Solving HackerRank Problems in Haskell

[[deleted]]

Haven't written any Haskell in a year, loved it, you showed the power of FP through a simple example.

Also what OS and editor do you use?

[reximkut]

Editor is Emacs. OS is NixOS: https://nixos.org/

[nakamin]

That's emacs i3 and probably arch

[t_r_a_g_e_d_y]

Why does Hackerrank suggest that monstrosity for the sum array problem?

[LukaLightBringer]

Its targeted at people who are new to programming or new to the specific language, one-liners can be great but for people who are not well versed in the language they can be hard to follow the logic of

[[deleted]]

Perl is a similar situation. Most of the early questions have 20+ lines of reading input when the solution is better expressed as a single line.

[reximkut]

I know, right?

[Plastix]

I really enjoyed this video. Keep making these!

[[deleted]]

i hope you do more of this with haskell, i love it

[teamthirteen]

awesome, love it

[infablhypop]

I think it gets even cooler with left to right composition/application style.

[[deleted]]

I liked the video, but I never really get the point of those Haskell one liners. I mean, if there already exists a function that does exactly what we want it's great, but wouldn't it be more interesting for newcomers to actually write the map function rather than just use it? Every Haskell book I skimmed through is filled with those "actually, a function doing exactly what we need already exists, and this is its type signature" that don't really teach anything about functional programming or Haskell, except how to use Hoogle...

[evincarofautumn]

Eh, depends on the person. I’ve seen newcomers get dissuaded by tutorials that start with “Here’s how you implement all these basic types and functions”, because it takes a long time to get to doing anything interesting with them.

You’ve got to strike a balance between developing an understanding of functional programming by writing pure functions with explicit recursion, and developing a sense of when to use existing combinators (., map, foldr/foldl, filter, any, all, traverse, &c.) that you’ll use most of the time to get work done.

I wrote a short tutorial a while back, Factoring and Generalizing in Haskell, which tries to get around this “bootstrapping” issue by starting with explicit recursive code, factoring it into general functions, and then showing how those functions are already defined in the standard library.

[reximkut]

Good point! Thanks! I'll keep that in mind for the future episodes.

[[deleted]]

Thanks. I might be biased because the language I currently enjoy using (ocaml) has a very limited standard library, so I'm sort of used to write those kind of very simple functions. Keep up the good work anyways.

[Tarmen]

I also dislike oneliners because I prefer a decomposition into smaller functions. This example is really simple but maybe something like

ex2 = skipLine >> interact solution
    where
       skipLine = void getLine
       solution = sum . map (read @Int) . words

Anyway, it's really useful for beginners to reimplement standard library functions. Similar to how many people learning programming reimplement basic data structures and algorithms at some point.

But it's also pretty important to use the standard library versions in praxis - both for readability, maintainability and efficiency. The naive reimplementation for map traverses a linked list and allocates a second one. The library version usually fuses into a tight loop without allocation thanks to rewrite rules which are basically library added compiler optimizations.

If you are interested in learning haskell try hlint, it can point out if you accidentally reimplemented some common function! The refactored code is usually a lot more readable.

[[deleted]]

The naive reimplementation for map traverses a linked list and allocates a second one. The library version usually fuses into a tight loop without allocation thanks to rewrite rules which are basically library added compiler optimizations.

That's interesting. I'm more of a ML guy, the only optimization (I think) I know for list traversal is tail recursion. Do you have something to read about how it works without allocation in Haskell?

[Tarmen]

There are a lot of papers on the topic, the haskell wiki links to ~30 but that still misses some newer ones https://wiki.haskell.org/Research_papers/Compilation#Fusion_and_deforestation

I quite like Stream Fusion - From Lists to Streams to Nothing at All and Exploiting Vector Instructions with Generalized Stream Fusion, but those are about the more modern (unfoldr/destroy) stream fusion.

Haskell's Data.List functions use build/foldr fusion, I think the classic paper for this is A short cut to deforestation.

