Hi, i just came across this on clojure docs and was wondering why; > ;; it is tempting to try an index on a list > (get '(a b c) 1) > ;;=> nil > > ;; but you should use nth > (nth '(a b c) 1) > ;;=> b Is there some performance implication regarding nth on a list? I'm a bit confused why get is off limits here. Thanks,

get on a list always returns a nil.

https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/RT.java#L786

i guess i'm trying to decide if i should use nth and potentially incur a performance penalty, or coerce into a vector and then have get close at hand.

this is in the context of a list comprehension that i can fit into memory without too much issue

how many times do you need random access?

yeah, i guess thats the right question, i'm walking the entire sequence so i think i should shove it into a vector ?

i was getting at the opposite side of that. creating a vector walks the entire sequence. if you just need to find the nth thing once, just linear scan and don't pay the penalty for the datastructure creation on top of the O(n) behavior

if you need random access often, use a datastructure that provides the API and performance that you need

ok, thank you for the tip. I think i get it.

is finger-tree (https://github.com/clojure/data.finger-tree) . really that slow ?

I'm trying to containerize my dev environment, which involves connecting to a database at a separate host. Connection is refused because apparently JDBC is trying to connect to localhost, even though the correct DATABASE_URL env var is detected. Here is me debugging from inside the container:

www-data@d850df1729bc:/app$ clojure -A:dev -m 
Starting nREPL server at localhost:7000
Connecting to database at URL: jdbc:
log4j:WARN No appenders could be found for logger (com.zaxxer.hikari.HikariConfig).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See  for more info.
Apr 22, 2020 4:59:04 AM org.postgresql.core.v3.ConnectionFactoryImpl log
WARNING: ConnectException occurred while connecting to localhost:5432
.ConnectException: Connection refused (Connection refused)
...stack trace...

(defn connect! []
  (if-let [database-url (System/getenv "DATABASE_URL")]
    (do
      (println "Connecting to database at URL:" database-url)
      (conman/connect! {:jdbc-url database-url}))
    (throw
     (ex-info "Database exception!"
              {:causes #{:no-database-url}})))

See what dbhost resolves to in your container

is there any tool to find all unused vars (mostly functions) in a big clojure project?

i want to get rid of unused legacy stuff

Hey guys, is there any way to spread a vector/list into the argument arguments of function call without using apply?

There is a third party macro that I want to spread into, since its a macro I cannot use apply. Specifically, in the compojure library, there is a defroutes macro that I would like to call, with a list of pre-compiled routes from various modules. Instead of manually entering each and every module into the call, I'd like to do something like (apply defroutes app-routes list-of-all-my-routes)

you can write your own macro that wraps it and use ~@

Don't use defroutes

https://github.com/weavejester/compojure/blob/master/src/compojure/core.clj#L187

I found this behavior quite weird

(defn test-chan [] (let [c (a/chan 10)] (a/onto-chan c [1 2]) (a/close! c) (println (a/<!! c)) ; prints nil (println (a/<!! c)) ; prints nil (println (a/<!! c)))) ; prints nil

If I omit this sexp (a/close! c), then everything is fine. wh closing on a channel twice will fail the takes ?

"Puts the contents of coll into the supplied channel.
  Returns a channel which will close after the items are copied."

@i onto-chan is async

you need to (a/<!! (a/onto-chan ...)) before you try to read from c

aha. because the goblock of onto-chan hasn’t been dispatched.

it's been dispatched, but not guaranteed to be done

yes.

does using go blocks causing much latency? I previously heard on this channel someone saying go block under jvm (clojure) is quite tricky regarding the performance.

by itself, adding a go block will slow down your code (if that's the only change). If you are doing something parallel in order to have better throughput then need to do complex coordinataion, a go block can facilitate that speedup if that makes sense

async is a debt / detriment you take on (in terms of both perf and debugability) that should be balanced by some other greater benefit which happens to need async

Hey! Do you have any general suggestions how to transform XML (possibly parsed via data.xml) efficiently (both in terms of performance and code readability and manageability) into nested map, extracting specific keys and values from tags and content at specific levels?

I thought about Specter, but I'm not sure if it's actually possible to collect stuff from multiple places into map with it.

I dunno about efficiency but I always start with for

Do you want to do anything special with each tag? Do you want to flatten it?

I worked with XMPP in Clojure and ended up using transducers inspired by this blog post: https://juxt.pro/blog/posts/xpath-in-transducers.html

In general I need to build map from various nested properties of this XML. I've got multiple "objects" in this XML which I need to extract.

So it's something like: first go to tag x > tag y > tag x of id y and then take every children of tag z and build map out of it's tag a and b and c.

Sounds like you could just use xpath type tools

(for [maybe-x (tree-seq map? :content xml) :when (= (:tag maybe-x) :x) maybe-y (:content maybe-x) :when (= (:tag maybe-y) :y) :when (= (:id (:attr maybe-y)) (:id (:attr maybe-x)))] whatever) or something

Yeah, something like that probably.

for is a real swiss army knife for querying nested data

you can even do recursive queries with it by just wrapping it in a recursive function

(fn f [whatever] (for [x whatever i (f x)] i))

For server-side access to Firebase, should I just use the java admin library or is there a preferred pre-existing wrapper, or should I use something like taika, a wrapper around the REST API?

Hey, does Clojure have an official/unofficial motto or philosophy? For example: Java: "Write once, run anywhere" Erlang: "Write once, run forever" Haskell: "Avoid success at all costs"

@yosevuk Simple made Easy

I've created a macro:

(defmacro defelem [name element]
  `(ws/defcard ~name
    (ct.react/react-card
     (r/as-element
      ~element
      )
     )
    )
  )

Macros have to be executed in Clojure, even for ClojureScript, since they rely on the Clojure compiler for expansion and then the resulting code can be compiled by ClojureScript. I think.

@pshar10 Depending on which ClojureScript REPL you're using, asking in #clojurescript #lumo #planck #devcards ... may yield better answers about how to deal with macros in those environments.

Say I have a list '(1 3 4 5), and I want to append to it some number n only if it already doesn't exist in it. How to do that in clojure elegantly?

(let [coll '(1 3 4 5)]
  (if ((set coll) 4)
    coll
    (conj coll 4)))

(let [coll '(1 3 4 5)]
  (if (some #{4} coll)
    coll
    (conj coll 4)))

@nikolavojicic why vectors and not lists?

Vector data structure is more performant. List is a linked list, its purpose in Clojure is code writing mostly...

I have some code that gives a reflection warning when run on OpenJDK 8 but does not give a reflection warning on OpenJDK 11 or OpenJDK 14. I found that surprising so I'd be interested to hear suggestions/explanations of what might have changed between JDK versions for that to happen. It's with this image metadata extractor library com.drewnoakes/metadata-extractor {:mvn/version "2.13.0"}

(I haven't checked whether it's a multi-release JAR which I suppose could account for it -- actually being different JVM bytecode on 8 vs 11/14)

(nope, that's not it -- no MR stuff in that lib)

clojure.lang.Reflector (which the compiler also uses to resolve methods) actually has a check to see if it is running on 1.8 or not

it may be a module thing

9+ respects module accessibility, so something that may be making things ambiguous in 8 might be ruled out because of module something or other in 11