STAR_$traverse_mobile_cps__6694$fn__6695$fn__6696$fn__6697 cannot be cast to java.lang.Number
Numbers.java:126 clojure.lang.Numbers.add
.../git/sicp-clojure/src/sicp_clojure/2_1_exercises_2_24_2_32.clj:185 sicp-clojure.2-1-exercises-2-24-2-32/total-weight*[fn]
core.clj:5801 clojure.core/trampoline
core.clj:5806 clojure.core/trampoline
RestFn.java:439 clojure.lang.RestFn.invoke
.../git/sicp-clojure/src/sicp_clojure/2_1_exercises_2_24_2_32.clj:186 sicp-clojure.2-1-exercises-2-24-2-32/total-weight*
The code using trampoline, following the excellent link, is:
(defn total-weight* [mobile]
(letfn
[(branch-weight-cps
[branch kont]
(let [structure (branch-structure branch)]
(if (mobile? (branch-structure branch))
(do (println "then " structure) (kont (traverse-mobile-cps structure identity)))
(do (println "else " structure) (kont structure)))))
(traverse-mobile-cps
[mobile kont]
(branch-weight-cps (left-branch mobile)
(fn [left-weight]
(branch-weight-cps (right-branch mobile)
(fn [right-weight] #(kont (+ left-weight right-weight)))))))]
(trampoline traverse-mobile-cps mobile identity)))
And finally some sample data:
(def branch11 (make-branch 1 1))
(def branch22 (make-branch 2 2))
(def branch36 (make-branch 3 6))
(def branch43 (make-branch 4 3))
(def mobile11-43 (make-mobile branch11 branch43))
(def mobile36-22 (make-mobile branch36 branch22))
(def branch5m1143 (make-branch 5 mobile11-43))
(def branch7m3622 (make-branch 7 mobile36-22))
(def mobile5m1143-7m3622 (make-mobile branch5m1143 branch7m3622))
(total-weight* mobile5m1143-7m3622)
Why does it blow up?
Following the same link in my post, I have solved turning my implementation in:
(defn total-weight* [mobile]
(letfn
[(branch-weight-cps
[branch kont]
(let [structure (branch-structure branch)]
(if (mobile? (branch-structure branch))
(fn [] (traverse-mobile-cps structure kont))
(fn [] (kont structure)))))
(traverse-mobile-cps
[mobile kont]
(branch-weight-cps (left-branch mobile)
(fn [left-weight]
(branch-weight-cps (right-branch mobile)
(fn [right-weight] #(kont (+ left-weight right-weight)))))))]
(trampoline traverse-mobile-cps mobile identity)))
Related
I saw a talk about railroad oriented programming (https://www.youtube.com/watch?v=fYo3LN9Vf_M), but i somehow do not get how to work this out, if i use reduce, because reduce has two or even three arguments.
How am i able to to put the following code like a railroad? I seems to me hard, because of reduce taking a function as an argument in addition to the game object.
(defn play-game-reduce []
(let [game-init
(->>
(io/initialize-cards-and-players)
(shuffle-and-share-cards myio/myshuffle)
(announce))
play-round
(reduce play-card (assoc-in game-init [:current-trick] '()) [:p1 :p2 :p3 :p4])]
(reduce play-round game-init (range (get game-init :round-count)))))
The whole code is here:
https://github.com/davidh38/doppelkopf/blob/master/src/mymain.clj
The code should more look like this:
(->> (io/initialize-cards-and-players)
(shuffle-and-share-cards myio/myshuffle)
(announce)
reduce (play-round .. )
reduce (play-card ...))
That would look to me much more explicit.
That video was made for a different language and you can't directly transfer these ideas to Clojure.
I looked at your source code and there are some things to improve:
(defn play-card-inp []
(eval (read-string (read-line))))
You shouldn't use eval in production code.
Read-string is unsafe and you should use clojure.edn/read-string instead. I'm not sure what is expected input here and what is the result of the evaluation, maybe you should use just clojure.edn/read here.
