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498 points azhenley | 146 comments | | HN request time: 1.717s | source | bottom
1. EastLondonCoder ◴[] No.45770007[source]
After a 2 year Clojure stint I find it very hard to explain the clarity that comes with immutability for programmers used to trigger effects with a mutation.

I think it may be one of those things you have to see in order to understand.

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2. ◴[] No.45770426[source]
3. m_rpn ◴[] No.45770485[source]
salutes from a WestLondonCoder
4. emil0r ◴[] No.45770884[source]
The way I like to think about is that with immutable data as default and pure functions, you get to treat the pure functions as black boxes. You don't need to know what's going on inside, and the function doesn't need to know what's going on in the outside world. The data shape becomes the contract.

As such, localized context, everywhere, is perhaps the best way to explain it from the point of view of a mutable world. At no point do you ever need to know about the state of the entire program, you just need to know the data and the function. I don't need the entire program up and running in order to test or debug this function. I just need the data that was sent in, which CANNOT be changed by any other part of the program.

replies(1): >>45772025 #
5. rendaw ◴[] No.45770924[source]
I think the explanation is: When you mutate variables it implicitly creates an ordering dependency - later uses of the variable rely on previous mutations. However, this is an implicit dependency that isn't modeled by the language so reordering won't cause any errors.

With a very basic concrete example:

x = 7

x = x + 3

x = x / 2

Vs

x = 7

x1 = x + 3

x2 = x1 / 2

Reordering the first will have no error, but you'll get the wrong result. The second will produce an error if you try to reorder the statements.

Another way to look at it is that in the first example, the 3rd calculation doesn't have "x" as a dependency but rather "x in the state where addition has already been completed" (i.e. it's 3 different x's that all share the same name). Doing single assignment is just making this explicit.

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6. ◴[] No.45770972[source]
7. skeezyjefferson ◴[] No.45771110[source]
whats the difference between immutable and constant, which has been in use far longer? why are you calling it mutable?
replies(4): >>45771178 #>>45771248 #>>45773965 #>>45774112 #
8. jstimpfle ◴[] No.45771163[source]
The immutable approach doesn't conflate the concepts of place, time, and abstract identity, like in-place mutation does.

In mutating models, typically abstract (mathematical / conceptual) objects are modeled as memory locations. Which means that object identity implies pointer identity. But that's a problem when different versions of the same object need to be maintained.

It's much easier when we represent object identity by something other than pointer identity, such as (string) names or 32-bit integer keys. Such representation allows us to materialize us different versions (or even the same version) of an object in multiple places, at the same time. This allows us to concurrently read or write different versions of the same abstract object. It's also an enabler for serialization/deserialization. Not requiring an object to be materialized in one particular place allows saving objects to disk or sending them around.

replies(1): >>45774510 #
9. Thorrez ◴[] No.45771178{3}[source]
Immutable and constant are the same. rendaw didn't use the word mutable. One reason someone might use the word "mutable" is that it's a succinct way of expressing an idea. Alternative ways of expressing the same idea are longer words (changeable, non-constant).
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10. EastLondonCoder ◴[] No.45771234[source]
I agree that the explicit timeline you get with immutability is certainly helpful, but I also think its much easier to understand the total state of a program. When an imperative program runs you almost always have to reproduce a bug in order to understate the state that caused it, fairly often in Clojure you can actually deduct whats happening.
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11. inanutshellus ◴[] No.45771248{3}[source]
"Constant" implies a larger context.

As in - it's not very "constant" if you keep re-making it in your loop, right?

Whereas "immutable" throws away that extra context and means "whatever variable you have, for however long you have it, it's unchangeable."

replies(1): >>45773019 #
12. kgwxd ◴[] No.45771258{4}[source]
They aren't the same for object references. The reference can't be changed, but the properties can.
replies(1): >>45781637 #
13. a4isms ◴[] No.45771300{4}[source]
In languages like JavaScript, immutable and constant may be theoretically the same thing, but in practice "const" means a variable cannot be reassigned, while "immutable" means a value cannot be mutated in place.

They are very, very different semantically, because const is always local. Declaring something const has no effect on what happens with the value bound to a const variable anywhere else in the program. Whereas, immutability is a global property: An immutable array, for example, can be passed around and it will always be immutable.

JS has always hade 'freeze' as a kind of runtime immutability, and tooling like TS can provide for readonly types that provide immutability guarantees at compile time.

