I’ve been using a lot of Elixir, and in that language you can use pattern matching to dispatch to different implementations of a function depending on the arguments. In my case right now, I just want to dispatch based on whether the first arg is a string or a list. What’s the standard way to do this?

What is what I’m talking about called?

polymorphic dispatch?

Both multimethods and protocols seem like too much for what I’m trying to do. I assume there is something simpler?

Is this preferred for something this simple?

(defn something [arg]
  (cond
    (str? arg)  arg
    (list? arg) (other-fn arg)))

that is fine, although it is string? not str? and seq? or sequential? is likely what you want not list?

but I would avoid that kind of thing, a function where an arg can be different types

isn’t that the essence of a dynamically typed language?

or you mean just dispatching based on type?

I mean it is weird to have a function that takes a string or a list as the same argument

multi method seems the right choice though

the only thing I would thing if is some really annoying apis will return a string value instead of a list containing one string

that’s my situation

sometimes it’s a string, sometimes its a list of strings

I would undo that as soon as possible, meaning take the api response and box it in a vector if it is not already a list

and then everything after it is fixed up doesn't have to worry about it

I guess I’ll do that, but I was more interested in learning a general lesson rather than solving this specific problem

and box is a fairly common function to write, and it looks sort of similar to your something function

But maybe that is the lesson

All right thanks for your help guys

ring middleware is kicking my ass 😄

I want to check if the session variables are set, if so give you the app, if not give you the login screen

but I get lots of Null Pointer Exceptions ;-;

@chrysalis129 feel free to ask your beginner questions here. I have to ask : is this a class homework problem? I've seen several people ask for help with the exact same problem in various places.

yes it is its an assignment due on sunday 😧

if anyone can help me with debugging this that would be great

https://pastebin.com/4rfjbZZ1

That explains why your class mates are all posting the same question then 😁

I'm eating right now but will be happy to help later -- or any time over the weekend

ok sounds good 🙂

Seems like everyone is struggling with it. Makes me think maybe the teacher isn't doing a very good job ...

yea the prof usually only teaches syntax when it's not relevant to an assignment.. plus he often goes on tangents during lectures so it's hard to pay attention

first assignment he gave = super easy and now this one super hard 😧

I sympathize -- my first year comp Sci prof was terrible 🙃

Is it working but taking too long?

And what are y’all going over in class?

it's not i got most of the functions to work but cities doesnt print all the csv values anymore and i'm trying to find a way to do closest-city-pairs without the use of ->>

https://pastebin.com/9bhrf08R

also theres another function i need to implement

its to take all the :population key values from all the hashmaps in cities and sum them up some how

im trying to map the populations to a vector but it doesnt seem to want to print too

(defn total-population [cities] (for [x (map :pop (cities)) :let [y x]] (print y)))

You have parens around cities -- are you passing in function?

yea

cities returns the hash map of each row in the csv file

Sorry I’m down to help and explain but I still don’t see a question or what you working. Just the whole file

ok nevermind i got the total population working

now for the last function i need to do

i need to print :province and :pop from all the hash maps in cities and sort them in order from smallest :pop to largest

any idea where i can start with that?

sort-by :pop

ok

how would i go about printing just :province and :pop from cities(returns a vector of all hash maps in a csv file))

also is there anyway to complete this function without using ->> and (filter large) dpsutton if i might ask?

Why do you want to avoid the threading macro?

And which function?

Because theres a thread with the solution on reddit and i feel all my classmates are going to copy it so I want to find a different way to do the function

(defn closest-city-pairs [] (let [y (->> (cities) (filter large) vec)] (for [a (butlast y) b (rest y) :when (and (not= a b) (close a b))] [(:city a) (:city b) (distance a b)])))

i tried doing it this way

Well you can remove the vec and just use (filter large (cities)) but I think the for loop is the more tell tale expression there

but i get unsupported binding form :when

yea it is lol

What is your current version? Let’s see what’s up with the :when binding error. If I had to guess you’ve got that in the let rather than the for

(I'm back from dinner and have a beer and a warm REPL!)