(defn myshuffle [cards]
(shuffle cards)
)
(defn initialize-cards-and-players []
; init cards
(def cards '([0 :c], [1 :c],[2 :c], [3 :c], [0 :s], [1 :s], [2 :s], [3 :s]))
(def players '(:p1 :p2 :p3 :p4))
;(def round-players (take 4 (drop (who-won_trick tricks) (cycle (keys players)))))
; mix and share cards
{:players (zipmap players (repeat {:cards () :tricks ()}))
:current-trick ()
:round-start-player :p1
:cards cards
:round-count (/ (count cards) (count players))
:mode ""
})
You should delete myshuffle and use directly shuffle where needed. Ending parenthesis shouldn't be on a separate line.
Don't use def (creates global variable) inside defn, use let (creates local variables). I would rewrite this as:
(defn new-deck []
(for [letter [:c :s]
number (range 4)]
[number letter]))
(defn new-game []
(let [cards (new-deck)
players [:p1 :p2 :p3 :p4]]
{:players (zipmap players (repeat {:cards () :tricks ()}))
:current-trick ()
:round-start-player :p1
:cards cards
:round-count (/ (count cards) (count players))
:mode ""}))
Notes for mymain.clj:
(defn who-won-trick [trick]
(eval (read-string (read-line))))
Some unused function, same problems as above.
(defn share-card-to-player [game players-cards]
(assoc game
:players
(assoc
(get game :players)
(first players-cards)
(assoc (get (game :players) (first players-cards))
:cards
(second players-cards)))))
Use assoc-in and some destructuring, something like this:
(defn share-card-to-player [game [player cards]]
(assoc-in game [:players player :cards] cards))
Your next function:
(defn shuffle-and-share-cards [myshuffle game]
(reduce share-card-to-player game
(map vector
(keys (get game :players))
(->> (get game :cards)
(myshuffle)
(partition (/ (count (get game :cards))
(count (get game :players))))))))
You can also destructure hash-maps, so I would rewrite this as:
(defn shuffle-and-share-cards [{:keys [players cards] :as game}]
(let [card-piles (->> cards
shuffle
(partition (/ (count cards)
(count players))))]
(reduce share-card-to-player game
(map vector
(keys players)
card-piles))))
Next functions:
(defn announce [game]
game)
(defn play-card [game curr-player]
(println curr-player)
(println game)
(let [played-card (io/play-card-inp)]
(->
(assoc-in game [:players curr-player :cards]
(remove #(= played-card %) (get-in game [:players curr-player :cards])))
(assoc-in [:current-trick]
(conj (game [:current-trick]) played-card)))))
announce is useless and update and update-in are better here:
(defn play-card [game curr-player]
(println curr-player)
(println game)
(let [played-card (io/play-card-inp)]
(-> game
(update-in [:players curr-player :cards] #(remove #{played-card} %))
(update :current-trick conj played-card))))
And finally, the last two functions:
(defn play-game-reduce []
(let [game-init
(->>
(io/initialize-cards-and-players)
(shuffle-and-share-cards myio/myshuffle)
(announce))
play-round
(reduce play-card (assoc-in game-init [:current-trick] '()) [:p1 :p2 :p3 :p4])]
(reduce play-round game-init (range (get game-init :round-count)))))
(defn play-game []
(let [game-init
(->>
(io/initialize-cards-and-players)
(shuffle-and-share-cards io/myshuffle)
(announce))]
(loop [round 1 game game-init]
(let [game-next (loop [curr-player 1 game-next game]
(if (> curr-player 4)
game-next
(recur (inc curr-player)
(play-card game-next (keyword (str "p" curr-player))))))]
(if (> round 2)
game-next
(recur (inc round) game-next))))))
loop/recur will be probably more readable, but two reduce should also work:
(defn play-game-reduce []
(let [game-init (-> (io/new-game)
shuffle-and-share-cards)]
(reduce (fn [game round]
(reduce play-card (assoc-in game [:current-trick] '()) [:p1 :p2 :p3 :p4]))
game-init
(range (get game-init :round-count)))))
(play-game-reduce)
Version with one reduce:
(defn play-game-reduce []
(let [game-init (-> (io/new-game)
shuffle-and-share-cards)
turns (for [round (range (:round-count game-init))
player [:p1 :p2 :p3 :p4]]
[round player])]
(reduce (fn [game [round player]]
(let [state (cond-> game
(= player (:round-start-player game)) (assoc-in [:current-trick] '()))]
(play-card state player)))
game-init
turns)))
And I also noticed that there's no validation of whether the current player can really play inserted card.