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14. zelphirkalt ◴[] No.45771438[source]
Made a similar experience with Scheme. I could tell people whatever I wanted, they wouldn't really realize how much cleaner and easier to test things could be, if we just used functions instead of mutating things around. And since I was the only one who had done projects in an FP language, and they only used non-FP languages like Java, Python, JavaScript and TypeScript before, they would continue to write things based on needless mutation. The issue was also, that using Python it can be hard to write functional style code in a readable way too. Even JS seems to lend itself better to that. What's more is, that one will probably find oneself hard pressed to find the functional data structures one might want to use and needs to work around recursion due to the limitations of those languages.

I think it's simply the difference between the curious mind, who explores stuff like Clojure off the job (or is very lucky to get a Clojure job) and the 9 to 5 worker, who doesn't know any better and has never experienced writing a FP codebase.

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15. ndr ◴[] No.45771558[source]
Clojure also makes it very easy, it'd require too much discipline to do such a thing in Python. Even Carmack, who I think still does python mostly by himself instead of a team, is having issues there.
replies(1): >>45774914 #
16. ErroneousBosh ◴[] No.45771722[source]
I guess I'm not that good a programmer, because I don't really understand why variables that can't be varied are useful, or why you'd use that.

How do you write code that actually works?

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17. mleo ◴[] No.45771824[source]
It forces you to consider when, where and why a change occurs and can help reason later about changes. Thread safety is a big plus.
replies(1): >>45771845 #
18. ErroneousBosh ◴[] No.45771845{3}[source]
Okay, so for example I might set something like "this bunch of parameters" immutable, but "this 16kB or so of floats" are just ordinary variables which change all the time?

Or then would the block of floats be "immutable but not from this bit"? So the code that processes a block of samples can write to it, the code that fills the sample buffer can write to it, but nothing else should?

replies(1): >>45773139 #
19. ape4 ◴[] No.45771937[source]
I would be nicer if you gave x1 and x2 meaningful names
replies(1): >>45772547 #
20. DrScientist ◴[] No.45772025[source]
Sure modularity, encapsulation etc are great tools for making components understandable and maintainable.

However, don't you still need to understand the entire program as ultimately that's what you are trying to build.

And if the state of the entire programme doesn't change - then nothing has happened. ie there still has to be mutable state somewhere - so where is it moved to?

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21. rafaelmn ◴[] No.45772048[source]
I would say it's more than immutability - it's the "feel" of working with values. I've worked with at least 6 languages professionally and likely more for personal projects over last 20 years. I can say that Clojure was the most impactful language I learned.

I tried to learn Haskel before but I just got bogged down in the type system and formalization - that never sat with me (ironically in retrospect Monads are a trivial concept that they obfuscated in the community to oblivion, yet another Monad tutorial was a meme at the time).

I used F# as well but it is too multi paradigm and pragmatic, I literally wrote C# in F# syntax when I hit a wall and I didn't learn as much about FP when I played with it.

Clojure had the lisp weirdness to get over, but it's homoiconicty combined with the powerful semantics of core data structures - it was the first time where the concept of working with values vs objects 'clicked' for me. I would still never use it professionally, but I would recommend it to everyone who does not have a background in FP and/or lisp experience.

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22. Tarean ◴[] No.45772126[source]
Sometimes keeping a fixed shape for the variable context across the computation can make it easier to reason about invariants, though.

Like, if you have a constraint is_even(x) that's really easy to check in your head with some informal Floyd-Hoare logic.

And it scales to extracting code into helper functions and multiple variables. If you must track which set of variables form one context x1+y1, x2+y2, etc I find it much harder to check the invariants in my head.

These 'fixed state shape' situations are where I'd grab a state monad in Haskell and start thinking top-down in terms of actions+invariants.

23. everforward ◴[] No.45772243{5}[source]
Arrays are a very notable example here. You can append to a const array in JS and TS, even in the same scope it was declared const.

That’s always felt very odd to me.

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24. ◴[] No.45772345{3}[source]
25. StopDisinfo910 ◴[] No.45772446[source]
I think the advantage is often oversold and people often miss how things actually exist on a continuum and just plainly opposing mutable and immutable is sidestepping a lot of complexity.