In that for comprehension you're going still compare N-2 of those cities against themselves so I don't think you're saving much with butlast and rest -- you already need (not= a b) so you might just as well do (for [a y b y :when (and (not= a b) (close a b))] [(:city a) (:city b) (distance a b)])

(since close calls distance anyway and just checks it against 600, I'd probably have :let [d (distance a b)] :when (< d 600) inside the for and then [(:city a) (:city b) d] in the body of it)

(or have a predicate near? that takes a distance and check it against 600km: (defn near? [d] (< d 600)) -- then :when (near? d) in the for per above)

Lots of ways to write it.

You can also use (< 0 d 600) and then you don’t have to check not=

Since distance is a metric distance is zero if and only if the points are the same

Per @dpsutton’s comment above, since you're using for and it only cares about sequences, you definitely don't need to call vec on the filtered sequence of cities:

(defn closets-city-pairs []
  (let [large-cities (filter large (cities))]
    (for [a large-cities b large-cities
          :let [d (distance a b)]
          :when (< 0 d 600)]
      [(:city a) (:city b) d])))

^ @chrysalis129

thanks that helps

okay now for some reason the distance between two cities isnt accurate anymore

Use your repl with two simple cities and investigate

nevermind it works fine scratch that

You can use Pythagorean triples so the distances are nice easy numbers. (0,0) and (3,4) will give you a distance of 5

how can i get print out a vector of hash maps that just returns the :province and :population

if i have cities function that returns the vector with all the hashmaps and keys

select-keys will help

nice

Also, if you want to map a function over a sequence and get back a vector instead of a sequence, you can use mapv (there's a filterv as well)

ok but i need to iterate over a vector with multiple hash maps.. how do i just get the :pop and :province

select-key doesnt seem to work if i try to pass (cities) into it

Select keys takes a single map. So you can map it over a sequence of maps

hmm

(mapv #(select-keys % [:pop :province]) (cities))

(map inc [1 2 3]) will call inc on each item in the vector. Likewise you want to select keys for each map

@chrysalis129 Some good advice about map and functions https://stuartsierra.com/2015/08/10/clojure-donts-redundant-map (when you're ready to think about idiomatic code as opposed to just working code).

okay thanks

ill take a look

i tried this

(print (mapv #(select-keys % [:province :pop]) (cities)))

it seems to only be returning the :pop for each tho

Are you sure that all the city maps have :province as a key?

o true lol

had it as :admin

for some reason

Administrative Region 🙂

😮

Yeah, I've seen that coming back from Google Geo APIs.

okay so it prints like this

{:admin Nunavut, :pop 0}]

is it possible to get rid of :admin and :pop so it just prints the values?

also i tried sorting it in terms of population

(print (mapv #(select-keys % [:admin :pop]) (cities)) (sort-by :pop))

its giving me wrong number of args passed thgo

(mapv vals coll) will give you sequences of just the values from hash maps

You might not always get them in the same order tho' since hash maps are unordered. If you always want :admin then :pop, you might look at the juxt function

(mapv (juxt :admin :pop) (cities)) will give you a vector of vectors, each subvector containing the :admin value, followed by the :pop value.

okay yeah that works

thanks

ok it goes from largest population to lowest

where do i put sort-by to flip it?

You can either sort the city collection, then map over and select just the two fields, or you can do it the other way around.

In the first case you would sort-by :pop since you have hash maps with the key :pop

In the second case you would sort-by second since you now have vectors with the population in the second position

ok

is there any function that can parse 20% to 0.2?

How is 20% represented @haiyuan.vinurs ?

