Is there some built in function or middleware to add no-cache headers for a ring app? a search didn't find anything

Well.. it is sending them now in FF and chrome. not Edge. So guess ignore this unless someone has some valuable info I should be aware of. Thanks!

Ok, so I read it wrong. Apparently ff and chrome send nocache headers in the request. IE does not. So i'll have to add the response headers to force it not to cache. So back to my original question. Any ring middleware for this yet? I'll keep looking, then make my own.

@stvnmllr2 It doesn't seem like it warrants middleware for that? Are you saying you want all requests to respond with nocache headers? That's a one-line function... a middleware library would be overkill.

ring.util.response/header or ring.util.response/update-header is all you need, unless I'm misunderstanding your question?

This SO answer seems to be fairly comprehensive in terms of what headers to actually set https://stackoverflow.com/questions/49547/how-to-control-web-page-caching-across-all-browsers

When I start a REPL, how come it doesn't already know the functions from my code? I'm starting it in the same directory as the source code.

I don't even know if I'm wording that question correctly. Total newb here 😬

Because when you load a REPL by default it's not in the namespace of your code

There are flags you can pass when starting the REPL to make it be in that namespace

But you can always just load your code with a (require ...) call

oh I see. I read lein help repl but it didn't seem to explain those flags

do you know where I could find the correct help file to learn about these flags?

I'm afraid I use boot, so I don't know much about how to do it via lein

I can't remember the flags off the top of my head, but the default namespace for a boot trigger repl is boot.user

Is there any existed "search engine" library for Clojure? I want to build a simple meta search engine which combine some search engines like Google, Bing etc results.

I did a search on http://clojars.org, only find one related library called "web-search", but it only use Google. I wish to create a meta search engine which combines difference sources results.

What is idiomatic Clojure for the pattern “decorate a collection, find the max decorated value, return the original value”? So for example, I have a vector [:a :b :c :d] and a function f so that (mapv f [:a :b :c :d]) yields [6 9 8 7]. I want to combine those together so that I get :b.

@orestis (max-key f coll)

http://clojuredocs.org/clojure.core/max-key

@schmee Thanks, I’ve seen that and even tried it but for some reason never realized that’s what it’s doing 🙂

haha, there are a lot of function that do a lot of things in clojure, it’s not easy to keep track of them all 😄

OK, another question; how can I produce a lazy sequence in the same style as Python’s generators? i.e., yield a value, do some computation, yield another, and so on?

Seems like for is the mental equivalent…

Reference: I’m working on past AoC problems, and as part of one I have to generate list of length N of non-negative numbers that must add up to K. For N=2, K=10, I can do: (for [i (range 11) j (range 11) :when (= 10 (+ i j))] [i j]) - but how I can do it for the general case for arbitrary N? Write a macro?

@orestis A macro doesn't help you if you don't know the parameter at compile time. You'll need to use a different approach

does with-redefs-fn see past function boundaries?

Hello, I’m having problems running clojure.spec. When I do (s/def ::key val?) it fails to compile with this error:

Caused by: java.lang.IllegalStateException: Attempting to call unbound fn: #'clojure.core/ident?

@orestis for generation: look at iterate and some of the collection transformations e.g. filter

clojure doesn't have arbitrary suspension like yield in python

but you don't need it for like 99% percent of usecases

@rauh @ghadi Thanks. I guess with python I was used to “this is the next value, throw it over the wall”. Now I have to think a bit more 😉

clojure tends to favor functional transformations over potentially irregular control flow

i am trying to format my ClojureScript file

soz wrong channel 🙂

@stvnmllr2 you could consider alternatively

(update (handler request) :headers
        assoc
        "Pragma" "no-cache"
        "Cache-Control" "no-cache, no-store, must-revalidate"
        "Expires" "0")

I actually find the code with explicit calls to header more readable in terms of intent. YMMV.