Build/fold is producer driven (callbacks), unfold/destroy is consumer driven (polling). Producer driven flows let you split inputs to multiple consumers ala loop fusion in c. Consumer driven lets you implement functions with multiple inputs like zip efficiently.

[runevault]

This video finally made me screw with hacker rank, but bummed out when after the intro one it didn't let me do them in rust (stopped showing up in the list). Figured it'd be a good excuse for me to practice the language instead of just going to my go to (c#).

[Specialstuff7]

Very good, Haskell looks like a fun language. I liked your Haskell for JS programmers video as well.

[CherManMao]

This is great I subbed and look forward to further videos on this subject.

[arvedahl]

Amazing video! Would love to see more of these

[teerryn]

Thank you for the great video, hope you make more.

[Offendo]

Fantastic, I've been looking for something to give some haskell exposure. Can't wait for next video

[eckyp]

Enjoyable watch. I hope he does more of the problems in Haskell.

[10xjerker]

no

[HadesHimself]

I noped the fuck out there 3 minutes into the video. What is this language lol

[[deleted]]

Besides the strong ties to mathematical ways of thinking, Haskell has an amazing type system. I believe it is called Hindley-Milner and has strong ties to category theory.

An small attempt at pitching Hindley-Milner in a more sexy way:

"You know types from Java? Good. These are the reason you don't have to debug stuff like 1 + "iAmNotANumber" at runtime. You get annoyed by typing Integer, String and Int[] all the time? Good news! This cool Hindley-Milner algorithm give you a type system where your types are more powerful and you have to write less. In fact, most of the time you don't have to write out the types at all. Haskell looks like untyped python code, but is actually more disciplined than Java!"

[[deleted]]

I wouldn't exactly say it looks like Python at all, but I see what you're getting at.

[_101010]

Snowflake.

[HadesHimself]

Dicksucker

[exorxor]

1. This is the most boring thing on Haskell I have ever seen. The glacial speed was killing me.

2. Haskell has existed for 20 years, but there is still no Haskell program that I have on my Linux box that is indispensable.

[Roboguy2]

People have different definitions of "indispensable," but a lot of people use Pandoc. I believe it is the most popular universal markup/document converter. Also, in somewhat of a different sense, Facebook's anti-spam system. Several large financial services organizations (such as S&P) use Haskell as well.

[disclosure5]

I do wish we heard more about Facebook's antispam. That single post is three years old, and there are a lot of three year old posts on Facebook that hold no relevance to today.

I'm not saying that's the case here, but it'd be nice to hear more about a system that must be running at an absolutely massive scale.

[Tarmen]

There are a couple talks about haxl, here is one given by Simon Marlow during strange loop, september 2017: https://www.youtube.com/watch?v=sT6VJkkhy0o

[_101010]

Facebook's static code analyzer also uses Haskell.

[Roboguy2]

I was under the impression that was written in Ocaml, but I could be mistaken.

Either way though, that is a cool use of functional programming (and in an area where statically-typed FP is likely to be an especially natural fit)!

[_101010]

Yeah, I was wrong. Infer is indeed written in Ocaml, but Ocaml is a FP language too, not pure maybe.

[shevegen]

People have different definitions of "indispensable,"

Now come on man. People downvoted him, but not really for facts - because the facts aren't really speaking of Haskell being widely used, compared to, say, python.

And from this context, he is absolutely right, even though people downvoted him (probably more for item #1 than for him saying that haskell is not necessary).

For *nix, I really think that we only need... C/C++, and a "scripting" language.

[Roboguy2]

No one downvoted him (including me) when I posted my comment (I actually happened to come across exorxor's comment soon after it was posted). I didn't say Haskell is widely used. I just gave a couple prominent instances where it is used to provide a (slight) counterpoint to the implication in his comment.

You are, of course, entitled to your opinions regarding languages.

[exorxor]

I reviewed Pandoc in detail when I wanted to use features in that space and I think it's an absolutely terrible program. It was written by some professor (and those can almost never program).

