Clojurians Log v2

It might be a stupid question. And it might me formulated incorrectly. But how do I run a channel in endless loop in my main function? I can do that in repl with (async/go-loop ...) but not with lein run

Ok. I just figured out. It was not a problem of my channels misunderstaning, but wrong config.

Now another question. A file being read in my code. How do I say it not to look for the same file to read after compilation, but rather have it "remembered"?

Ok. I just get this. The problem is still with the first question.

Having this function: (defn start [x y] (async/go-loop [] ...)) I want go make it work being into main :

(defn -main [& args]
  (start))

I'm using next.jdbc to connect to postgres which is running as a docker container. I am able to connect to it using intellij and psql, but not via the repl. I have the following in my project.clj

[org.postgresql/postgresql "42.2.5"]
[seancorfield/next.jdbc "1.1.613"]

(comment
  (require '[next.jdbc :as jdbc])
  (def db {:dbtype    "postgresql"
           :classname "org.postgresql.Driver"
           :dbname    "foo-dev"
           :username  "foo"
           :password  "foo"
           :host      "localhost"
           :port      7432})
  (def ds (jdbc/get-datasource db))
  (jdbc/execute! ds ["select * from some-table"]))

psql -h localhost -p 7432 --username foo -d foo-dev

Execution error (ConnectException) at java.net.PlainSocketImpl/socketConnect (PlainSocketImpl.java:-2).
Connection refused (Connection refused)

This is the docker-compose.yml part for the postgres container

db:
    image: postgis/postgis:13-master
    volumes:
      - db_data:/var/lib/postgresql/data
    ports:
      - 7432:5432
    environment:
      POSTGRES_USER: foo
      POSTGRES_PASSWORD: foo
      POSTGRES_DB: foo-dev

Change username to user

user=> (def db {:dbtype "postgres" :classname "org.postgresql.Driver" :dbname "foo-dev" :user "foo" :password "foo" :host "localhost" :port 7432})
#'user/db
user=> (jdbc/execute! (jdbc/get-datasource db) ["select now()"])
[{:now #inst "2020-12-26T11:25:58"}]

❯ docker ps                CONTAINER ID   IMAGE                       COMMAND                  CREATED         STATUS         PORTS                    NAMES
04d4199d1cd8   postgis/postgis:13-master   "docker-entrypoint.s…"   4 minutes ago   Up 4 minutes   0.0.0.0:7432->5432/tcp   david_db_1

Is clojure.lang.PersistentQueue still the appropriate data structure to use if you want a FIFO queue? i.e. conj to the back, pop off the front?

still the same error 😕

Interesting. Works for me, and I basically copy and pasted your code to my terminal

AFAIK that is its primary purpose

Tried it in the older clojure.jdbc and it works, guessing it's a problem with next.jdbc and my system

(def idb {:classname                                  "org.postgresql.Driver"
          :subprotocol                         "postgresql"
          :subname                             "//localhost:7432/foo-dev"
          :user                                "foo"
          :password                            "foo"})

Glad you got it sorted! 🙂

Hey, everyone. I'm coming in from the Java/Kotlin world, and I'd like to try building a REST API using Clojure. In Java, you download Spring Boot and get to work, but I'm reading that in the Clojure world you tend to put together the pieces that you want, with everyone using Ring. Is there a de facto standard for building APIs nowadays? I I've seen samples with Reitit and Compojure; where is the industry heading?

compojure and reitit are routers, ring provides an http server (which can be combined with the router of your choice). none of these are REST specific

@abyala for "batteries included" opinionated setups, the biggest player is luminus, they have a project template generator which accepts args reflecting the setup you are looking for (eg. cljs webapp support, database backend...) https://luminusweb.com/ - the excellent book "web development with clojure" uses a luminus template, walks through building a full app, and is worth the price https://pragprog.com/titles/dswdcloj3/web-development-with-clojure-third-edition/

but depending on what kind of REST API you are trying to make, there are tools that simplify that as well (and those can be combined with the big pile of code that luminus hands you)

NB - don't ever use first / rest / etc. seq operations on a queue, it can quietly turn your fifo into a lifo

only use into, peek, conj, and pop

(and other tools built on those)

Awesome, I'll check that out. Thanks!

