Clojurians Log v2

I was checking documentation about do in the cheat sheet and found that it executes all the function one -by-one, but clojure is smart enough to handle execution one by one right!, what is the purpose of do then?

@popeyepwr Consider if. It can only accept a single expression for "then" and a single expression for "else". What would you do if you wanted to print a message in both arms and also return a value?

yeah, I am able to connect the dots now, if we have if and we need multiple statement to be executed then we use do

There's a few other places where something expect a single expression, but if you want to pass in more than one you can wrap it in a do, but if is one of the main one.

Hello! How do I write a spec definition to say ::lrow must be larger or equal to ::row ? Currently have (s/def ::lrow (s/and nat-int? #(<= % max-rows) #(<= % ::row))) and

(s/def ::coord
  (s/keys :req-un [::row ::col]
          :opt-un [::sheet ::lrow ::lcol]))

you have to keep in mind, spec is to some degree not oriented around specing maps

when you define ::lrow you are specing what ::lrow should mean in every map everywhere in your program

you are not defining it in relation to other things it may be aggregated with, because the set of things it may be aggregated with is open

if you have a constraint based on the relationship between things in an aggregate (::cord) you spec it on the aggregate

I think I get what you mean, but is there an example of how I can do this? Understand the idea but not the syntax

(s/def ::coord
  (s/and
    (s/keys :req-un [::row ::col]
            :opt-un [::sheet ::lrow ::lcol])
    (<= ::row ::lrow)
    (<= ::col ::lcol)))

::row and ::lrow are keywords

<= takes numbers

<= returns true or false, s/and takes specs / predicates

You can use proper clojure functions in specs

Including anonymous functions. So you can have something like (s/and (s/keys ...) (fn [m] "do some operation on the map"))

Apologies, I'm on the phone. The point of the function in this case is to describe a property, not do any arbitrary operation, sorry if the above was confusing. As a more simple example, you can definitely say (s/def ::int (s/and ::number integer?))

So is there a way to say that one must be greater than the other then? :o

Yes. Disregard specs for a moment. If you had a map, how would you check whether the value of key :k1 is greater than the value of :k2?

(when #(> :k1 :k2)
do-something))

i have defined a variable (def abc {:a "b" :c "d" :e "f"})

why ({:keys ["a" "b" "c"]} abc) returning nil

the :keys destructuring needs symbols, not strings. So try ({:keys [a b c]} abc)

#(> (:k1 %) (:k2 %))

% is your map

because that's what the function expects as input, it needs the whole map to check this property

So how would you rewrite the spec now?

that is assuming you want to destructure. Just calling it like that is creating a new map with the key :keys and value [a b c] associated with that key. Then that map is used to lookup if anything in that map is associated with abc . To which the answer is no, since the only key is :keys .

Hence you get nil.

({:keys [a c e]} abc ) - this gave unable to resolve symbol

#(<= (::row %) (::lrow %))

I see I see!

destructuring is make sence only in some context such as let ’s binding form and argument declaration for function

like this

(let [{:keys [a c e]} abc]
  ;; you can use a, c, and e here
  )

Thanks 🙂

yeah got it !!! thanks @d.wetzel86 @delaguardo

I was doing this exercise from the braveandtrue book, the aim is to create a macro that will create a number of function that get a specific key from a map eg. c-int is defined as (defn c-int [map] (:intelligence map)) etc.. and the macro should be used like this (create-attribute-fs c-int :intelligence c-str :strength c-dex :dexterity) my solution looks like this

(defmacro create-attribute-fs
  ([] nil)
  ([fn-name key & nks]
   `(conj (create-attribute-fs ~@nks)  
          (defn ~fn-name [m#] (~key m#)))))

I figured that it's because the elements of the list created by conj are evaluated

🙂 I just did and finisch that one . Which chapter was that ? @antbbn

Chapter 8

aha, I see it

I did it something else

feel free to post your solution 🙂

key is for you the function and nks the arguments

I could but then you would not learn anything

but I begin with :

(defmacro defattrs
  [& assignments]

what can you use and have we learned to iterate trough a collection ? @antbbn

map?

yep

and I want to give you a hint I also get. Look at partition

@antbbn

yeah I would imagine that mapping over the partition works too, i was just curious about the need for a (do)

could maybe also work instead of a map. I do not know

@d.wetzel86 do you know if we need a do ?

in my solution where I use a map I do not use a do @antbbn

yeah me neither

you will need a do for that macro. Since you are effectively returning a piece of code from a macro, and you want to do multiple things in that piece of code (any number of defn in this case), those need to be in a do.