(if it's just an integer -- 20 -- you can just divide by 100, or by 100.0 if you specifically want a double result rather than a ratio)

ok i tried doing the first option sean

its saying im giving the wrong number of inptus tho for my function

(defn provincial-population [] (let [y cities] (sort-by :pop) (mapv (juxt :admin :pop) (cities)) ))

(sort-by :pop coll) <-- needs a collection

Remember that Clojure is a) expression-based and b) all immutable. Sorting produces a new collection that is a sorted version of the input collection.

i see

@seancorfield i read the string 20% from file, so i need to parse it

So you're either going to have (sort-by ... (mapv ... (cities))) or (mapv ... (sort-by ... (cities)))

->> can make the order of operations clearer, if you want.

@haiyuan.vinurs so you have "20%" as a string?

ok i got it to print

but its still doing largest populations first

(defn provincial-population [] (let [y cities] (sort-by :pop (mapv (juxt :admin :pop) (y))) ))

now i want convert the string 20% to float 0.2

@chrysalis129 Read what I said above about the order of sort and select keys. You're trying to sort on :pop after you've produced a vector of vectors.

ohhh

@haiyuan.vinurs are you guaranteed all the values are <some sequence of digits> always followed by %? If so, you could just do a substring operation to remove the last character, then parse it as a long, then divide by 100.0

(Long/parseLong "20") will produce 20, for example.

thanks that worked sean

Much love to you guys here 🙂

finished my assignment

this is a great place

Assignments are never "finished" 🙂

🙂

Let us know how the marking goes on it.

I will 🙂 thanks again

actually im not done my apologies -_-

i just realized we have to sum up each province because theres hash maps with the same province

and then sort that

hmm

i think ill need a for loop yea?

Excellent time to look into reduce

Or you could look into group-by

(defn distinct-provincial-population [] (for [[p xs] (group-by :province (cities))] [p (reduce + (map :pop xs))])) is what i got so far

any ideas how to proceed from there

Sounds about right -- but isn't it :admin rather than :province?

Hi, everyone I keep running into a problem with records and protocols : We have a couple of protocols in separate namespaces, the record implementing those protocols resides in a separate ns. Whenever I uberjar and import it as a lib in another project it gives the exception : *no impelmentation of method ... of protocol ... found for class*. Is there sth I am missing about protocols & records? Does it have to do with which ns s are imported where I am using them?

Probably. You need to load (require) any namespaces that contain implementations before you can use them

I imported the record and the exception went away. (The namespace containing the implementation was already required). Do I always have to import the record or there is some other thing at work here?

It depends what you’re doing but you should not generally need to import

I have a middleware for checking logins, I want to check the session variable and redirect to login if it's not set

but i'm stuck in an infinite loop of login checking middleware

actually it seems to work now but i keep getting null pointer exceptions on RegEx matches

I'm having a beginner moment. Why does the following always return "dunno"?

(defn color->name [color]
  (case color
    "#ffffff" "white"
  	"#000000" "black"
    "dunno"))

Using a switch case in Java "works". How is Clojure seeing the "#" differently?

😄

try it without the dunno and see if you get "no matching clause"

oh

hmm

Actually your code works for me when I put in "#ffffff"

jpdrills.server=> (defn color->name [color]
             #_=>   (case color
             #_=>     "#ffffff" "white"
             #_=>       "#000000" "black"
             #_=>     "dunno"))
#'jpdrills.server/color->name
jpdrills.server=> (color->name "#ffffff")
"white"
jpdrills.server=>

Ok, so I just copy/pasted it again, and now it's working for me.

not your question, but why not just make a hashmap of it, then use that as a function?

@tws makes a great point, could you provide an example for our new friend?

(def color->name {"#ffffff" "white", ...})

(map color->name color-coll)

you’ll have nil instead of "dunno"

I like that solution. I was just confounded why it wasn't working for me.

you’re calling the old function

I found my problem!

