elbrujohalcon

elbrujohalcon

How do List Functions Fail in Erlang?

A long time ago, I wrote an article about The Asymmetry of ++, thanks to
Fede Bergero’s findings. Let’s add a few more asymmetries to that list…

Most Liked

OvermindDL1

OvermindDL1

As to the originally referenced article, ++ isn’t asymmetric in the way it was shown but rather it’s a function that would be written like this in elixir:

def ++(left, right), do: append(:lists.reverse(left), right, [])
def shift_cells([], acc), do: acc
def shift_cells([e|r], acc), do: [e, acc]

Which is precisely what it is defined to do. Lists on the beam in erlang and elixir are not typed lists, they are not full “proper” Cons lists, you can potentially make lists in the first Cons element, the second, zig zag every which way, etc… The ending element isn’t special, it doesn’t need to be a list. Now sure ++ would not make much sense in a statically typed language, but erlang/elixir are not statically typed languages. It’s not an asymmetry as it is not a ‘prepend’ operator, it is more of a ‘shift cells over’ operator.


As for this article, looks good. ^.^

You really should put what OTP version you were working with in Erlang as a lot of error responses for BIF’s have changed in recent versions (more information in the exceptions! ^.^).

I wouldn’t opt for the exception catching of the lists calls but rather a pre-check, or toss the check ‘up’ the callstack by requiring, for example, the list argument to your function to be List=[_|_] instead of just List to enforce a Cons cell instead of a Nil cell.

Basically, behave like lists:foldl/3 but don’t treat empty lists as a special case.

I’m not sure I agree, those are different issues with different exceptions. More I would argue that the exceptions should not be caught at all to begin with as malformed input was supplied to the function and thus who knows what other bad data there is, this is part of OTP’s Let It Crash philosophy, and exceptions are indeed “exceptional” events, not for standard control flow like they are being used here. Plus adding those guards may seem easy to something like map, but that is going to incur a cost on one of the hottest code paths in the entire system, not sure it’s worth it (although with the new JIT in OTP24, who knows, benchmark?).

OvermindDL1

OvermindDL1

Hear hear! I really like the recent changes. ^.^

Lol, why do I want a link to this discussion? ^.^

In this case yes. Most good type systems can enforce non-emptiness, and erlang likes to pretend it does as well. It’s on the caller to ensure they are passing in good data in that case. I’m a fan of static typed systems that can actually enforce this though, lol. Dialyzer helps a little bit at least. Hmm, does dialyzer catch that case actually?

elbrujohalcon

elbrujohalcon

Thank you for the super-detailed answer(s), @OvermindDL1 !!

I should’ve stated that I was testing this on OTP23, you’re right.

In any case, Lukas Larsson (from the OTP team) already replied in Medium with that regarding what they’re doing to improve error descriptions… and it’s GREAT!!

Finally, to some things in your message…

100% agree! I actually had to explain this very same thing when discussing the robot butt article on lobsters recently.

To be clear: Are we talking about the same different issues here? What I tried to say was that calling a function that works normally with empty lists, with an empty list and another wrong argument, should not behave as if it was called with something that’s not a list. Instead, it should behave as if it was called with a non-empty list. Do you still think that calling it with a bad fun and an empty list is a different issue than calling it with a bad fun and a non-empty list?

Of course, you’re right. But I was exemplifying.

Yeah, I agree again. That’s why the section in the article is called Is this a Problem? and not This is a Problem. This is clearly a made up problem just for the sake of arguing, except for the confusing error descriptions in the shell, which is what Lukas and the OTP Team are fixing right now :tada: :exclamation:

Where Next?

Popular Backend topics Top

dimitarvp
Apparently he decided to live-stream how he’s going to create a semver library.
New
tomekzawada
Greetings from Membrane Framework team! Check out our case study based on our latest projects at Software Mansion. https://blog.swmansi...
New
First poster: bot
Julia is a scientific programming language that is free and open source.1 It is a relatively new language that borrows inspiration from l...
New
AstonJ
If you’re interested in Rust this is worth a read :smiley: Technology from the past come to save the future from itself Hi I have be...
New
First poster: bot
Rails Best Practices I. Today I share some of my favorite practices applicable to Ruby on Rails (and to web development on small teams g...
New
paulanthonywilson
Post on using UDP multicasting with Elixir to broadcast presence, and listen for peers, on a local network. I have found this approach us...
New
First poster: bot
Just a small test with lists in cython. Considering echosystem, multithreading and ease of use, Julia is a clear winner here.
New
CommunityNews
Have you ever wanted to write a structurally typed function in Rust? Do you spend a lot of time and effort getting your Rust struct s jus...
New
wolf4earth
Tej Pochiraju joins the mix to discuss Progressive Web Apps and how you can support them using Elixir and Phoenix to control IoT devices....
New
fullstackplus
The Ruby ecosystem is rich with tools that make us developers more productive at what we do. Both Rails and Sinatra have been used to bui...
New

Other popular topics Top

AstonJ
There’s a whole world of custom keycaps out there that I didn’t know existed! Check out all of our Keycaps threads here: https://forum....
New
AstonJ
I ended up cancelling my Moonlander order as I think it’s just going to be a bit too bulky for me. I think the Planck and the Preonic (o...
New
AstonJ
This looks like a stunning keycap set :orange_heart: A LEGENDARY KEYBOARD LIVES ON When you bought an Apple Macintosh computer in the e...
New
AstonJ
Do the test and post your score :nerd_face: :keyboard: If possible, please add info such as the keyboard you’re using, the layout (Qw...
New
PragmaticBookshelf
Create efficient, elegant software tests in pytest, Python's most powerful testing framework. Brian Okken @brianokken Edited by Kat...
New
AstonJ
Saw this on TikTok of all places! :lol: Anyone heard of them before? Lite:
New
PragmaticBookshelf
Build efficient applications that exploit the unique benefits of a pure functional language, learning from an engineer who uses Haskell t...
New
AstonJ
If you want a quick and easy way to block any website on your Mac using Little Snitch simply… File > New Rule: And select Deny, O...
New
hilfordjames
There appears to have been an update that has changed the terminology for what has previously been known as the Taskbar Overflow - this h...
New
Fl4m3Ph03n1x
Background Lately I am in a quest to find a good quality TTS ai generation tool to run locally in order to create audio for some videos I...
New