??? we did not use a do but a map

We did splice unquote the map into a (do ...` in the end. 🙂

@antbbn then I apolize for this error of saying I did not use a do

but my macro does define all the functions it's supposed to

that's why I was confused

could be, there are more ways to solve a problem

@antbbn what does your macro look like atm?

like here https://clojurians.slack.com/archives/C053AK3F9/p1610448385038400

okay, your macro works, but I don't know if you intended that behaviour. Let's look at a call like this:

(create-attribute-fs c-int :intelligence c-str :strength)

(conj
  (conj
    nil
    (defn c-str [m] (:strength m)))
  (defn c-int [m] (:intelligence m)))

(create-attribute-fs c-int :intelligence c-str :strength)
=> (#'user/c-int #'user/c-str)

A question I have is if the resulting functions work with your character map from Chapter 5? So does (c-int character) work?

yep

but returning the sequence of functions might not be the intended behaviour

It's not a bad thing either though, so you can just roll with it 😄

ehehe all right 🙂

Edit: solved 😄 Hello, I have a question around repl set up in intellij with the cursive plugin. I used the lein new app app_name command, and can run the app from my terminal. I haven’t changed anything at all in the scaffolded app. When I try to run the repl, I get Error: Could not find or load main class clojure.main Caused by: java.lang.ClassNotFoundException: clojure.main and all the function names are highlighted as not resolved. Any idea what might be causing that?

Probably best asked in the #cursive channel.

What I always do when creating a new leiningen project is create it at the command line like you did, and then in IntelliJ, select File -> New -> Project From Existing Sources. That registers the project as a Leiningen project and loads all the dependencies, etc. If you didn’t do that, that might be why it’s giving you problems.

Do you have clojure in project.clj :dependencies? Ie.: [org.clojure/clojure "1.10.1"]

short answer: yes

long answer - here is my project.clj

(defproject comparer "0.1.0-SNAPSHOT"
  :description "FIXME: write description"
  :url "<http://example.com/FIXME>"
  :license {:name "EPL-2.0 OR GPL-2.0-or-later WITH Classpath-exception-2.0"
            :url "<https://www.eclipse.org/legal/epl-2.0/>"}
  :dependencies [[org.clojure/clojure "1.10.1"]]
  :main ^:skip-aot comparer.core
  :target-path "target/%s"
  :profiles {:uberjar {:aot :all
                       :jvm-opts ["-Dclojure.compiler.direct-linking=true"]}})

I will try that now!

actually, just looking at the file tab, there was an option to add it as leiningen project, and that did the trick!

thank you!

for the future, you can right click on project.clj or deps.edn files and import it

if I want to do web developement in clojure as soon as i finished the brave book, can I then better make a leiningen or a clojure-clj project ?

I personally just use clj and shadow-cljs. But getting familiar with leinigen is beneficial. Also for web dev have a look at: https://luminusweb.com/ And consider reading: https://www.amazon.com/Web-Development-Clojure-Build-Bulletproof/dp/1680500821

clj; back or front -end, or both?

Here's a small (server-side) web app that uses the Clojure CLI and deps.edn: https://github.com/seancorfield/usermanager-example -- I would suggest making sure you understand how this works and that you can modify and enhance it yourself before trying to work with Luminus (or the Web Development book). Luminus is a lot of moving parts and uses a lot of different libraries. It's a huge step up from what you're currently working with in Brave&True.

When I evaluate a line referencing a function, after changes were made to that function, those changes aren't being reflected when evaluating the line. Evaluating that function and then evaluating the line referencing it shows the changes. Why is that?

can you be a bit more specific what "changes were made to that function" means?

Yeah, so say I have a function that adds 2 to a provided number, then I realize it should be adding 3. I'll change that number to 3 inside of the function, evaluate a line referencing that function, and it'll still add 2.

But if I evaluate the function and then evaluate the reference, that change will be reflected.

right. you need to reevaluate the function

until you reevaluate it with 3, the function exists but with the value of 2.

Cool, thank you! Just taking some getting used to on my part.

If it helps: Clojure is always compiled, even in the REPL, and compilation happens as you evaluate each top-level form. It's a good practice to get into to instinctively "eval top-level block" after every change you make to any function.

Funnily enough, I just gave a talk to the London Clojurians about this very topic... recording to be available soon!

Awesome, that does help! I'll keep an eye out for the recording.