Why calling this function doesn't print anything?
(defn test-go-loop []
(go (for [a (cycle [:a :b :c])]
(do (println a) (<! (timeout 1000))))))
for is lazily evaluated, and nothing in your code is asking for the result of that for. Try doseq:
(defn test-go-loop []
(go (doseq [a (cycle [:a :b :c])]
(println a)
(<! (timeout 1000)))))
Related
I have a namespace like this:
(ns foo.core)
(def ^:dynamic *debug-fn*
"A function taking arguments [bar baz]"
nil)
(defn bar-info
[bar _]
(println bar))
(defn baz-info
[_ baz]
(println baz))
(defn do-stuff
[bar baz]
(when *debug-fn* (*debug-fn* bar baz)))
(defn -main
[& {:keys [debug-fn]}]
(binding [*debug-fn* (symbol debug-fn)] ;; THIS WON'T WORK!
(do-stuff 27 42)))
What I would like to do is allow a debug function to be specified from the command line like this: lein run bar-info or lein run baz-info.
I'm not sure how to take the string specified as a command-line argument and turn it into the namespace-qualified function to bind. Do I need a macro to do this?
Use ns-resolve, you will need to specify namespace where your function is defined though.
user=> (defn f [n] (* n n n))
#'user/f
user=> ((ns-resolve *ns* (symbol "f")) 10)
1000
Use alter-var-root:
user=> (doc alter-var-root)
-------------------------
clojure.core/alter-var-root
([v f & args])
Atomically alters the root binding of var v by applying f to its
current value plus any args
nil
user=> (alter-var-root #'*debug-fn* (fn [v] (fn [x] (println x) x)))
#<user$eval171$fn__172$fn__173 user$eval171$fn__172$fn__173#7c93d88e>
user=> (*debug-fn* 1)
1
1
Though I've accepted Guillermo's answer above, I figured that it might also be useful to add the solution I ended up going with:
(def debug-fns
{:bar-info (fn [bar _] (println bar))
:baz-info (fn [_ baz] (println baz))
(def active-debug-fns (atom []))
(defn activate-debug-fn!
[fn-key]
(let [f (debug-fns fn-key)]
(if f
(swap! active-debug-fns conj f)
(warn (str "Debug function " fn-key " not found! Available functions are: "
(join " " (map name (keys debug-fns))))))))
(defn debug-fn-keys
[args]
(if (args "--debug")
(split (or (args "--debug") "") #",")
[]))
(defn do-stuff
[bar baz]
(doseq [f #active-debug-fns]
(f bar baz)))
(defn -main
[& args]
(let [args (apply hash-map args)]
(doseq [f (debug-fn-keys args)]
(activate-debug-fn! (keyword k)))
(do-stuff 27 42)))
So now you can say something like lein run --debug bar-info to get info on bars, or lein run --debug bar,baz to get info on both bars and bazes.
Any suggestions to make this more idiomatic will be happily accepted and edited in. :)
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)
I want to create a function (thunk) that will return successive elements in a list. What is the best way to do this? I wrote this code based on an apparently flawed understanding of how local variables in clojure work:
(defn reader-for [commands]
(with-local-vars
[stream commands]
(fn []
(let
[r (var-get stream)]
(if (empty? r)
nil
(let
[cur (first r)
_ (var-set stream (rest r))]
cur))))))
In this code I get:
#<CompilerException java.lang.IllegalStateException: Var null/null is unbound. (Chapel.clj:1)>
which seems to suggest that with-local-vars is dynamically scoped. Is that true? Is there any lexically scoped alternative? Thanks for any help.
If you require mutable state, use one of the clojure reference types:
user=> (defn reader-for [coll]
(let [a (atom coll)]
(fn []
(let [x (first #a)]
(swap! a next)
x))))
#'user/reader-for
user=> (def f (reader-for [1 2 3]))
#'user/f
user=> (f)
1
user=> (f)
2
user=> (f)
3
user=> (f)
nil
Also, let is for lexical scoping, binding is for dynamic scoping.
Edit: the thread-safe version as pointed out by Alan.
(defn reader-for [coll]
(let [r (ref coll)]
#(dosync
(let [x (first #r)]
(alter r next)
x))))
And just for fun, a thread-safe version with atoms (don't do this):
(defn reader-for [coll]
(let [a (atom coll)]
(fn []
(let [ret (atom nil)]
(swap! a (fn [[x & xs]]
(compare-and-set! ret nil x)
xs))
#ret))))
The following code appears to force line-seq to read 4 lines from file. Is this some kind of buffering mechanism? Do I need to use lazy-cat here? If so, how can I apply a macro to a sequence as if it were variadic arguments?
(defn char-seq [rdr]
(let [coll (line-seq rdr)]
(apply concat (map (fn [x] (println \,) x) coll))))
(def tmp (char-seq (clojure.contrib.io/reader file)))
;,
;,
;,
;,
#'user/tmp
Part of what you're seeing is due to apply, since it will need to realize as many args as needed by the function definition. E.g.:
user=> (defn foo [& args] nil)
#'user/foo
user=> (def bar (apply foo (iterate #(let [i (inc %)] (println i) i) 0)))
1
#'user/bar
user=> (defn foo [x & args] nil)
#'user/foo
user=> (def bar (apply foo (iterate #(let [i (inc %)] (println i) i) 0)))
1
2
#'user/bar
user=> (defn foo [x y & args] nil)
#'user/foo
user=> (def bar (apply foo (iterate #(let [i (inc %)] (println i) i) 0)))
1
2
3
#'user/bar
What I want to do is like following.
(def mystream (stream (range 100)))
(take 3 mystream)
;=> (0 1 2)
(take 3 mystream)
;=> (3 4 5)
(first (drop 1 mystream))
;=> 7
The stream function make sequence side-effectfull like io stream.
I think this is almost impossible.
Here is my attempt.
(defprotocol Stream (first! [this]))
(defn stream [lst]
(let [alst (atom lst)]
(reify Stream
(first! [this]
(let [[fs] #alst]
(swap! alst rest)
fs)))))
(let [mystream (stream (iterate inc 1))]
(map #(if (string? %) (first! mystream) %)
[:a "e" "b" :c "i" :f]))
;=> (:a 1 2 :c 3 :f)
Unfotunately this approach need to implement all function I will use.
Judging by your followup comment to Maurits, you don't need mutation, but rather simply need to emit a new sequence with the elements in the right place.
For example:
(defn replace-when [pred coll replacements]
(lazy-seq
(when (seq coll)
(if (seq replacements)
(if (pred (first coll))
(cons (first replacements)
(replace-when pred (rest coll) (rest replacements)))
(cons (first coll)
(replace-when pred (rest coll) replacements)))
coll))))
user=> (def seq1 [:a :b :c])
#'user/seq1
user=> (def seq2 [:x "i" "u" :y :z "e"])
#'user/seq2
user=> (replace-when string? seq2 seq1)
(:x :a :b :y :z :c)
This won't work with the standard take and drop, but you could quite easily write your own to work on a mutable atom, e.g. you could do something like this:
(def mystream (atom (range 100)))
(defn my-take [n stream]
(let [data #stream
result (take n data)]
(reset! stream (drop n data))
result))
(my-take 3 mystream)
=> (0 1 2)
(my-take 3 mystream)
=> (3 4 5)