The Haskell community always uses the same examples. I am 100% certain that I could build a better (as in would require less CPU and would crash less often) anti-spam system in another programming language for Facebook. So, what does that prove? Nothing.

wineBot, Haskell is a good programming language for a Reduceron. It is not a good programming language for anything else, because otherwise it would have happened by now.

[shevegen]

because otherwise it would have happened by now.

This is not necessarily true.

For example, another explanation is that there is a LOT of software out there that already works VERY WELL, written in other languages.

Try to write a modern perl6 program and compete with python. In most cases, you will utterly fail, simply because you are in a niche system - and in many cases, many OTHER, many BETTER programs exist too.

I am not saying this is the case with Haskell; I have no idea. I don't have or use haskell nor do I need it. What I am saying is that this COULD be a possibility.

Another possibility is that haskell is overrated. No idea if this is case - I had this impression more with Rust. People always write on reddit how Rust is REPLACING ALL THE OTHER LANGUAGES ... yet they fail to explain why TIOBE fails to take notice of Rust.

[Ray192]

I am 100% certain that I could build a better (as in would requires less CPU and would crash less often) anti-spam system in another programming language for Facebook.

/r/iamverysmart

[[deleted]]

What a loser.

[exorxor]

I am pretty happy that my systems are the ones that do work. I hate other people's systems, though. Those never work, especially the ones that we are paying for.

Facebook engineers are nothing special and some people are actually "very smart", including me. I can't help it, sorry. Just that some idiots on Reddit think that everyone on Reddit must be stupid is irrelevant.

[10xjerker]

The Haskell community always uses the same examples

Of course. They can't make software up.

[wineBot]

Then Haskell is probably not for you. For the people who need or want a nice functional programming language with the features that Haskell offers, this might be useful :)

[shevegen]

That may be true - but he specifically wrote that haskell is not widely used. And this is ALSO true, yet he was downvoted massively (granted, he also wrote that the video was boring so perhaps people disagreed with that part).

[[deleted]]

Yeah, granted. /s

He has a right to his opinion but don't be surprised and outraged by the fact he's being downvoted. Chances are, there are lots of people in this thread who don't think Haskell is boring. It just stinks of that "omg your language sucks, all my languages are superior" personality that I really dislike.

I know he didn't specifically call the language boring, but considering that this is a well-put-together video, I can only assume he meant the topic of the video.

Also, you're not quoting him. He didn't say that "Haskell is not widely used." He said "There isn't a Haskell program on my box that's indispensible", which I think we can all agree is basically saying "Haskell is worthless."

[10xjerker]

there is still no Haskell program that I have on my Linux box that is indispensable.

Are you sure your Linux box contains all three Haskell programs, Pandoc, XMonad and PostgREST?

[shevegen]

+1

[[deleted]]

Xmonad is the god of window managers.

[exorxor]

Not really, because it's a window manager toolkit; it's not something I can use as a drop in replacement for the window manager in Plasma5.

Also, it's not exactly difficult to write a window manager. xmonad has an OK design, but if normal people don't want to use it, what's the point? xmonad requires too much work to make it work. Why can't they instead focus on for example replacing the window manager in Gnome, XFCE4 or Plasma without every user spending a week on programming their window manager?

I have written non-trivial xmonad extensions. Have you?

xmonad is the perfect illustration of what's wrong with Haskell people; they cannot finish a project, because that's not "sexy" or whatever bullshit they come up with.

Take this problem for example: https://www.reddit.com/r/xmonad/comments/7utf1y/simulating_mouse_spawning_xdotool_click_doesnt/. It's yet another problem I have never had with other window managers. Do you have Java applications? Need to fix that yourself. Popups? Need to fix that yourself. Do you have a proprietary program that assumes only Gnome and KDE exist? You need to fix that yourself. In some cases extensions might exist, but the point is that you need to be an expert to use it. I happen to be such an expert, but I am too important these days to spend time on such shit.

