Clojurians Log v2

Hello! I struggle finding a good way how to perform parametrized initialization only once in the face of concurrent calls. What I have now is:

(defn get-or-init-adoc [atom params]
  (compare-and-set! atom nil (delay (some-expensive-init-reading-files params)))
  @@adoc)

what's wrong with just having a (def x (delay (some-expensive-init params))) ?

where are the params coming from?

there's also promise which you can deliver once and will block all defref s until it's delivered ... but you need to be careful not to create deadlocks where the read calls block all the running threads before any thread can be started to deliver the promise

or maybe (defn get-or-init-adoc [atom params] (swap! atom #(when (nil? %) (some-expenisive-init params)) ?

I guess you're trying to make sure that the init only gets run once, and have the function lazily initialise some value?

so maybe you could memoize the function?

note that memoize is not suited for non-referentially transparent functions

and it doesn't guarantee that the underlying function will be executed once

you could use locking to lock some sentinel, which will probably be the least headachy

Do you know what's a free service to quickly deploy a Clojure Deps.edn application?

I would normally use Heroku but they do not support Clj tools out of the box and I need to do to many changes to my project

Make an uberjar and deploy that one

Yeah I guess that's the easiest way. Any chance you have a link about producing an uberjar with clj.tools @zilti

Yes: https://github.com/seancorfield/depstar

2020-10-29T14:47:15.148369+00:00 app[web.1]: Error: Invalid or corrupt jarfile app.jar

Interestingly enough, the deployed jar on Heroku seem to be broken or something :thinking_face:

@vincenz.chianese Does it work locally though?

Yeah, I think I made a mistake in the jar execution, let me try again 🙂

I was specifying a wrong namespace

Ok that works now, but it seems like I'm having issues running datomic dev-local on heroku

the name doesn't suggest you should run it in production maybe? just guessing

No, I think it's because I'm using a wrong directory name, let me triple check…

Ok I got it working. It takes ages to startup but eventually it works

😂

I don't really see the benefit of putting an atom outside the delay here, the delay already locks and will only execute once

The problem is I need to do the initialization inside a function that gets 'params' as it's argument. And I don't want to unnecessarily create a new delay upon each its call.

swap! doesn't work because it can execute the function multiple times.

agents never retry, fwiw

so sending an fnil to an agent should work

Thanks! I guess careful use of locking is the simplest solution here...

Hi does anyone have an idea of how I would do this? I’m trying to conditionally define functions using a global lein profile depending on whether I have certain libraries loaded, ex. something like this where I’m developing several libraries at once, some of which are childrens of the others. Say if the dependencies looked something like

[my.lib5 [my.lib1 my.lib2 my.lib4]
 my.lib4 [my.lib1 my.lib2]
 my.lib3 [my.lib2]]

;; load libs if they exist
(let [repl-deps ['[my.lib1 :as l1]
                 '[my.lib2 :as l2]
                 '[my.lib3 :as l3]
                 '[my.lib4 :as l4]
                 '[my.lib5 :as l5]
                 '[clj-async-profiler :as prof]
                 '[criterium.core :as crit]]]
  (doseq [dep repl-deps]
    (try
      (require dep)
      (catch Exception e nil))))

;; Create custom functions depending on what's loaded
(let [namespaces (->> (all-ns) (map ns-name) (map name) set)
      loaded? (fn [& args] (every? namespaces args))]
  (when (loaded? "my.lib1")
    (defn custom-fn-1 (l1/foo) )
  (when (loaded? "my.lib2")
    (l2/init)
    (defn custom-fn-2 (l2/bar)))
  (when (loaded? "my.lib2" "my.lib4")
    (l4/init l2/options)
    (defn custom-fn-3 (l4/baz)))

@elton.law This is called the Gilardi scenario: https://technomancy.us/143

Gilardi issue is secondary symptom - the root issue is you are throwing away your exceptions @elton.law . Who knows what that is suppressing

But a good hypothesis is that lib4 is not loading initially, being skipped silently, causing a subsequent compilation error later (failure to resolve symbol l4/init )

but also good to understand the gilardi scenario

Umm forgive me if I’m misunderstanding but I don’t think the Gilardi scenario is related, I’m working on a project that has about 20 different individual projects, and basically, sometimes I will be able to do a require on that my.lib4 but sometimes I won’t (because it’s not a dependency)

and I want specific blocks using variables from my.lib4 to not be run

you can't compile a form containing the symbol l4/init even if its behind a when form if the l4 alias isn't present

yeah

but you are attempting to

I’d like to, that way I can have it all in one startup.clj file

and then say if I have multiple projects that use criterium, I can have those all load

only when its avaialble

if that makes sense

right. and that seems to be exactly the Gilardi scenario

hmm okay, maybe i’ll take another look at it, thanks everyone

i think you could easily get around this with requiring-resolve

^ I was about to suggest that.

Bear in mind that it will throw an exception if the namespace can't be required.

Right right, because it would be a runtime thing and okay, after a second reading it totally makes sense now thanks again everyone

(when-let [foo (resolve 'l1/foo)]
  (defn custom-fn-1 (foo)))

Steve, my man!

That way, it will only resolve if l1 is loaded. Well, if it's loaded and l1/foo resolves 🙂

I could have sworn he was on Slack here as well as @sgilardi

with the understanding that it's not the best practice and for good reason, is there a way to fetch the latest commit of a branch via deps.edn, rather than a specific sha?

@joshkh I have an example in my dot-clojure repo, if you've seen that?

nope! searching now

https://github.com/seancorfield/dot-clojure/blob/develop/deps.edn#L175-L183

(relies on the add-lib3 branch of t.d.a.)

exactly what i was looking for. just to test my understanding, extra-deps brings in tda:add-lib3, which then handles the :add-libs key of deps.edn? nvm i misread the indentation

@scgilardi

i actually asked him (since i was pairing with him at the time) if "Gilardi scenario" was a coincidence. totally wasn't 😛

why would it be a co-incidence?

I figured given the community it’d be pretty unlikely it WASNT him. But still wanted to ask