For exemple, it's endlessly amusing to me to see all the efforts the Haskell community does to basically reinvent mutability in a way which is somehow palatable to their type system. Sometimes they even fail to even realise that it's what they are doing.

In the end, the goal is always the same: better control and warranties about the impact of side effects with minimum fuss. Carmack approach here is sensible. You want practices which make things easy to debug and reason about while mainting flexibility where it makes sense like iterative calculations.

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26. ryandv ◴[] No.45772528{3}[source]
That's right - immutability enables equational reasoning, where it becomes possible to actually reason through a program just by inspection and evaluation in one's head, since the only context one needs to load is contained within the function itself - not the entire trace, where anything along the thread of execution could factor into your function's output, since anybody can just mutate anybody else's memory willy-nilly.

People jump ahead using AI to improve their reading comprehension of source code, when there are still basic practices of style, writing, & composition that for some reason are yet to be widespread throughout the industry despite already having a long standing tradition in practice, alongside pretty firm grounding in academics.

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27. catlifeonmars ◴[] No.45772547{3}[source]
What would those names be in this example?
replies(1): >>45775677 #
28. jayd16 ◴[] No.45772583[source]
If you need new values you just make new things.

If you want to do an operation on fooA, you don't mutate fooA. You call fooB = MyFunc(fooA) and use fooB.

The nice thing here is you can pass around pointers to fooA and never worry that anything is going to change it underneath you.

You don't need to protect private variables because your internal workings cannot be mutated. Other code can copy it but not disrupt it.

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29. eyelidlessness ◴[] No.45772941[source]
> Sometimes they even fail to even realise that it's what they are doing.

Because that’s not what they’re doing. They’re isolating state in a systemic, predictable way.

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30. skeezyjefferson ◴[] No.45773019{4}[source]
> As in - it's not very "constant" if you keep re-making it in your loop, right?

you cant change a constant though

replies(1): >>45773195 #
31. skeezyjefferson ◴[] No.45773072{4}[source]
but we already had the word variable for values that can change. on both counts it seems redundant
replies(1): >>45781429 #
32. raddan ◴[] No.45773093{6}[source]
That's because in many languages there is a difference between a stored reference being immutable and the contents of the thing the reference points to being immutable.
33. stickfigure ◴[] No.45773139{4}[source]
Sounds like you have a data structure like `Array<Float>`. The immutable approach has methods on Array like:

   Array<Float> append(Float value);
   Array<Float> replace(int index, Float value);
The methods don't mutate the array, they return a new array with the change.

The trick is: How do you make this fast without copying a whole array?

Clojure includes a variety of collection classes that "magically" make these operations fast, for a variety of data types (lists, sets, maps, queues, etc). Also on the JVM there's Vavr; if you dig around you might find equivalents for other platforms.

No it won't be quite as fast as mutating a raw buffer, but it's usually plenty fast enough and you can always special-case performance sensitive spots.

Even if you never write a line of production Clojure, it's worth experimenting with just to get into the mindset. I don't use it, but I apply the principles I learned from Clojure in all the other languages I do use.

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34. maleldil ◴[] No.45773150{3}[source]
> there still has to be mutable state somewhere - so where is it moved to?

This is one way of thinking about it: https://news.ycombinator.com/item?id=45701901 (Simplify your code: Functional core, imperative shell)

35. raddan ◴[] No.45773166{3}[source]
In functional programs, you very explicitly _do not_ need to understand an entire program. You just need to know that a function does a thing. When you're implementing a function-- sure, you need to know what it does. But you're defining it in such a way that the user should not know _how_ it works, only _what_ it does. This is a major distinction between programs written with mutable state and those written without. The latter is _much_ easier to think about.

I often hear from programmers that "oh, functional programming must be hard." It's actually the opposite. Imperative programming is hard. I choose to be a functional programmer because I am dumb, and the language gives me superpowers.

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36. veilrap ◴[] No.45773195{5}[source]
He’s implying that the variable it’s being defined within the loop. So, constant, but repeatedly redefined.
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37. raincole ◴[] No.45773250[source]
Yet even Rust allows you to shadow variables with another one with the same name. Yes, they are two different variables, but for a human reader they have the same name.

I think that Rust made this decision because the x1, x2, x3 style of code is really a pain in the ass to write.

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38. jimbokun ◴[] No.45773254{3}[source]
> However, don't you still need to understand the entire program as ultimately that's what you are trying to build.