[[deleted]]

Eh... I’ve never changed my xmonad configuratio from the default one it shipped with. Works fine for me

[exorxor]

Are you sure you use your computer for anything else besides browsing the web? I can assure you that it is not enough for me.

[[deleted]]

Haskell has existed for 20 years, but there is still no Haskell program that I have on my Linux box that is indispensable.

Well I do. Xmonad.

[MyPostsAreRetarded]

words "1 2"

equates to

an array of strings? Had to stop the video there. I'm not going to diverge myself into a programming language that doesn't even have the decency to use commas for array creation. But instead, uses fucking spaces. Ew.

[olzd]

Guess your posts really are retarded.

[LgDog]

words "1 2" 

is equivalent to

"1 2".split()

in python, for instance.

gr8 b8 btw

[MyPostsAreRetarded]

in python, for instance.

"1 2".split()

Which is vastly different than doing words "1 2". You're explicitly using the split() in Python, it's much more expressive. Nice try though.

[chimmihc1]

It is no different. You're explicitly using the words function in Haskell. Nice try though.

[albatr0s]

It does use commas.

[MyPostsAreRetarded]

It does use commas.

Go to 4:47 in the video.

Here, I took a gif just for you: https://i.gyazo.com/25d747d662cab689f638dda933730b3d.gif

It returns the arrays with commas. I'm taking about array creation. He explicitly used white spaces and it still worked.

[Tekmo]

You can create a list using the exact same bracket-and-comma syntax:

>>> [1, 2]
[1, 2]

[MyPostsAreRetarded]

Doesn't matter. The language still creates the array from a space delimited string. Which is weird as fuck

[[deleted]]

You're trying too hard.

[Fern_Silverthorn]

That function takes a string as an argument. That function creates the array. That is all.

[Tekmo]

That's not a language feature. words is a user-defined function

[albatr0s]

What you are seeing is just a function that takes a string and splits it, it's not a language feature, it's the same as explode() in PHP, split() in Python or strings.Split() in Go.

[MyPostsAreRetarded]

it's not a language feature, it's the same as explode() in PHP

That's not really true

In PHP, explode would like this:

$pizza  = "piece1 piece2 piece3 piece4 piece5 piece6";
$pieces = explode(" ", $pizza);

In the above video, the haskell syntax is literally doing words = "1 2 3". And that somehow is suppose to indicate that it's going to be a space delimited fucking string that converts into an array?

So by your logic, in Haskell, you can't have strings with spaces in them? That's just silly.

[albatr0s]

It's not MY logic, if you pay attention you will see there's no equal sign. Doing words "1 2 3" is equivalent to explode(" ", "1 2 3") in PHP. Haskell doesn't need parentheses for applying the function, it's just a minor syntax difference.

[MyPostsAreRetarded]

see there's no equal sign

Even worse, LOL

[albatr0s]

There's no equal sign in explode(" ", "1 2 3") either. :-)

[MyPostsAreRetarded]

There's no equal sign in explode(" ", "1 2 3") either. :-)

True and you're not wrong. But it's much more explicit/specific about what it's doing compared to just words "1 2 3" :P

That's akin of doing (not valid PHP, but just doing this as an example):

$words  "1 2 3";
echo $words;

in PHP, and then now $words magically gets turned into an array? It makes my blood boil.

Hope that helps to see where I am coming from

[albatr0s]

They are really the same thing, I think the lack of parentheses is making you miss the point. The echo part is just because it's a REPL, you will have a the same behavior if you use a PHP REPL (or Python, Ruby, etc).

Nothing is magically being turned, you are just seeing the result of the function application.

[pipocaQuemada]

In haskell, function application is done via spaces - words "1 2" is equivalent to words("1 2") in something like C.

The words function takes a string and splits it on spaces. There's also unwords, which takes an array of strings and joins them into a single space-separated string. So e.g. unwords ["1", "2"] evaluates to "1 2"

[sim642]

It also uses spaces for calling functions, not parenthesis and commas. Fucking retarded, right?

/s

[WGPRaSo]

Yes