Thanks for the help. 😀

Wow I was missing an end-brace in my one authentication handler

err it was misplaced in the map, the ending }

😄

So yeah! don't pull out your hair if you have been working all day and you suddenly "break everything" you probably accidentally put an ending brace } someplace inopportune and you can find it with clear eyes in the morning.

Is it possible to do (swap! jpc-users-atom update-in [email :kanji-stumbles] conj kanji) and then simply do (swap! jpc-users-atom update-in [email :kanji-stumbles] disj kanji) to remove one?

Ignore swap. Does the day structure you’re updating allow conj and disj?

If it’s a set, then yes.

Swap just calls your function on the value of the atom. You can test everything outside of atoms and swapping and then know it will work in the context of atoms

Gotcha

can someone help me troubleshoot flycheck-clj-kondo? i've got flycheck installed (`M-x fly` brings up flycheck commands), I've got clj-kondo on my $PATH as evidenced by being able to fun it from the command line, but it's not doing any linting

M-x flycheck-verify-setup shows "no checkter to run in this buffer"

and running clj-kondo --lint my-source.clj returns 0 without seeming to have done anything

is there not a default configuration for clj-kondo?

@tstirrat what operating system are you on? what do you see when you do which clj-kondo and clj-kondo --version?

➜  ~ which clj-kondo 
/usr/local/bin/clj-kondo
➜  ~ clj-kondo --version
clj-kondo v2019.10.11-alpha

this is on ubuntu

can you maybe first upgrade to the newest version?

sure

and how did you install this version?

https://github.com/borkdude/clj-kondo/blob/master/doc/install.md#installation-script-macos-and-linux

installation script

i suppose i could try yarn/npm

hi i'm back

so i tried implementing this and have been trying to sort it from least to most

(for [[p xs] (group-by :admin (cities))] [p (apply + (map :pop xs))]))

but when i try applying sort-by in the manner i did before i get wrong number of args error

(for [[p xs] (group-by :admin (cities))] [p (apply + (map :pop xs (sort-by :pop)))]))

@chrysalis129 https://clojuredocs.org/clojure.core/sort-by

sort-by requires a key-fn for comparison

i see

you could take the result you have from the for loop and wrap that whole result in (sort-by > val ... )

basically it needs to know what you think matters for sorting

dont know what u mean

but i tried adding cities

but its still largest to smallest

flip the operator

You have (sort-by :pop). There is no collection in that function call

which operator?

And if you sort it and then map and sum everything the sort is unnecessary

right, the order of operations works from inside-out, so if you want it sorted you need to put sort-by on the outside

ok i did

(for [[p xs] (group-by :admin (cities))] ((sort-by :pop (cities)) [p (apply + (map :pop xs ))])))

getting arrayseq cannot be cast to lang.ifn

is cities a (function) ?

yea

what's the latest line of code you had that was working?

(for [[p xs] (group-by :admin (cities))] [p (apply + (map :pop xs))]))

nice, it has a result you like, but it's not sorted correctly?

its sorted from largest to smallest

need smallest to largest

what does the result look like my friend?

oh yea its not even sorted

it just lists the provinces

and their pops

it adds all the hashmap province populations correctly

just need to figure out where to put sort-buy

Excellent work so far

sort-by i mean

(sort-by second (for [[p xs] (group-by :admin (cities))] [p (apply + (map :pop xs))]))

Assuming you want to sort the pairs of province / population.

Say it out loud in words. “Once I compute the pairs of city name and counts, I need to sort by the population count”

thanks sean that did it 🙂

Then it’s obvious the sort goes outside of the computation

i see so like last operation that needs to be done goes on the outside

Right, because function arguments are evaluated before the function is called.

(sort-by second (for ,,,))
;; evaluates (for ,,,) first
;; then calls `sort-by` on the result of that evaluation

And what does Second do? just grabs the second parameter of each thang?

Yes, because we have a sequence of pairs (province, population)

Second in this case is the population ?