(defn headers [& kvs] (fn [response] (apply update response :headers assoc kvs))

Hey all, would love some feedback on these palindrome functions and tests. 🙂

http://app.klipse.tech/?cljs_in=%3B%3B%20Palidrone%20Functions%20by%20Jim%20Lynch%0A%0A(defn%20is-palindrome-brute-force%20%5Bs%5D%0A%20%20%3B%3B%20Checks%20whether%20a%20string%20is%20a%20Palindrome%20by%20comparing%20the%20string%20with%20the%20reverse%20of%20string.%0A%20%20(%3D%20s%20(apply%20str%20(reverse%20s))))%0A%0A%0A(defn%20is-palindrome-half-and-half%20%5Bs%5D%0A%20%20%3B%3B%20Checks%20whether%20a%20string%20is%20a%20Palindrome%20by%20comparing%20the%20first%20half%20with%20the%20reverse%20of%20the%20second%20half.%0A%20%20(if%20(%3D%20s%20nil)%0A%20%20%20%20false%0A%20%20(%3D%20(take%20%20(.floor%20js%2FMath%20(%2F%20(count%20(seq%20s))%202))%20(seq%20s))%0A%20%20%20%20%20(reverse%20(take-last%20(.floor%20js%2FMath%20(%2F%20(count%20(seq%20s))%202))%20(seq%20s))))))%0A%20%20%0A%20%20%0A(require%20'%5Bcljs.test%20%3Arefer%20%5Bis%20deftest%5D%5D)%0A%0A(deftest%20palindrome-6-letters%0A%20%20(is%20(%3D%20true%20(is-palindrome-brute-force%20%22lollol%22)))%0A%20%20(is%20(%3D%20true%20(is-palindrome-half-and-half%20%22lollol%22))))%0A%0A(deftest%20palindrome-7-letters%0A%20%20(is%20(%3D%20true%20(is-palindrome-brute-force%20%22racecar%22)))%0A%20%20(is%20(%3D%20true%20(is-palindrome-half-and-half%20%22racecar%22))))%0A%0A(deftest%20non-palindrome-6-letters%0A%20%20(is%20(%3D%20false%20(is-palindrome-brute-force%20%22leller%22)))%0A%20%20(is%20(%3D%20false%20(is-palindrome-half-and-half%20%22leller%22))))%0A%0A(deftest%20non-palindrome-7-letters%0A%20%20(is%20(%3D%20false%20(is-palindrome-brute-force%20%22whatsup%22)))%0A%20%20(is%20(%3D%20false%20(is-palindrome-half-and-half%20%22whatsup%22))))%0A%0A(deftest%20palindrome-single-letter%0A%20%20(is%20(%3D%20true%20(is-palindrome-brute-force%20%22a%22)))%0A%20%20(is%20(%3D%20false%20(is-palindrome-half-and-half%20%22a%22))))%0A%0A(deftest%20palindrome-nil%0A%20%20(is%20(%3D%20false%20(is-palindrome-brute-force%20nil)))%0A%20%20(is%20(%3D%20false%20(is-palindrome-half-and-half%20nil))))%0A%0A%0A(palindrome-6-letters)%0A(palindrome-7-letters)%0A(non-palindrome-6-letters)%0A(non-palindrome-7-letters)%0A(palindrome-single-letter)%0A(palindrome-nil)%0A%0A(println%20%22Tests%20finished%20running!%22)%0A%0A

Is the "brute-force" one better because it's shorter, or is the other one better because it's more efficient (and is it actually more efficient)?

That link won't even open in my browser (and it's a really ugly link to paste into chat). Can you put the code in a Gist and provide the link to that instead? That way folks can comment on it, fork it, etc.

@derpocious I bet the most performant thing would use (-> s (StringBuilder.) (.reverse) (str))

its cljs i think.

oh, I should have seen that, thanks

yep, cljs

Here's a gist link: https://gist.github.com/JimTheMan/c62b226ec2bfee1fd5caa21837c899e4

@derpocious some things to try - comparing (seq s) to (reverse s) is less code, and likely faster than making a string in order to compare it, if you care about performance use let blocks to bind computations instead of doing them twice, clojure doesn’t optimize that sort of thing automatically

(also using a let instead of doing ops inline twice is more readable, and will rarely be less performant)

true, good points!

Bear in mind that (let [s ""] (= (seq s) (reverse s))) is false which seems wrong to me...

oh that’s weird

So I think you might want (= (seq s) (seq (reverse s))) so that an empty string (and nil) are both considered palindromes.

@seancorfield good catch, I did not expect that!

