Yes it's a very good book, the most comprehensive one on Clojure web development I know of. And I'd say more generally applicable than the Luminus docs because people tend to pick and choose libraries in practice.

That is using http://slf4j.org logging facade and it is complaining about missing logging implementation. To fix this you’ll need to include depency that includes the dependency

logback is one such implementation [ch.qos.logback/logback-classic "1.2.3"]

The linked page has instructions,

If you are responsible for packaging an application and do not care about logging, then placing slf4j-nop.jar on the class path of your application will get rid of this warning message.

But if you care about logs at all, better configure the logger properly.

I actually was oblivious to what SLF4J was, I thought this was a logging library, as this is an abstraction layer I'm more comfortable with this

yup, I'm going to do that

The purpose of SLF4J is to give you the power to choose the right implementation for your app yet capture the logs from the library you pulled.

Without an implementation in path, it cannot log anything. But for any app in production, you will want to at least capture WARN and ERROR from every library you pull.

So, I don't really recommend slf4j-nop.jar.

How about ring?

@U010FU3Q9N0 don't know how to do that. Can you please elaborate or enlighten me about this?

I can just tell you that I am just a noob in web development,and I found ring-clojure is a simple,good framework.Yo You can search it on github

read-line and the prompt share the same input, it's automatic

if you don't readline in another thread, it won't interfer

@U9J50BY4C This might be a good use case for trampoline.

Hmmm, ok. I'm confused on the syntax and all that involved too though.

I doubt the jvm will run out of stack frames before the user runs out of patience 😛

I don't see how trampoline would help

I was approaching it like this:

(defn get-num []
  (println "Number: "
    (let [num (read-line)]
      (try 
        (Long/parseLong num)
        (catch Exception e)))))

Trampoline is a solution to a problem several steps ahead

But I don't actually know how to do this. I'm reading clojuredocs but could use a more extensive resource on try/catch if you folks have it.

E.g. you can't recur from the catch

Yeah, once the user inputs a valid number the program runs some calculations and that's that. Nothing crazy going on here.

You don't need to use try/catch. A simple predicate would do

@U051SS2EU To repeatedly execute read till a number is found. EDIT: But recur can do that as well.

(if-not (is-number? x) (get-num))

Yup. Ok. Seems obvious now that I think of it. Lol. But I would still like to learn more about exceptions and error handling all that jazz if anyone has Clojure resources on that.

I'm not experienced enough with Clojure to say much about exceptions, but I would search for articles on idiomatic error handling. My guess is that their primary purpose is for integration with Java/JS code and not to be used more generally

@UVDD67FFX In the JVM world, exceptions are the canonical way to handle exceptional cases. try/catch is not unidiomatic.

so now when I use this:

(defn get-num []
  (println "Number: "
    (let [num (read-line)]
      (if (number? (Long/parseLong num))
        (valid-cc? num)
        (get-num)))))

it doesn't print out "Number: " first. It just sits there waiting for the input with no prompt. and then afterwards does "Number: nil" after my other calculations do there thing.

you might need to add a call to (flush)

@hindol.adhya In Java world, definitely. You wouldn't typically use exceptions in Scala

@U9J50BY4C the problem is the entire body is inside the println

ok. Also if it isn't a number it gives me the NumberFormatException and terminates instead of calling the function again

you need to take the let out of the println

@UVDD67FFX In Clojure though, it is not frowned upon or anything.

@U9J50BY4C right, that's where you do the try/catch and return nil

@hindol.adhya thanks, that's interesting.

Ok, definitely re-fixed that println problem.