Of course not, that's impossible. Modern programs are way to large to keep in your head and reason about.

So you need to be able to isolate certain parts of the program and just reason about those pieces while you debug or modify the code.

Once you identify the part of the program that needs to change, you don't have to worry about all the other parts of the program while you're making that change as long as you keep the contracts of all the functions in place.

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39. fwip ◴[] No.45773339{3}[source]
It's moved toward the edges of your program. In a lot of functional languages, places that can perform these effects are marked explicitly.

For example, in Haskell, any function that can perform IO has "IO" in the return type, so the "printLine" equivalent is: "putStrLn :: String -> IO". (I'm simplifying a bit here). The result is that you know that a function like "getUserComments :: User -> [CommentId]" is only going to do what it says on the tin - it won't go fetch data from a database, print anything to a log, spawn new threads, etc.

It gives similar organizational/clarity benefits as something like "hexagonal architecture," or a capabilities system. By limiting the scope of what it's possible for a given unit of code to do, it's faster to understand the system and you can iterate more confidently with code you can trust.

40. jimbokun ◴[] No.45773343[source]
The concept is actually pretty simple: instead of changing existing values, you create new values.

The classic example is a list or array. You don't add a value to an existing list. You create a new list which consists of the old list plus the new value. [1]

This is a subtle but important difference. It means any part of your program with a reference to the original list will not have it change unexpectedly. This eliminates a large class of subtle bugs you no longer have to worry about.

[1] Whether the new list has completely new copy of the existing data, or references it from the old list, is an important optimization detail, but either way the guarantee is the same. It's important to get these optimizations right to make the efficiency of the language practical, but while using the data structure you don't have to worry about those details.

replies(1): >>45775639 #
41. suspended_state ◴[] No.45773392{3}[source]
Or they got inspired by how this is done in OCaml, which was the host language for the earliest versions of Rust. Actually, this is a behaviour found in many FP languages. Regarding OCaml, there was even a experimental version of the REPL where one could access the different variables carrying the same name using an ad-hoc syntax.
42. pxc ◴[] No.45773464[source]
If you read through the Big Red Book¹ or its counterpart for Kotlin², it's quite explicit about the goals with these techniques for managing effects, and goes over rewriting imperative code to manage state in a "pure" way.

I think the authors are quite aware of the relationship between these techniques and mutable state! I imagine it's similar for other canonical functional programming texts.

Besides the "pure" functional languages like Haskell, there are languages that are sort of immutability-first (and support sophisticated effects libraries), or at least have good immutable collections libraries in the stdlib, but are flexible about mutation as well, so you can pick your poison: Scala, Clojure, Rust, Nim (and probably lots of others).

All of these go further and are more comfortable than just throwing `const` or `.freeze` around in languages that weren't designed with this style in mind. If you haven't tried them, you should! They're really pleasant to work with.

----

1: https://www.manning.com/books/functional-programming-in-scal...

2: https://www.manning.com/books/functional-programming-in-kotl...

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43. ghurtado ◴[] No.45773473{6}[source]
That's the opposite of what any reasonable engineer means by "constant".
replies(2): >>45774034 #>>45774230 #
44. dwwoelfel ◴[] No.45773479[source]
Carmack is talking about variable reassignment here, which Clojure will happily let you mutate.

For example:

  (let [result {:a 1}
        result (assoc result :b 2)]
    ...)

He mentions that C and C++ allow const variables, but Clojure doesn't support that.

clj-kondo has a :shadowed-var rule, but it will only find cases where you shadow a top-level var (not the case in my example).

replies(2): >>45773913 #>>45774145 #
45. adrianN ◴[] No.45773559{4}[source]
In theory it’s certainly right that imperative programs are harder to reason about. In practice programmers tend to avoid writing the kind of program where anything can happen.
replies(1): >>45773696 #
46. ◴[] No.45773609{6}[source]
47. ryandv ◴[] No.45773696{5}[source]
> In practice programmers tend to avoid writing the kind of program where anything can happen.

My faith in this presumption dwindles every year. I expect AI to only exacerbate the problem.

