Of Monads and Men - 2017-10-08

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3:48 AM

Hello again, I'm having trouble with the following code f a=[a!!i<$[1..(f a)!!i]|i<-[0..]]

Theoretically it should take a[i] from array a, such as [2, 3, 1] and append a[i] to the new list a[i] times, using n<$[1..n]. Is there a way to concatenate n<$[1..n] instead of adding as a list inside a list?

4:34 AM

Ah nuts. I forget that a!!i in the beginning should be (cycle a)!!i

3 hours later…

7:58 AM

@Sherlock9 I'm not exactly sure what you are trying to do, but you can add concat (or id=<<) in front of the list comprehension to get it to type: Try it online!

@Laikoni Thanks for the help. I'm trying to do this challenge

There was also a $ missing behind take 25.

Hm, as I expected I do need to initialize the array. That's going to eat a lot of bites

Also how would I put take in f?

Exactly, otherwise the self reference can not be resolved.

Well, this almost worked

8:04 AM

@Sherlock9 You probably can't because of the self reference, but take 25.f would work. Or is the 25 also an input?

25 is an input

8:35 AM

Hm, maybe I can do take n in a function below the main list comprehension instead

@Laikoni Any idea what's going wrong here with f a=(a!!0)<$[1..(a!!0)]++concat[(cycle a)!!i<$[1..(f a)!!i]|i<-[1..]]?

9:00 AM

I think (cycle a) is the problem, it producs an infinite list

(not sure yet)

@Sherlock9 <$ binds weaker than ++

You need to enclose the initialisation in parentheses.

And as long you don't have any guards [...|i<-[1..]] can usually be shortened using map or <$>

For some reason I keep thinking about an ice cream cone whenever I see <$

Huzzah! f a=((a!!0)<$[1..(a!!0)])++concat[(cycle a)!!i<$[1..(f a)!!i]|i<-[1..]]

@flawr I'm having trouble figuring how to use map and <$> because the i variable is used in several different places

I may just need to learn more about lambda calculus reduction methods

ah right this seems to be getting longer

I also think about a double-scoop ice cream cone when I see <$

9:07 AM

@Sherlock9 high five:)

But you could simplify the a!!0 with f a|z<-a!!0=(z<$[1..z])...

Excellent. 84 bytes now since I have to put take in the main code.

and I think you don't need the parentheses around f a and cycle a

@Laikoni Before I forget, thank you very much :D

And we're down to 80 bytes. Is a shorter way to write ++concat?

id=<< is a shorter concat

Ah, sorry you already said and I forgot to try it

Hm. An error

9:12 AM

@Laikoni you need to add that to the tips!

@Sherlock9 Oh, ok, it does not work with the ++ in front

I think you have to put parens because of weaker binding :/ 80 bytes earlier vs 79 bytes now

a#n=take n$f a can be shortened to (.f).take if the arguments are reversed.

@flawr Apparently it is a tip

n#a=(.f).take is giving me another error

It's point free, you don't need n and a as arguments.

Like this:

[Try it online!](https://tio.run/##y0gszk7Nyfn/P00hsabKRjdRUdHANjk/LzmxREWjykYl2lBPrypW0‌yo6uTI5J1UBKJ0JEUwDs2NrMm10QdzYWK50Ww29NE29ksTs1P@5iZl5CrYKBUWZeSUKKgrpCkamCtFGOs‌Y6hrH/AQ "Haskell – Try It Online")

9:17 AM

Ah of course. Just need another single-character function name because #=... gives another error

@Sherlock9 well only as concatMap f, I the concat is probably used more often.

@Sherlock9 you'd have to write (#)=...

Which brings us to 79 bytes

@Laikoni how do you come up with solutions like that? I'm getting dizzy by just looking at it!

@flawr pointfree.io

@Laikoni that was not what I hoped to hear :D

Anonymous

9:23 AM

pointfree.io is how I make myself look like I know way more about Haskell than I do :P

Does anyone know, is it possible to add links in the room description?

Anonymous

Yep

@Sherlock9 Anonymous functions are allowed, so you don't need to count the (#)=.

Anonymous

See: TNB

I mean using []()?

Or can you just past URLs?

9:24 AM

@Sherlock9 70 bytes

Anonymous

Just paste URLs

Anonymous

No Markdown in room descriptions, unfortunately

@flawr Well, at least by now I usually understand the transformations and also do them myself in simple cases.

Hm, I'll have to practice a little bit then:)

@Laikoni Do you mind if I add the tips for golfing in haskell in the room description? (perhpas also pointfree.io?)