So then I can't just use the predicate? Which takes me back to the syntax of using the try/catch.

like this

(ins)user=> (try (Long/parseLong nil) (catch Exception _))
nil
(ins)user=> (try (Long/parseLong "1") (catch Exception _))
1

wrap the smallest possible block in the try

then have a conditional on whether the return was nil

or use some->

hmmm. Ok. Seems to be getting a bit difficult to just see if the input is a number or not and loop back around.

one moment I'll have an example

Maybe I'm not even understanding what exactly try/catch even does or means

(cmd)user=> (load-file "/tmp/get-num.clj")
#'user/get-num
(cmd)user=> (get-num)
(cmd)Number: alsdjf
(ins)Number: ajdsfk
(ins)Number: lllll
(ins)Number: 3
3

(defn get-num []
  (print "Number: ")
  (flush)
  (let [input (read-line)
        num (try 
             (Long/parseLong input)
             (catch Exception _))]
  (if num
    num
    (recur))))

that last line should probably be (or num (recur)) but that expands to the same thing anyway

Ahhh! Very informative. So you are saying the try will return nil if it is not a number which evaluates as false in the if statement?

try returns the body of the catch if it is invoked, here the catch implicitly returns nil as there's no body

you could make it explicit by adding false or nil to the catch

and yes, clojure treats nil as false in conditionals

awesome. thanks for all your help!

What exactly does flush do here?

it means you can get input on the same line as the prompt

it flushes partial output (newline automatically gets flushed)

perfect. That fits the problem description exactly as well. Thank you!

here's an alternate version (does the same thing, may or may not be clearer)

(defn get-num []
  (print "Number: ")
  (flush)
  (or (try (Long/parseLong (read-line))
           (catch Exception _
             false))
      (recur)))

also, I've seen (defn safe-long [s] (try (Long/parseLong s) (catch Exception _))) in multiple code bases, sometimes I write it, sometimes someone else does

I prefer the first one but both really clear things up for me

Awesome. Maybe I should start my own little util file and keep such things in it.

It is also possible to use clojure.edn/read-string and check if the resulting thing is a number?, int? or integer? depending on what you want. That avoids the try/catch as well.

but it will still throw on eg. ")"

Good point.

I don't like using read-string for user input

lojure 1.10.1
(cmd)user=> (read-string ")")
Execution error at user/eval1 (REPL:1).
Unmatched delimiter: )
(ins)user=> (read-string "#=(java.lang.System/exit 1)")
justin.smith@C02ZW014LVDN ~ %

that is, #= is read-eval, and it does things you probably don't want

I thought edn/read-string did not have the unsafe bits. I may be wrong.

oh, I missed that you specified clojure.edn

Well, someone's bored.

if you got a bit of time on your hand, i am wondering how to get sums of repeating keywords in a nested map. For instance if i have a map: {:a 1 :c { :a 2}} how do write a function (f map :a) -> 3

imagine the nested map has multiple levels and i am only interested in summing the values of a certain keyword that might be repeating at different nested level of the map, could also be in a vector

i tried postwalk but it doesnt seem to be fit for the task

I take back my statement. Typed without thinking.

user=> (->> (tree-seq coll? seq {:a 1 :c { :a 2}}) (filter map-entry?) (filter (comp #{:a} key)) (map val) (apply +))
3

I think this one is better, does the same thing

user=> (->> (tree-seq coll? seq {:a 1 :c { :a 2}}) (filter map?) (keep :a) (apply +))
3

Thanks, I was actually thinking about tree-seq. Forgot the name.

wow thanks, that works great! my postwalk involved atoms to track a counter but now i see that theres a much simpler way

tree-seq is great for when I want to process my tree as if it were a graph :D

that is, if only data types / keys are significant, and not the relative layer in the nest

thank you so much!

I am not completely sure how it applies, but in general in functional programming, fmap indicates an operation that applies a pure function to a value in a context. So fmap allows you to apply a function that evaluates a value coming from a generator to create a new value. From what I can see, gen/let allows you to evaluate a value coming from a generator to create a new generator.

In other languages, this would make let strictly more expressive.

But this is my ‘general’ functional programming experience speaking

Take a look at gen/bind, to which gen/let expands

(Hope this helps!)

Thanks a lot!!