Since we are on the topic of Carmack, "everything that is syntactically legal that the compiler will accept will eventually wind up in your codebase." [0]

[0] https://www.youtube.com/watch?v=1PhArSujR_A&t=15m54s

48. airstrike ◴[] No.45773898{3}[source]
I do find shadowing useful. If you're writing really long code blocks in which it becomes an issue, you are probably doing too much in one place.
49. didibus ◴[] No.45773913[source]
It's more nuanced, because the shadowing is block-local, so when the lexical scope exits the prior bindings are restored.

I think in practice this is the ideal middle ground of convenience (putting version numbers at the end of variables being annoying), but retaining mostly sane semantics and reuse of prior intermediate results.

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50. wongarsu ◴[] No.45773931{3}[source]
In idiomatic Rust you usually shadow variables with another one of the same name when the type is the only thing meaningfully changing. For example

   let x = "29"
   let x = x.parse::<i32>()
   let x = x.unwrap()
These all use the same name, but you still have the same explicit ordering dependency because they are typed differently. The first is a &str, the second a Result<i32, ParseIntError>, the third an i32, and any reordering of the lines would provide a compiler error. And if you add another line `let y = process(x)` you would expect it to do something similar no matter where you introduce it in these statements, provided it accepts the current type of x, because the values represent the "same" data.

Once you actually "change" the value, for example by dividing by 3, I would consider it unidiomatic to shadow under the same name. Either mark it as mutable for preferably make a new variable with a name that represents what the new value now expresses

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51. scott_w ◴[] No.45773965{3}[source]
In plenty of languages, there's not really a difference. In Rust, there is a difference between a `let var_name = 10;` and `const var_name: u64 = 10;` in that the latter must have its value known at compile-time (it's a true constant).

> why are you calling it mutable?

Mostly just convention. Rust has immutable by default and you have to mark variables specifically with `mut` (so `let mut var_name = 10;`). Other languages distinguish between variables and values, so var and val, or something like that. Or they might do var and const (JS does this I think) to be more distinct.

52. fnordsensei ◴[] No.45774034{7}[source]
That’s the point, you’re just haggling about scopes now. All the way from being new per program invocation to new per loop.

Immutability doesn’t have this connotation.

replies(1): >>45774245 #
53. scott_w ◴[] No.45774040{3}[source]
> However, don't you still need to understand the entire program as ultimately that's what you are trying to build.

Depends on what I'm trying to do. If what I'm trying to handle is local to the code, then possibly not. If the issue is what's going into the function, or what the return value is doing, then I likely do need that wider context.

What pure-functional functions do allow is certainty the only things that can change the behaviour of that function are the inputs to that function.

54. StopDisinfo910 ◴[] No.45774070{3}[source]
Lenses is mutation by another name. You are basically recreating states on top of an immutable system. Sure, it's all immutable actually but conceptually it doesn't really change anything. That's what makes it hilarious.

In the end, the world is stateful and even the purest abstractions have to hit the road at some point. But the authors of Haskell were fully aware of that. The monadic type system was conceived as a way to easily track side effects after all, not banish them.

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55. munificent ◴[] No.45774112{3}[source]
"Constant" is ambiguous. Depending on who you ask, it can mean either:

1. A property known at compile time.

2. A property that can't change after being initially computed.

Many of the benefits of immutability accrue properties whose values are only known at runtime but which are still known to not change after that point.

56. manoDev ◴[] No.45774145[source]
That's not mutation though.

The `assoc` on the second binding is returning a new object; you're just shadowing the previous binding name.

This is different than mutation, because if you were to introduce an intermediate binding here, or break this into two `let`s, you could be holding references to both objects {:a 1} and {:a 1 :b 2} at any time in a consistent way - including in a future/promise dereferenced later.

replies(2): >>45774328 #>>45778688 #
57. davrosthedalek ◴[] No.45774230{7}[source]
No? It has a lifetime of one loop duration, and is constant during that duration. Seems perfectly fine to me.
58. davrosthedalek ◴[] No.45774245{8}[source]
How? I think the same argument applies: If it's changing from loop to loop, seems mutable to me.
replies(2): >>45774345 #>>45776074 #
59. ◴[] No.45774256{3}[source]
60. bcrosby95 ◴[] No.45774298{3}[source]
It lets you refine when and where it happens more than other methods of restricting state change, such as in imperative OOP.
61. ◴[] No.45774328{3}[source]
62. fnordsensei ◴[] No.45774345{9}[source]
I think you’re after something other than immutability then.

You’re allowed to rebind a var defined within a loop, it doesn’t mean that you can’t hang on to the old value if you need to.