OK, I watched the talk (for the record, it gives a 5 minute overview of FP, then discusses error handling in pipelines in F#.
I didn't really care for the content of the video.
Clojure uses Exceptions for error handling, so a Clojure function always has only one output. Therefore the whole bind and map thing in the video doesn't apply.
I haven't looked at F# much before, but after watching that video I think it over-complicates things without much benefit.
I'm writing an agar.io clone. I've lately seen a lot of suggestions to limit use of records (like here), so I'm trying to do the whole project only using basic maps.*
I ended up creating constructors for different "types" of bacteria like
(defn new-bacterium [starting-position]
{:mass 0,
:position starting-position})
(defn new-directed-bacterium [starting-position starting-directions]
(-> (new-bacterium starting-position)
(assoc :direction starting-directions)))
The "directed bacterium" has a new entry added to it. The :direction entry will be used to remember what direction it was heading in.
Here's the problem: I want to have one function take-turn that accepts the bacterium and the current state of the world, and returns a vector of [x, y] indicating the offset from the current position to move the bacterium to. I want to have a single function that's called because I can think right now of at least three kinds of bacteria that I'll want to have, and would like to have the ability to add new types later that each define their own take-turn.
A Can-Take-Turn protocol is out the window since I'm just using plain maps.
A take-turn multimethod seemed like it would work at first, but then I realized that I'd have no dispatch values to use in my current setup that would be extensible. I could have :direction be the dispatch function, and then dispatch on nil to use the "directed bacterium"'s take-turn, or default to get the base aimless behavior, but that doesn't give me a way of even having a third "player bacterium" type.
The only solution I can think of it to require that all bacterium have a :type field, and to dispatch on it, like:
(defn new-bacterium [starting-position]
{:type :aimless
:mass 0,
:position starting-position})
(defn new-directed-bacterium [starting-position starting-directions]
(-> (new-bacterium starting-position)
(assoc :type :directed,
:direction starting-directions)))
(defmulti take-turn (fn [b _] (:type b)))
(defmethod take-turn :aimless [this world]
(println "Aimless turn!"))
(defmethod take-turn :directed [this world]
(println "Directed turn!"))
(take-turn (new-bacterium [0 0]) nil)
Aimless turn!
=> nil
(take-turn (new-directed-bacterium [0 0] nil) nil)
Directed turn!
=> nil
But now I'm back to basically dispatching on type, using a slower method than protocols. Is this a legitimate case to use records and protocols, or is there something about mutlimethods that I'm missing? I don't have a lot of practice with them.
* I also decided to try this because I was in the situation where I had a Bacterium record and wanted to create a new "directed" version of the record that had a single field direction added to it (inheritance basically). The original record implemented protocols though, and I didn't want to have to do something like nesting the original record in the new one, and routing all behavior to the nested instance. Every time I created a new type or changed a protocol, I would have to change all the routing, which was a lot of work.
You can use example-based multiple dispatch for this, as explained in this blog post. It is certainly not the most performant way to solve this problem, but arguably more flexible than multi-methods as it does not require you to declare a dispatch-method upfront. So it is open for extension to any data representation, even other things than maps. If you need performance, then multi-methods or protocols as you suggest, is probably the way to go.
First, you need to add a dependency on [bluebell/utils "1.5.0"] and require [bluebell.utils.ebmd :as ebmd]. Then you declare constructors for your data structures (copied from your question) and functions to test those data strucutres:
(defn new-bacterium [starting-position]
{:mass 0
:position starting-position})
(defn new-directed-bacterium [starting-position starting-directions]
(-> (new-bacterium starting-position)
(assoc :direction starting-directions)))
(defn bacterium? [x]
(and (map? x)
(contains? x :position)))
(defn directed-bacterium? [x]
(and (bacterium? x)
(contains? x :direction)))
Now we are going to register those datastructures as so called arg-specs so that we can use them for dispatch:
(ebmd/def-arg-spec ::bacterium {:pred bacterium?