(seq "") => nil

if we extended IReversible to String (which I bet is totally doable) then we’d likely get near optimum performance out of the seq / seq reverse method

as it is, it appears cljs currently does an eager reduce to reverse a string into a sequence, which is potentially wasteful for this task

though seq on String is reversible, so in this case (= (seq s) (seq (reverse (seq s)))) could potentially perform better? I’d just test it if criterium existed for cljs heh

… oh I should try this some time https://github.com/michalmarczyk/criterium/tree/cljs

excellent catch @seancorfield !

@noisesmith for me, (= (seq s) (seq (reverse s)))) is still returning false when s is empty string

(ins)dev:cljs.user=> (= (seq "") (seq (reverse "")))
true

sorry, it's failing for me on palindrome-nil

we wanted (isPalindrome nil) to return false

oh - then you need an extra check for that, I didn’t see that expectation

if you drop the seq on the reverse call it will

reverse returns a seq, an empty one on ""

but then you are back to the first error - “” not being a palindrome

then it will find nil != ()

^ right

ah my bad

this what I have now, only failing on the nil

https://gist.github.com/JimTheMan/0b85f6c58c0bf69183792f63f9f773b8

@derpocious the easy thing might be (and (string? %) …)

hmm how would you use that?

(and (string? s) (= (seq s) (seq (reverse s))))

or, if all you care about is the nil case, and not eg. lists of strings, just (and s ...)

ah ok, so then any time you pass a non-string into it then it just automatically returns false

right

I think that works. 🙂

And I found out how to use "run-all-tests" too!

(ins)dev:cljs.user=> (time (dotimes [_ 1000] (= (seq "hello") (seq (reverse "hello")))))
"Elapsed time: 9.400000 msecs"
nil
(ins)dev:cljs.user=> (time (dotimes [_ 1000] (= (seq "hello") (seq (reverse (seq "hello"))))))
"Elapsed time: 6.090000 msecs"
nil

the js JIT makes the numbers change (I think that’s what’s doing it) but with repeated usage the second seems to stay faster

quite counterintuitive

(mind-blown)

it’s about the IReversible thing - it allows short-circuiting instead of eagerly building a list

(I think)

interesting...

Well here's my code now. Is it so beautiful? 🙂

https://gist.github.com/JimTheMan/7f70b91b87a25c436f3ef5ca3d73af38

yeah, for a counted / associative input, it can be lazy it looks like https://github.com/clojure/clojurescript/blob/d45012273029bd5df3973ea716394f368e1c44cc/src/main/cljs/cljs/core.cljs#L1701

I would argue that nil is a palindrome in the same way that (and) is true.

But if you want it to return true only for strings, why not (and (string? s) (= (seq s) (seq (reverse s)))) ?

That was (palindrome? []) is also false...

(I don't think anyone has mentioned yet that is-something is not very idiomatic Clojure -- something? is better)

Hah, the conversation moved on and @noisesmith already said the same (re string?)! That'll teach me to scroll back and read everything properly! 🙂

yeah, I never understood why some functions ended in a question mark. Does is always take the place of "is"?

it means the result of calling it is a boolean

? indicates a predicate (that returns true or false specifically).

If a function returns just truthy/falsey, then you wouldn't have a ? (in general -- based on several discussions I've seen on the Clojure mailing list over the years).

ok, that makes sense. So if I just name a function "palindrome?" it would be clear to people that it takes an input and returns true or false whether or not the input is a palindrome?

(defn palindrome? "Returns true or false depending upon whether input is a palindrome. Nil is not considered a palindrome" [s] ...)

make it clear 🙂

@dpsutton "Returns true for a string that is a palindrome. Returns false for all other arguments." 🙂

even better

but @derpocious I'm a big fan of docstrings and well named functions. palindrome? does indicate to me that it returns true or false as we have discussed. And Sean's message there would clear up any corner issues I might have. Then there's always the source

true, great tips. Thanks a lot guys!

https://gist.github.com/JimTheMan/695b1041f9d5658bc8b9b3a8461df201

Hi, is there a good Clojure idioms page to know all idioms commonly used (I mean no just random 5 of them but most of them)?

https://github.com/bbatsov/clojure-style-guide, not all idioms, but definitely more than 5

It's tough to give an example of every idiom. It's more about what you are trying to do and whether your code is written in an idiomatic way, I think.

Thanks @smith.adriane, @derpocious! 🙂