With mutability, you actively can’t hang on to the old value, it’ll change under your feet.

Maybe it makes more sense if you think about it like tail recursion: you call a function and do some calculations, and then you call the same function again, but with new args.

This is allowed, and not the same as hammering a variable in place.

63. ymyms ◴[] No.45774362{4}[source]
Another idiomatic pattern is using shadowing to transform something using itself as input:

let x = Foo::new().stuff()?; let x = Bar::new(x).other_stuff()?;

So with the math example and what the poster above said about type changing, most rust code I write is something like:

let x: plain_int = 7

let x: added_int = add(x, 3);

let x: divided_int = divide(x, 2);

where the function signatures would be fn add(foo: plain_int, int); fn divide(bar: added_int, int);

and this can't be reordered without triggering a compiler error.

64. ◴[] No.45774454{3}[source]
65. SpaceNoodled ◴[] No.45774510{3}[source]
The hardware that these programs are running on store objects in linear memory, so it doesn't not make sense to treat it as such.
replies(2): >>45774729 #>>45780295 #
66. DrScientist ◴[] No.45774575{4}[source]
I think you missed the point. I understand that if you writing a simple function with an expected interface/behaviour then that's all you need to understand. Note this isn't something unique to a functional approach.

However, somebody needs to know how the entire program works - so my question was where does that application state live in a purely functional world of immumutables?

Does it disappear into the call stack?

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67. eyelidlessness ◴[] No.45774704{4}[source]
But there isn’t anything hilarious about that.

It’s a clear-minded and deliberate approach to reconciling principle with pragmatic utility. We can debate whether it’s the best approach, but it isn’t like… logically inconsistent, surprising, or lacking in self awareness.

replies(2): >>45778219 #>>45780526 #
68. DrScientist ◴[] No.45774720{4}[source]
> Once you identify the part of the program that needs to change,

And how do you do that without understanding how the program works at a high level?

I understand the value of clean interfaces and encapsulation - that's not unique to functional approaches - I'm just wondering in the world of pure immutability where the application state goes.

What happens if the change you need to make is at a level higher than a single function?

replies(1): >>45776162 #
69. wtallis ◴[] No.45774729{4}[source]
DRAM is linear memory. Caches, less so. Register files really aren't. CPUs spend rather a lot of transistors and power to reconcile the reality of how they manipulate data within the core against the external model of RAM in a flat linear address space.
70. MetaWhirledPeas ◴[] No.45774869{5}[source]
It didn't disappear; there's just less of it. Only the stateful things need to remain stateful. Everything else becomes single-use.

Declaring something as a constant gives you license to only need to understand it once. You don't have to trace through the rest of the code finding out new ways it was reassigned. This frees up your mind to move on to the next thing.

replies(1): >>45777402 #
71. MetaWhirledPeas ◴[] No.45774914[source]
> it'd require too much discipline to do such a thing in Python

Is Python that different from JavaScript? Because it's easy in JavaScript. Just stop typing var and let, and start typing const. When that causes a problem, figure out how to deal with it. If all else fails: "Dear AI, how can I do this thing while continuing to use const? I can't figure it out."

replies(3): >>45775194 #>>45780834 #>>45781656 #
72. MetaWhirledPeas ◴[] No.45774945{3}[source]
> If you read through the Big Red Book

This is a thoughtful response, but I can't help but chuckle at a response that starts with, just read this book!.

replies(1): >>45778177 #
73. MetaWhirledPeas ◴[] No.45774996{3}[source]
> If you need new values you just make new things. > If you want to do an operation on fooA, you don't mutate fooA. You call fooB = MyFunc(fooA) and use fooB.

The beautiful thing about this is you can stop naming things generically, and can start naming them specifically what they are. Comprehension goes through the roof.

replies(1): >>45777602 #
74. SatvikBeri ◴[] No.45775098{3}[source]
A pretty basic example: I write a lot of data pipelines in Julia. Most of the functions don't mutate their arguments, they receive some data and return some data. There are a handful of exceptions, e.g. the functions that write data to a db or file somewhere, or a few performance-sensitive functions that mutate their inputs to avoid allocations. These functions are clearly marked.

That means that 90% of the time, there's a big class of behavior I just don't need to look for when reading/debugging code. And if it's a bug related to state, I can pretty quickly zoom in on a few possible places where it might have happened.