:pos [(new-bacterium [9 8])]
:neg [3 4]})
(ebmd/def-arg-spec ::directed-bacterium {:pred directed-bacterium?
:pos [(new-directed-bacterium [9 8] [3 4])]
:neg [(new-bacterium [3 4])]})
For each arg-spec, we need to declare a few example values under the :pos key, and a few non-examples under the :neg key. Those values are used to resolve the fact that a directed-bacterium is more specific than just a bacterium in order for the dispatch to work properly.
Finally, we are going to define a polymorphic take-turn function. We first declare it, using declare-poly:
(ebmd/declare-poly take-turn)
And then, we can provide different implementations for specific arguments:
(ebmd/def-poly take-turn [::bacterium x
::ebmd/any-arg world]
:aimless)
(ebmd/def-poly take-turn [::directed-bacterium x
::ebmd/any-arg world]
:directed)
Here, the ::ebmd/any-arg is an arg-spec that matches any argument. The above approach is open to extension just like multi-methods, but does not require you to declare a :type field upfront and is thus more flexible. But, as I said, it is also going to be slower than both multimethods and protocols, so ultimately this is a trade-off.
Here is the full solution: https://github.com/jonasseglare/bluebell-utils/blob/archive/2018-11-16-002/test/bluebell/utils/ebmd/bacteria_test.clj
Dispatching a multimethod by a :type field is indeed polymorphic dispatch that could be done with a protocol, but using multimethods allows you to dispatch on different fields. You can add a second multimethod that dispatches on something other than :type, which might be tricky to accomplish with a protocol (or even multiple protocols).
Since a multimethod can dispatch on anything, you could use a set as the dispatch value. Here's an alternative approach. It's not fully extensible, since the keys to select are determined within the dispatch function, but it might give you an idea for a better solution:
(defmulti take-turn (fn [b _] (clojure.set/intersection #{:direction} (set (keys b)))))
(defmethod take-turn #{} [this world]
(println "Aimless turn!"))
(defmethod take-turn #{:direction} [this world]
(println "Directed turn!"))
Fast paths exist for a reason, but Clojure doesn't stop you from doing anything you want to do, per say, including ad hoc predicate dispatch. The world is definitely your oyster. Observe this super quick and dirty example below.
First, we'll start off with an atom to store all of our polymorphic functions:
(def polies (atom {}))
In usage, the internal structure of the polies would look something like this:
{foo ; <- function name
{:dispatch [[pred0 fn0 1 ()] ; <- if (pred0 args) do (fn0 args)
[pred1 fn1 1 ()]
[pred2 fn2 2 '&]]
:prefer {:this-pred #{:that-pred :other-pred}}}
bar
{:dispatch [[pred0 fn0 1 ()]
[pred1 fn1 3 ()]]
:prefer {:some-pred #{:any-pred}}}}
Now, let's make it so that we can prefer predicates (like prefer-method):
(defn- get-parent [pfn x] (->> (parents x) (filter pfn) first))
(defn- in-this-or-parent-prefs? [poly v1 v2 f1 f2]
(if-let [p (-> #polies (get-in [poly :prefer v1]))]
(or (contains? p v2) (get-parent f1 v2) (get-parent f2 v1))))
(defn- default-sort [v1 v2]
(if (= v1 :poly/default)
1
(if (= v2 :poly/default)
-1
0)))
(defn- pref [poly v1 v2]
(if (-> poly (in-this-or-parent-prefs? v1 v2 #(pref poly v1 %) #(pref poly % v2)))
-1
(default-sort v1 v2)))
(defn- sort-disp [poly]
(swap! polies update-in [poly :dispatch] #(->> % (sort-by first (partial pref poly)) vec)))
(defn prefer [poly v1 v2]
(swap! polies update-in [poly :prefer v1] #(-> % (or #{}) (conj v2)))
(sort-disp poly)
nil)
Now, let's create our dispatch lookup system:
(defn- get-disp [poly filter-fn]
(-> #polies (get-in [poly :dispatch]) (->> (filter filter-fn)) first))
(defn- pred->disp [poly pred]
(get-disp poly #(-> % first (= pred))))
(defn- pred->poly-fn [poly pred]
(-> poly (pred->disp pred) second))
(defn- check-args-length [disp args]
((if (= '& (-> disp (nth 3) first)) >= =) (count args) (nth disp 2)))
(defn- args-are? [disp args]
(or (isa? (vec args) (first disp)) (isa? (mapv class args) (first disp))))
(defn- check-dispatch-on-args [disp args]
(if (-> disp first vector?)
