Clojurians Log v2

Hi team, I’m trying to make concurrent calls to apis to gather paginated results, I’m thinking of using atom, future and loop for that, my take so far is as follows, but not sure how do I ensure not breaking out of loop before all future calls finish? Any advise please? Thanks

(let [panda-mappings (atom {})
      range 200]
  (loop [offset 0]
    (future (swap! panda-mappings assoc offset "val"))
    (if (> offset range)
      (println @panda-mappings)
      (recur (+ offset 50))))
  (println @panda-mappings))

results is

I’m trying to get something like this instead…

{100 val, 200 val, 150 val, 250 val, 0 val, 50 val}

clojure.core.async has some ideas about how to communicate like this. you could check out clojure.core.async/pipeline-blocking

you mean future is not the right tool for this kind of problem?

there are many tools for this. but core.async can give you good coordination tools for your problem of waiting until all the requests finish

pipeline-blocking will actually use thread underneath it but with async channels to communicate results and pending results

Am following a Clojure Tutorial - Anyone has an idea why (shutdown-agents) causes .... : "Exception updating the ns-cache #error"? I can skip this but am just wondering o:

(defn agent-ex []
  (def tickets-sold (agent 0))
  (send tickets-sold + 15)
  (println)
  (println "Tickets " @tickets-sold)
  
  (send tickets-sold + 10)
  (await-for 100 tickets-sold)
  (println "Tickets " @tickets-sold)
  (shutdown-agents))

(defn -main [& args]
  (agent-ex)
  )

That's not a message from Clojure itself, must be something from your tooling. If something in your tooling is using agents or futures, shutdown-agents could be breaking the tooling itself possibly

Right! Maybe NREPL itself then o:

https://github.com/clojure-emacs/cider-nrepl/blob/master/src/cider/nrepl/middleware/track_state.clj#L123

looks like it

Right! Thanks for your help! 🙂

@zackteo def should only be used at the top-level. If you want a local binding, use let instead.

And, yeah, most REPL tooling will break if you call (shutdown-agents) since it will kill off stuff inside the tooling, not just your program.

The safer idea is to move the (shutdown-agents) call into -main so it's

(defn -main [& args]
  (agent-ex)
  (shutdown-agents))

I'm trying to use Double/isNaN as a predicate in some, but clojure interprets it as a static field. Is this the idiomatic way of dealing with such things?

rules-engine.thrashing02=> (Double/isNaN ##NaN)
true

rules-engine.thrashing02=> (some Double/isNaN [##NaN 2])
Syntax error compiling at (form-init5367077877367404867.clj:1:1).
Unable to find static field: isNaN in class java.lang.Double

rules-engine.thrashing02=> (some #(Double/isNaN %) [##NaN 2])
true

#(Double/isNan %) is the idiomatic way

i believe the reason is that methods are not first class objects on the JVM.

Ok, thanks. Only quirk I have is that this is already in an anonymous function, which I cannot nest, so I have to do something else. But this makes sense, thank you.

make either the outer or the inner a (fn [thing] ...) version. probably the outer since #(Double/isNan %) is quite simple

yep, that's what I did:

(fn [it] (Double/isNaN it))

and yep, it's simple, so the fn construct works well.

Hey

it might be worth making a nan? function in core

(def first-test '(not x))
(def second-test '(not (not a)))
(def third-test '(not (not (and a b))))
(def fourth-test '(not (not (not (not c)))))
(defn not-elmination
  [expression]
  (cond
    (= first-test expression) set '{}
    (= second-test expression) set '{a}
    (= third-test expression) set '{(and a b)}
    (= fourth-test expression) set '{(not (not c))}

    :else set '{})

  )

Anyone know why?

#{} is the set literal, {} is the map literal

set there makes no sense, what are you trying to do

So if the user enters '(not x), the cond checks and i want to return a set {(not x)}

ok, set '{(not x)} makes no sense, you want #{'(not x)}

'#{(not x)} also works, it'll quote the entire set instead of the specific element but in this case they're the same thing

So i should be returning #'{a} for example

no

#{'a}

or '#{a}

those are equivalent to #{(quote a)} and (quote #{a}) respectively

#'{ is not valid syntax

the syntax for a set is #{ elements.. }

so you can use ' either in front of the whole set to quote all the values in the set, or in front of an element in the set

Ok thanks for the information, I will apply that to my code rn

hmm now im getting "cond requires an even number of forms"

(def second-test '(not (not a)))
(def third-test '(not (not (and a b))))
(def fourth-test '(not (not (not (not c)))))
(defn not-elmination
  [expression]
  (cond
    (= second-test expression)  #{'a}
    (= third-test expression) #{'(and a b)}
    (= fourth-test expression) #{'(not (not c))}
    :else set '#{})

  )

@ntrut Parentheses are important in Clojure and mean "function call" (in general) so set '{} is two unrelated expressions but (set '{}) would be a function call -- and would call set on '{}

:else set '#{}is wrong

(because it is three separate expressions)

Sorry, its hard getting used to clojure, just recently starting learning it in one of my classes

No worries. Lisps are always a bit tricky to get used to.

And it also depends what languages you're used to.

In Java or similar: obj.method(arg) In Clojure (method obj arg)

Defiantly not used to this, I code in Java/C/Pyhon

New territory for me here

What is wrong with my else?

:else expresssion but you have :else a-function an-empty-set

(also, you can just use #{} as an empty set. there are no forms in it that you are quoting)

Yep using #{} fixed my else problem and now my program works

thanks people

Regarding the infer-externs cljs compiler option: Are you intended to move the generated externs in inferred_externs.js to a file that you use for advanced compile? Or is it supposed to be enough to use type hinting and infer-externs when you use advanced compile?

@mitchell_clojure If no one answers here, you might try #clojurescript -- I'm not sure how many active folks here are familiar with ClojureScript compilation issues.

Thanks Sean! Hard to know sometimes if I am missing something simple sometimes

if i have an defrecord instance, is there a simple way to create a new one with just one value substituted, sort of like update-in does for maps?

you can use update

records are maps

or assoc into it

user=> (defrecord Point [x y])
user.Point
user=> (def r (->Point 1 2))
#'user/r
user=> (update r :x inc)
#user.Point{:x 2, :y 2}
user=> (assoc r :x 10)
#user.Point{:x 10, :y 2}