75. codethief ◴[] No.45775194{3}[source]
I agree that Python is not too different and in general I treat my Python variables as const. One thing, however, where I resort to mutating variables more often than I'd like is when building lists & dictionaries. Lambdas in Python have horrible DX (no multi-line, no type annotations, bad type checker support even in obvious cases), which is why the functional approach to build your list, using map() and filter() is much more cumbersome than in JS. As a result, whenever a list comprehension becomes too long, you end up building your list the old-fashioned way, using a for loop and the_list.append().
76. SoftTalker ◴[] No.45775382[source]
JS is much more of a functional language than it was given credit for a long time. It had first-class functions and closures from day one if I'm not mistaken.
replies(1): >>45776085 #
77. waffletower ◴[] No.45775439{4}[source]
In a Clojure binding this is perfectly idiomatic, but symbolically shared bindings are not shadowed, they are immutably replaced. Mutability is certainly available, but is explicit. And the type dynamism of Clojure is a breath of fresh air for many applications despite the evangelism of junior developers steeped in laboratory Haskell projects at university. That being said, I have a Clojure project where dynamic typing is throughly exploited at a high level, allows for flexible use of Clojure's rational math mixed with floating point (or one or the other entirely), and for optimization deeper within the architecture a Rust implementation via JVM JNI is utilized for native performance, assuring homogenous unboxed types are computed to make the overall computation tractable. Have your cake and eat it too. Types have their virtues, but not without their excesses.
78. ErroneousBosh ◴[] No.45775589{3}[source]
> If you want to do an operation on fooA, you don't mutate fooA. You call fooB = MyFunc(fooA) and use fooB.

This is the bit I don't get.

Why would I do that? I will never want a fooA and a fooB. I can't see any circumstances where having a correct fooB and an incorrect fooA kicking around would be useful.

replies(2): >>45775807 #>>45776159 #
79. ErroneousBosh ◴[] No.45775600{5}[source]
> The methods don't mutate the array, they return a new array with the change.

But then I need to update a bunch of stuff to point to the new array, and I've still got the old incorrect array hanging around taking up space.

This just sounds like a great way to introduce bugs.

replies(1): >>45775728 #
80. MarkMarine ◴[] No.45775611[source]
I have dreams of being at a “Clojure shop” but I fear daily professional use might dull my love for the language. Having to realize that not everyone on my team wants to learn lisp (or FP) just to work with my code (something I find amazing and would love to be paid to do) was hard.

On a positive note I have taken those lessons from clojure (using values, just use maps, Rich’s simplicity, functional programming without excessive type system abstraction, etc) and applied them to the rest of my programming when I can and I think it makes my code much better.

81. ErroneousBosh ◴[] No.45775639{3}[source]
> It means any part of your program with a reference to the original list will not have it change unexpectedly.

I don't get why that would be useful. The old array of floats is incorrect. Nothing should be using it.

That's the bit I don't really understand. If I have a list and I do something to it that gives me another updated list, why would I ever want anything to have the old incorrect list?

replies(2): >>45775956 #>>45776082 #
82. ape4 ◴[] No.45775677{4}[source]
In a real application meaningful names are nearly always possible, eg:

    const pi = 3.1415926
    const 2pi = 2 * pi
    const circumference = 2pi * radius
replies(2): >>45776064 #>>45777010 #
83. stickfigure ◴[] No.45775728{6}[source]
It ends up being quite the opposite - many, many bugs come from unexpected side effects of mutation. You pass that array to a function and it turns out 10 layers deeper in the call stack, in code written by somebody else, some function decided to mutate the array.

Immutability gives you solid contracts. A function takes X as input and returns Y as output. This is predictable, testable, and thread safe by default.

If you have a bunch of stuff pointing at an object and all that stuff needs to change when the inner object changes, then you "raise up" the immutability to a higher level.

    Universe nextStateOfTheUniverse = oldUniverse.modifyItSomehow();
If you keep going with this philosophy you end up with something roughly like "software transactional memory" where the state of the world changes at each step, and you can go back and look at old states of the world if you want.

Old states don't hang around if you don't keep references to them. They get garbage collected.

replies(1): >>45777578 #
84. Maxatar ◴[] No.45775781[source]
>For exemple, it's endlessly amusing to me to see all the efforts the Haskell community does to basically reinvent mutability in a way which is somehow palatable to their type system.

