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Comment by polotics | Hacker News Reader

This list is so deliciously sophomoric. The best one is, and I quote: """To many other weirdo bits of magic syntax, like [list comprehensions]""" Obviously without actually proposing how comprehensions could be made better one has to hope the author would say he likes the equivalent Haskell better, but there is a strong doubt that is not the case.

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Complaining about having to cast generators to list... seems like the kind of dev that has their code randomly run out of memory until a senior dev comes and fixes it.

It is annoying! And doesn't always save memory. In theory a "sufficiently smart language" should have a feature to understand that `thing[3]` can be translated to "call `next()` 3 times and gimme the last thing", not in "turn this completely to a list, that may take a ton of memory, although I only need that one third element".

Generators should be treatable as lazy lists in the end, and lists should have a common interface whether they are lazy or eager. Someone should figure out a way to have us write code at a higher abstraction level and have same interface for lazy vs eager data structures.

...but it's not gonna happen in a dynamic language like Python. And I can't say I like the "solution" of having the entire language be lazy like Haskell either :|

We're stuck with "casting generators" for now, I guess, but it really does suck!

You don't need to cast the generator, you can do: next(itertools.islice(my_generator, n, n+1))

With that said...

> In theory a "sufficiently smart language" should have a feature to understand that `thing[3]` can be translated to "call `next()` 3 times

This might be a newbie trap, because next() isn't the same as indexing. What happens if I perform `thing[7]` followed by `thing[5]`? Should performing `thing[7]` put 1-6 in memory and turn the object into a generator-list hybrid?

> Should performing `thing[7]` put 1-6 in memory and turn the object into a generator-list hybrid

You're right here. Probably can't work like that since generators are too general, you can't expect them to be rewindable or to not have side effects... prob you'd need a more specialized concept like a "lazy list" that would be a subtype of generator with some extra restrictions that would make it possible to implement the "hybrid" structure as an implementation detail without changing semantics.

Anyway... it would be too much work and probably would turn into a footgun.

that one is legit. sometimes I do something like map(int, [x,y,z]) and I always trip over that it's a generator

I personally find list comprehensions in python pretty horrible.

They seem to exist only to do lots of stuff in one single line of code. You end up with totally impenetrable unreadable perl-esq garbage write-once-read-never code that is too clever for its own good. And people say python is easy to learn and good for beginners...!

A better approach would be something like Java Streams/.net Lync/RxX pattern IMO. Explicit, clear, no magic, logical.

I personally find them extremely readable, and a lot clearer than for loops about the lack of side effects.

I disagree. It’s basically set-builder notation, which probably predates any programming language. It’s a concise way to express filtering.

" impenetrable unreadable perl-esq garbage write-once-read-never code " - I had about the same impression when I first saw it. I think ternary operator is as far as I am willing to go ;)

You can decompose any list comprehension into it's equivalent for loop fairly easily. I don't see what is so magic about them other than the language having a special compiler level optimization for them?

g = [i for i in list if x == 3] -> g = []; for i in list: if x == 3: g.append(i);

I see people frequently say that list comprehensions are special-cased and highly optimized by the compiler, but I did a few experiments recently with `timeit` and found that using `map()` is on the order of 1.5x faster in all the cases I tested. I think that either winds up being faster than a manual for loop, though

dic = {k: v for k, v in dic.items() if k in other_dic and v == "bar"}

How could that be improved? That's 3-4 LOC minimum in any other language

My main grip is python's ternary operators, since the True value is evaluated before the condition, if you are doing ternaries on things that might throw exceptions the False value has to come first

value = 0 if key not in dic else dic[key] * 5

rather than (throws indexerror if key isn't in dic)

value = dic[key] * 5 if key in dic else 0

    dic = {k: v for k, v in dic.items() if k in other_dic and v == "bar"}

> "How could that be improved? That's 3-4 LOC minimum in any other language"

If I'm understanding the comprehension correctly,

    (into {} (filter (fn [[k v]] (and (get other-dic k) (= v "bar"))) dic))

Though, for readability, I'd likely write it as:

  (->> dic
       (filter (fn [[k v]]
                 (and (get other-dic k)
                      (= v "bar"))))
       (into {}))

Legibility is in the eye of the beholder.

Each to their own, but both the snippets you posted crossed my threshold for headache inducing parentheses tracking

Similarly, my parsing of special-case syntax in the Python.

What special-case syntax? The only brazenly pythonic thing in there was the (iterative) tuple unpacking, as in

(for) k, v in dic.items()

which is just

k, v = (<key>, <value>) for every key value pair in the dictionary

There are a number of precedence rules you need to keep track of to parse the list comprehension. There are two different syntaxes that do the same thing. Arguably you need to do the same if you don't already know how the threading macro in the my second example works.

I posted due to your claim regarding all other languages necessitating increased verbosity. I should have left it lie, as I didn't intend to promote a language war, just to post a counter example. My apologies.

No language war intended, just curious how something I see as a wheel could be improved.

It appears textually first but the evaluation order is the same as the classic ?: ternary. ```[][0] if False else 'foo'``` will not raise an IndexError.

I like comprehensions, but think this style has some advantages (and some disadvantages):

dic = dic.where((k, v) => k in other_dic and v == "bar").todict()

> dic = {k: v for k, v in dic.items() if k in other_dic and v == "bar"}

dic.iter().filter(|k, v| other_dic.contains(k) && v == "bar").collect();

it's one line, though I'd format it as 3 for readability (list comprehension is hard to read and functional style composes better.

List comprehensions are one of those abuse subject features (like most things) where light simple use is fantastic but some people just take it too far into unreadable nastiness.

list comprehensions can be elegant and actually improve readability. However, I do agree that it can be easily misused. I have seen many junior Python programmers writing super long, complicated comprehensions that hurt my brain. They think it is pretty cool just because their solutions are one-liners.

"Please don't post shallow dismissals, especially of other people's work. A good critical comment teaches us something."

https://news.ycombinator.com/newsguidelines.html

For example, instead of putting someone's article down as sophomoric, you could explain what's different and possibly better about Haskell list comprehensions.

You could, but you'd lose the Socratic method and efficiency of thinking. There is a reason why many academics can be quite acerbic.

All these politeness comments are a speed bump that distracts from the real issues.

There's a huge difference between a classroom environment or academic symposium, and an internet forum. Here, when people attack and take swipes at each other, discussion slides downhill so fast that it becomes an existential issue for the forum. Having HN not destroy itself the way internet communities usually do has been the main goal here since pg created the site over a decade ago, and the guidelines here are written with all that experience in mind.

I used to think similarly about this to what you express, because I've always enjoyed reading about the sort of discourse in which devastating wit is exchanged. But eventually I realized that it doesn't translate into this context at all. This is a case of 'the medium is the message'. When you have millions of people who don't know each other all potentially interacting at the same time, the dynamics are so different as to be incommensurable with, say, small debates, literary journals, elite social events, and other places where the groups are small and highly cohesive. Here are some previous explanations about this:

https://hn.algolia.com/?dateRange=all&page=0&prefix=true&sor...

https://news.ycombinator.com/item?id=15378909

https://news.ycombinator.com/item?id=9378899

https://news.ycombinator.com/item?id=7906377

https://news.ycombinator.com/item?id=7742471

The bottom line is that having a forum like HN be open to everybody comes at the cost of some blandness.

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