Ah okay awesome

(first pair) => province; (second pair) => total population (the sum of the population of each city within that province

i see

So it's always a pipeline of operations, starting with the innermost and working outwards.

ok

House.Price.Erase in clojure talk becomes (Erase (Price (House))) right?

Collection.Second.Sort-by (Sort-by (Second (Collection))) not a perfect mapping but maybe some good examples could help illustrate how functions bloom from inside out

As Dan said above, say it out loud to get the order of operations -- and think about it as a single expression, rather than a sequence of imperative steps.

got it

mm that's true, functional does not rely so much on sequential procedure

but in the abstract what do you want this transformation on this collection to look like

ok so going back to a function i was doing previously

actually nevermind

i think im done 🙂

Rock n Roll

thanks for the help guys

I owe much of my sanity to the fine chaps here.

ill probably come back here when final exam season comes around 😛

lol

And that function composition / call ordering is also why ->> might make it easier to think about:

(->> (for ,,,) ; compute total population for each province as pairs
     (sort-by second)) ; then sort by the second element of each pair

hey that's a great time to introduce thread-thru and double-thread-thru

so that makes it not about order of operations?

->>?

think of a spool of thread

Is the class in clojure or just this particular homework?

its just an assignment

->> thread-last takes the expression and puts it into the last slot of each "step" in order -- turning it into the nested function call you saw above.

clojure class would be Dope

class is about a bunch of programming languages

ok makes sense

Which other PLs are you studying @chrysalis129?

do you mind saying which institution?

(Understand if you don’t want to)

were doing scala next

and some other stuff

but clojure and scala most indepth

yea sorry bout that dps 😧

Well I’m glad that clojure is in curriculum somewhere :) good luck with your studies

thanks dpsutton 🙂

Scala is... interesting... I did that for about 18 months before switching to Clojure. Do you already know Java @chrysalis129?

Yea I do

know it a lot better than clojure thats for sure xd

also know python C++ node.js

trying to learn some other frameworks so i can get a software engineering internship soon

hello again I have one more question

say I have a one string that is "Hello" and a second string that is "Apple"

is there anyway I can evaluate these two based on alphabetical order something like (> firstString secondString) evaluates to true?

Have you tried exactly that?

yea

doesnt work

java.base/java.lang.String cannot be cast to java.base/java.lang.Number

Try compare

i get -7

And (doc compare)

cant resolve doc

(clojure.repl/doc compare)

Not sure what your repl environment looks like sorry

yea doesnt work either 😧

How are you running clojure?

cmd with java -cp clojure-1.8.0.jar clojure.main filename.clj

But check out https://clojuredocs.org/clojure.core/compare

ok thanks i got it to work xd

@chrysalis129 you should consider running clojure with integration in your text editor - it’s a far smoother, richer experience

What platform are you on?

Hey all, in a philosophical moment I’m interested in a comparison or discussion of Clojure to other lisps/schemes/whatevers. Doesn’t need to be syntax focused, I’m as much interested in things like developer experience, “feeling”, purpose and historical accident results of various languages. Anything, basically. Any content anywhere to recommend? 🙂

One specific thing... what makes Clojure Clojure, as opposed to “almost Common Lisp” (or whatever). What subset of Clojure would be somehow enough to be thought of as Clojure, if at all possible or relevant?

(Yes, vague and fluffy. Any and all replies highly welcomed 😅)

@mattias504 That's probably a discussion better suited to #off-topic or perhaps #other-lisps rather than the #beginners channel (although I understand that beginners may be interested in this topic)

I would seed that discussion with the material here https://clojure.org/about/rationale and here https://clojure.org/about/lisp (the latter is very short tho') -- most of the left-hand links in that "About" section are worth reading as background to your question and provide quite a bit of insight toward answers for it.

Thanks a lot! I’ll dive in and use other channels going forward!

I'll keep an eye on both of those channels for any follow-up discussion -- it's a topic that interests me (and I have some opinions but not really in-depth knowledge of other lisps).