(-> disp (args-are? args))
(-> disp first (apply args))))
(defn- disp*args? [disp args]
(and (check-args-length disp args)
(check-dispatch-on-args disp args)))
(defn- args->poly-fn [poly args]
(-> poly (get-disp #(disp*args? % args)) second))
Next, let's prepare our define macro with some initialization and setup functions:
(defn- poly-impl [poly args]
(if-let [poly-fn (-> poly (args->poly-fn args))]
(-> poly-fn (apply args))
(if-let [default-poly-fn (-> poly (pred->poly-fn :poly/default))]
(-> default-poly-fn (apply args))
(throw (ex-info (str "No poly for " poly " with " args) {})))))
(defn- remove-disp [poly pred]
(when-let [disp (pred->disp poly pred)]
(swap! polies update-in [poly :dispatch] #(->> % (remove #{disp}) vec))))
(defn- til& [args]
(count (take-while (partial not= '&) args)))
(defn- add-disp [poly poly-fn pred params]
(swap! polies update-in [poly :dispatch]
#(-> % (or []) (conj [pred poly-fn (til& params) (filter #{'&} params)]))))
(defn- setup-poly [poly poly-fn pred params]
(remove-disp poly pred)
(add-disp poly poly-fn pred params)
(sort-disp poly))
With that, we can finally build our polies by rubbing some macro juice on there:
(defmacro defpoly [poly-name pred params body]
`(do (when-not (-> ~poly-name quote resolve bound?)
(defn ~poly-name [& args#] (poly-impl ~poly-name args#)))
(let [poly-fn# (fn ~(symbol (str poly-name "-poly")) ~params ~body)]
(setup-poly ~poly-name poly-fn# ~pred (quote ~params)))
~poly-name))
Now you can build arbitrary predicate dispatch:
;; use defpoly like defmethod, but without a defmulti declaration
;; unlike defmethods, all params are passed to defpoly's predicate function
(defpoly myinc number? [x] (inc x))
(myinc 1)
;#_=> 2
(myinc "1")
;#_=> Execution error (ExceptionInfo) at user$poly_impl/invokeStatic (REPL:6).
;No poly for user$eval187$myinc__188#5c8eee0f with ("1")
(defpoly myinc :poly/default [x] (inc x))
(myinc "1")
;#_=> Execution error (ClassCastException) at user$eval245$fn__246/invoke (REPL:1).
;java.lang.String cannot be cast to java.lang.Number
(defpoly myinc string? [x] (inc (read-string x)))
(myinc "1")
;#_=> 2
(defpoly myinc
#(and (number? %1) (number? %2) (->> %& (filter (complement number?)) empty?))
[x y & z]
(inc (apply + x y z)))
(myinc 1 2 3)
;#_=> 7
(myinc 1 2 3 "4")
;#_=> Execution error (ArityException) at user$poly_impl/invokeStatic (REPL:5).
;Wrong number of args (4) passed to: user/eval523/fn--524
; ^ took the :poly/default path
And when using your example, we can see:
(defn new-bacterium [starting-position]
{:mass 0,
:position starting-position})
(defn new-directed-bacterium [starting-position starting-directions]
(-> (new-bacterium starting-position)
(assoc :direction starting-directions)))
(defpoly take-turn (fn [b _] (-> b keys set (contains? :direction)))
[this world]
(println "Directed turn!"))
;; or, if you'd rather use spec
(defpoly take-turn (fn [b _] (->> b (s/valid? (s/keys :req-un [::direction])))
[this world]
(println "Directed turn!"))
(take-turn (new-directed-bacterium [0 0] nil) nil)
;#_=> Directed turn!