@Laikoni Many thanks :D I'm going to write up my answer. Is it alright if in my answer I write the following: Many thanks to Laikoni and flawr for their help in debugging and golfing this answer in the PPCG Haskell chatroom, Of Monads and Men

9:27 AM

@flawr Sure, go ahead.

Anonymous

Credit and advertisement, nice double-whammy

2

room topic changed to Of Monads and Men: For discussion about, learning of and golfing in Haskell codegolf.stackexchange.com/questions/19255/… pointfree.io [haskell]

too bad it doesn't just show the title of the question =/

Anonymous

I thought you actually did it there, and was confused why it didn't change on the side

The Guide to Golfing Rules in Haskell is also a good place to link to.

Oh yeah, is there something else to add?

One could also link these in the haskell description

ok this is strange: I recently pinned this CMS you can still see on the starboard, without actually starring it - now that I've unpinned it it looks like I starred my own message (but cannot unstar it)

and if I try to unstar it it says "you cannot star your own message" as if I haven't starred it yet.

9:32 AM

@Sherlock9 I like that. Also 68 bytes

Anonymous

@flawr Yep, known bug.

Anonymous

As with all chat bugs, it has existed for quite a while, and SE has made no visible effort to fix it or even acknowledge it

@Laikoni Well, I was starting to explain the 70-byte one, but that's gone and confused me

Oh wait

@Mego ok at least it was not me doing something wrong:)

a@(z:_) is just defining a and defining z as the head

9:33 AM

Also shouldn't ROs be able to clear stars?

Anonymous

They should be able to

Anonymous

You use the cancel stars button from the starboard, not unstarring

Ah that was what I was looking for! Thanks a lot!

"Remove the stars against this post."

Anonymous

Glad to help :P

I just added some links to the haskell tag description: codegolf.stackexchange.com/tags/haskell/info

9:39 AM

Answer is posted

Anonymous

I got to figure all this stuff out by myself when I was added as a TNB RO :P At least you're not having to try to find the button during a star spam wave

Ah, trial by fire

Anonymous

Trial by fire with gasoline

Or since it's stars, perhaps, trial by meteor shower

Anonymous

That's pretty accurate :P

Anonymous

9:41 AM

I can't remember the last time I had to cancel stars, though, so things have gotten better

Anonymous

Either that or I've just gotten luckier and not been around for it

flawr has made a change to the feeds posted into this room

flawr has made a change to the feeds posted into this room

@Feeds Oh, nice.

32

A: Point Free problems in Haskell

Converting to pointfree style can be done entirely mechanically, though it's hard without being comfortable with the fundamentals of Haskell syntax like left-associative function application and x + y being the same as (+) x y. I will assume you are comfortable with Haskell syntax; if not, I sug...

@Laikoni @Mego ^ provides an algorithm for making thins pointless

Anonymous

9:59 AM

@flawr Oh, I'm aware :) pointfree.io (and the pointfree package it uses) uses that algorithm, I believe

Thanks for the link. Reading it now

Did we actually get 5 Haskell questions at once, or is this Feeds trying to catch up?

Anonymous

Probably the latter

Oh no, can I delete these messages somehow?

Anonymous

You can move them to Trash

Anonymous

You might need to get access, though, since it's a gallery room

Anonymous

10:04 AM

I think ArtOfCode is the one to ping

Anonymous

Apparently this guy is the sole RO of Trash, but I think Art used his diamond powers to give the TNB ROs write access so we could move trash there

user image

^ this is what I feel like :D

Anonymous

Too much RO power? :P

10:28 AM

0

A: Tips for golfing in Haskell

Converting to Pointfree Als @Laikoni already mentioned there is pointfree.IO, but if you want to do this conversation manually you can check out this answer on stackoverflow by @luqui, which provides an algorithm: You need the following combinators, which are in the standard library. I hav...

@flawr May edit your post in Haskell tips for clarity?

@Sherlock9 sure!

Done!

1 hour later…

11:56 AM

→ 14 messages moved to Trashcan

@flawr I'm not reading the contents of that room this time... It's scary.

Is it possible to trash things as RO?

@flawr It's possible to move things from any room you are RO in to any room you have access to, iirc.

Try moving this message to the Sandbox.

→ 1 message moved to Sandbox

Ah now I see, thanks a lot!

But that should never be done.

Best if we don't tell anyone :)

Thanks a lot for your help!

12:04 PM

It's one of the less important abuses because it's obvious when it's being done.

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Oct '178

Of Monads and Men

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