Hello , When can we use apply and when can we use reduce? what is the difference between them? I searched in internet before posting here, But did not get good understanding!

@popeyepwr for many functions that take variable argument counts, adding an arg to the function is equivalent to another call - in that case apply and reduce do the same thing

so "can" - for many vararg functions that treat their arguments homogenously, you can use either

in practice reduce has some optimizations that the function might not, so it's often preferable

concretely: (+ 1 2 3) will always be the same as (+ (+ 1 2) 3), so apply and reduce will do the same thing with + and [1 2 3]

also conj

(ins)user=> (apply conj [] [1 2 3])
[1 2 3]
(ins)user=> (reduce conj [] [1 2 3])
[1 2 3]

but with apply, it can be smarter about building the result, with reduce it needs to do more allocations (it builds a bunch of immutable vectors it only needs once)

for correctness they both work, it is case by case which is more efficient

To add to @noisesmith’s answer, I'll try and illustrate the difference: (apply f coll) will call f once with all elements of given collection supplied to it as arguments; (reduce f val coll) will call the function once for each element in the collection. Consider this:

(vector 1 2)                   ;; => [1 2]
(vector 1 2 3 4 5)             ;; => [1 2 3 4 5]
(apply vector [1 2 3 4 5])     ;; => [1 2 3 4 5]
(reduce vector 1 [2 3 4 5])    ;; => [[[[1 2] 3] 4] 5]

(reduce vector [1 2 3 4 5])    ;; => [[[[1 2] 3] 4] 5]

(reduce #(apply update %1 %2) {:left 0 :right 0} [[:left dec]
                                                  [:right inc]
                                                  [:right inc]])
;; => {:left -1, :right 2}

Can we say reduce is equivalent to apply if f is associative?

that's the word I almost used earlier, yes

Associativity is somewhat different, it is only defined for binary functions (but see below). And yes, reduce in its basic form [1] repeatedly applies a binary function to a sequence. If the function is associative, then the result will be the same for reversed sequence. But apply works with functions of different arities for which associativity makes no sense, e.g. (apply update [{:a 0} :a inc]). [1] https://en.wikipedia.org/wiki/Fold_(higher-order_function)

right, and a foolish person could make a vararg function that is associative for two args, but does something non-associative for a higher arg count

Well, for higher arg count, the function cannot be associative or non-associative -- e.g. above (update :a inc) would make no sense Edit: I think we can translate associativity to variadic functions if we consider them as functions of one argument which operate on finite sequences, i.e. associative then meaning (using common mathematical notation) f((x1, f((x_2, ..., x_n)))) = f((f((x_1, ..., x_{n-1})), x_n)) for all such sequences. For apply and reduce to be equivalent on the other hand would mean that f((f((x_1, f(( ... )), x_{n-1})), x_n)) = f((x_1, x_2, ... x_n))._

a version of plus that took three args would still be "associative" in the loose sense that you could start simplifying anywhere in the expression and get the same answer

sorry, no, that's incoherent, clojure functions based on known associative functions are often varargs, and if so they usually do the same thing under reduce / apply, that's all

I think the reason apply and reduce seem similar is that clojure provides many functions with their reduce'd extensions to higher arities. Consider addition as commonly defined in mathematics: it is a binary operation which can be extended to sequences via formal equivalent of reduce. Since it's associative, it can even be extended to sets. And in many programming languages addition remains just a binary operation, and to get a sum of more than two numbers one has to reduce or fold it. It's just that clojure conveniently provides this already -- if we look at the source of +, we'll see

(defn +
  ;; snip
  ([x y] (. clojure.lang.Numbers (add x y)))
  ([x y & more]
     (reduce1 + (+ x y) more)))

I think it's because it's commutative that you can do that on sets

(ins)user=> (apply / [1 2 3 4])
1/24
(ins)user=> (reduce / [1 2 3 4])
1/24

I think I mixed up associative and commutative above myself

@max.deineko reduce1 is a less efficient and less flexible version of reduce used before the clojure compiler has implemented reduce

You're right, for the reduction to be well defined on at least some interesting sets, the operation has to both be associative and commutative. But I shouldn't have mentioned it -- we don't want to drift too far I reckon 🙂

@popeyepwr I found this very useful https://stackoverflow.com/questions/3153396/clojure-reduce-vs-apply