;nil
(defpoly take-turn :poly/default [this world]
(println "Aimless turn!"))
(take-turn (new-bacterium [0 0]) nil)
;#_=> Aimless turn!
;nil
(defpoly take-turn #(-> %& first :show) [this world]
(println :this this :world world))
(take-turn (assoc (new-bacterium [0 0]) :show true) nil)
;#_=> :this {:mass 0, :position [0 0], :show true} :world nil
;nil
Now, let's try using isa? relationships, a la defmulti:
(derive java.util.Map ::collection)
(derive java.util.Collection ::collection)
;; always wrap classes in a vector to dispatch off of isa? relationships
(defpoly foo [::collection] [c] :a-collection)
(defpoly foo [String] [s] :a-string)
(foo [])
;#_=> :a-collection
(foo "bob")
;#_=> :a-string
And of course we can use prefer to disambiguate relationships:
(derive ::rect ::shape)
(defpoly bar [::rect ::shape] [x y] :rect-shape)
(defpoly bar [::shape ::rect] [x y] :shape-rect)
(bar ::rect ::rect)
;#_=> :rect-shape
(prefer bar [::shape ::rect] [::rect ::shape])
(bar ::rect ::rect)
;#_=> :shape-rect
Again, the world's your oyster! There's nothing stopping you from extending the language in any direction you want.
I'm implementing Sleeping barber using core.async. My current code is:
(def workingtime 10000)
(defn barber [in waiting-room]
(go-loop [served-customers 0]
(let [[v] (alts! [waiting-room in])]
(if (= v :close)
served-customers
(do (Thread/sleep 20)
(recur (inc served-customers)))))))
(defn customers [in waiting-room]
(go-loop [customers-overall 0]
(let [customer-arrival-interval (timeout (+ 10 (rand-int 20)))
[v] (alts! [in customer-arrival-interval])]
(if (= v :close)
customers-overall
(do (>! waiting-room :customer)
(recur (inc customers-overall)))))))
(defn -main [& args]
(let [in (chan)
waiting-room (chan (dropping-buffer 3))
barber-ch (barber in waiting-room)
customers-ch (customers in waiting-room)]
(println "opening the shop for 10 seconds...")
(Thread/sleep workingtime)
(>!! in :close)
(>!! in :close)
(println "closing the shop...")
(println (str "Served " (<!! barber-ch) " customers"))
(println (str "Overall " (<!! customers-ch) " customers came"))))
Is it a correct solution? Can it be improved to make it more Clojure-like?
I wanted to use alt! instead of alts! which makes code easier to read:
(defn barber [in]
(go-loop [served-customers 0]
(alt!
waiting-room (do (Thread/sleep 20)
(recur (inc served-customers)))
in served-customers)))
Runtime throws an exception: Can only recur from tail position. Can I still use alt!?
You could solve the alt!/recur problem by rewriting to:
(defn barber [in]
(go-loop [served-customers 0]
(if (= :waiting-room
(a/alt!
waiting-room ([result] :waiting-room) ;; you could also use result if needed
in ([result] :in))) ;; same here
(do (Thread/sleep 20)
(recur (inc served-customers)))
served-customers)))
I have written a game loop based on deWitter's game loop.
However, I am unsure how to transfer it to a more functional state. I realize that there may need to be some mutable state left within the code but are there any general principles for cleaning up extraneous defs?
(ns beepboop.core)
(def ticks-per-second 25)
(def skip-ticks (/ 1000 ticks-per-second))
(def max-frameskip 5)
(defn update []
(println "Updating."))
(defn display [delta]
(println "Displaying with delta: " delta))
(defn -main []
(def *next-tick* (System/currentTimeMillis))
(while true
(def *loops* 0)
(while (and
(> (System/currentTimeMillis)
*next-tick*)
(< *loops*
max-frameskip))
(update)
(def *next-tick* (+ *next-tick* skip-ticks))
(def *loops* (+ *loops* 1)))
(display
(/ (+ (System/currentTimeMillis) skip-ticks (* -1 *next-tick*))
skip-ticks))))
You should use loop and recur for updating your loop variables:
(defn -main []
(loop [next-tick (System/currentTimeMillis)]
(let [next-next
(loop [next-tick next-tick
loops 0]
(if (and (> (System/currentTimeMillis) next-tick)
(< loops max-frameskip))
(do (update)
(recur (+ next-tick skip-ticks) (+ loops 1)))
next-tick))]
(display (/ (+ (System/currentTimeMillis) skip-ticks (- next-next))
skip-ticks))
(recur next-next))))
I can launch two threads and they work, but synchronously. What am I missing to get these threads independently launched?
main, thread, and output
(defn -main
[& args]
(do
(let [grid-dim-in [0 5]
mr1-pos [\N 2 4]
mr2-pos [\N 1 5]
mr1-movs "LMLMMRMM"
mr2-movs "RMRMMMLM"]
(reset! grid-dim grid-dim-in)
(reset! mr1-id {:mr1 mr1-pos})
(reset! mr2-id {:mr2 mr2-pos})
(.start (Thread. (rover-thread mr1-id mr1-movs update-work-block)))
(.start (Thread. (rover-thread mr2-id mr2-movs update-work-block))))))
(defn rover-thread [id movs update-ref]
(let [id-key (keys #id)
id-vals (vals #id)]
(doseq [mov movs]
(println "Rover " id-key " is moving ")
(let [new-mov (determine-rover-move (first id-vals) mov)]
(move-rover id new-mov update-ref)
(print "Rover ")
(print (first id-key))
(print " is at ")
(println new-mov)
(Thread/sleep (rand 1000)))))
Rover :mr1 is at [E 2 4]
Rover (:mr1) is moving
Rover :mr1 is at [N 2 5]
Rover (:mr1) is moving
Rover :mr1 is at [N 2 5]
Finished on Thread[main,5,main]
Rover (:mr2) is moving
Rover :mr2 is at [E 1 5]
Rover (:mr2) is moving
Rover :mr2 is at [N 1 6]
Take a close look at these two lines:
(.start (Thread. (rover-thread mr1-id mr1-movs update-work-block)))
(.start (Thread. (rover-thread mr2-id mr2-movs update-work-block))))))
This code evaluates the (rover-thread mr1-id mr1-movs update-work-block) first, and passes the result of that to the constructor of Thread, which is not what you want.
Here's a simple function to illustrate the principle. This doesn't work, because the (f ...) is evaluated before its result it passed to the Thread constructor:
(defn run-thread-thing-wrong []
(let [f (fn [n s]
(doseq [i (range n)]
(prn s i)
(Thread/sleep (rand 1000))))]
(.start (Thread. (f 10 "A")))
(.start (Thread. (f 10 "B"))))
nil)
Here's a version that does work. A function is passed to the Thread constructor instead:
(defn run-thread-thing []
(let [f (fn [n s]
(doseq [i (range n)]
(prn s i)
(Thread/sleep (rand 1000))))]
(.start (Thread. (fn [] (f 10 "A"))))
(.start (Thread. (fn [] (f 10 "B")))))
nil)
Note: instead of (fn [] ....) you can use the short form #(....) for anonymous functions.
Here's another version that does the same, but with a future instead of manually creating threads:
(defn run-thread-thing []
(let [f (fn [n s]
(doseq [i (range n)]
(prn s i)
(Thread/sleep (rand 1000))))]
(future (f 10 "A"))
(future (f 10 "B")))
nil)
Note that in this case, you pass a form to future instead of a function.
This seems like a really good place to use Clojure's agent feature. I am not qualified to fully explain how to use them, but a really good example of their usage can be found here. Starting threads using agents is dead-easy, and I think it is more idiomatic.
The code would look something like,
(def rover1 (agent [mr1-posn mr1-movs mr1-id]))
(def rover2 (agent [mr2-posn mr2-movs mr2-id]))
(defn rover-behave [[posn movs id]]
(send-off *agent* #'rover-behave)
(. Thread (sleep 1000))
(let [new-mov (determine-rover-move posn movs id)
new-posn (posn-after-move posn new-mov)]
;return value updates state of agent
[new-posn movs id]
)
)
(send-off rover1 rover-behave)
(send-off rover2 rover-behave)