That's because Haskell is a predominantly a research language originally intended for experimenting with new programming language ideas.

It should not be surprising that people use it to come up with or iterate on existing features.

85. jayd16 ◴[] No.45775807{4}[source]
As Carmack points out, naming the intermediate values aides in debugging. It also helps you write code as you can give a name to every mutation.

But also keep in mind that correct and incorrect is not binary. You might want to pass a fooA to another class that does not want the fooB mutation.

If you just have foo, you end up with situations where a copy should have happened but didn't and then you get unwanted changes.

replies(1): >>45777571 #
86. ratelimitsteve ◴[] No.45775905[source]
the flash of enlightment I had when I understood the incredible power the rules of functional programming give you as a coder is probably the biggest one I've had in my career so far. idempotence, immutability and statelessness on their own let you build a thing once in a disciplined way and then use it all willy nilly anywhere you want without having to think about anything other than "things go into process, other things come out" and it's so nice.
87. b_e_n_t_o_n ◴[] No.45775956{4}[source]
State exists in time, a variable is usually valid at the point it's created but it might not be valid in the future. Thus if part of your program accesses a variable expecting it to be from a certain point in time but it's actually from another point in time (was mutated) that can cause issues.
88. tmtvl ◴[] No.45776064{5}[source]
Calling tau 2pi is the most cursed thing I've seen all day. Appropriate for Halloween.
replies(1): >>45777049 #
89. Zambyte ◴[] No.45776074{9}[source]
I can give a specific example.

    for (0..5) |i| {
        i = i + 1;
        std.debug.print("foo {}\n", .{i});
    }
In this loop in Zig, the reassignment to i fails, because i is a constant. However, i is a new constant bound to a different value each iteration.

To potentially make it clearer that this is not mutation of a constant between iterations, technically &i could change between iterations, and the program would still be correct. This is not true with a c-style for loop using explicit mutation.

90. jimbokun ◴[] No.45776082{4}[source]
You pass in an array to a function meant to perform a transformation on each item of the array and return the result.

You pass in an array of 10 values.

While the function is executing, some other thread adds two more values to the array.

How many values should the result of the function call have? 10 or 12? How do you guarantee that is the case?

replies(1): >>45777557 #
91. inopinatus ◴[] No.45776085{3}[source]
I’m fond of saying “JS is a Lisp”. It’s not a hill I’d bother dying on, however.
92. takinola ◴[] No.45776159{4}[source]
It is about being able to think clearly about your code logic. If your code has many places where a variable can change, then it is hard to go back and understand exactly where it changed if you have unexpected behavior. If the variable can never change then the logical backtrace is much shorter.
93. jimbokun ◴[] No.45776162{5}[source]
Yes, obviously a program with no mutability only heats up the CPU.

The point is to determine the points in your program where mutation happens, and the rest is immutable data and pure functions.

In the case of interacting services, for example, mutation should happen in some kind of persistent store like a database. Think of POST and PUT vs GET calls. Then a higher level service can orchestrate the component services.

Other times you can go a long way with piping the output of one function or process into another.

In a GUI application, the contents of text fields and other controls can go through a function and the output used to update another text field.

The point is to think carefully about where to place mutability into your architecture and not arbitrarily scatter it everywhere.

94. alain94040 ◴[] No.45776504[source]
That example is too simple for me to grasp it. How would you code a function that iterates over an array to compute its sum. No cheating with a built-in sum function. If you had to code each addition, how would that work? Curious to learn (I probably could google this or ask Claude to write me the code).
replies(2): >>45776593 #>>45776601 #
95. supergarfield ◴[] No.45776593{3}[source]
Carmack gives updating in a loop as the one exception:

> You should strive to never reassign or update a variable outside of true iterative calculations in loops.

If you want a completely immutable setup for this, you'd likely have to use a recursive function. This pattern is well supported and optimized in immutable languages like the ML family, but is not super practical in a standard imperative language. Something like

  def sum(l):
    if not l: return 0
    return l[0] + sum(l[1:])
Of course this is also mostly insensitive to ordering guarantees (the compiler would be fine with the last line being `return l[-1] + sum(l[:-1])`), but immutability can remain useful in cases like this to ensure no concurrent mutation of a given object, for instance.
replies(2): >>45776908 #>>45776939 #
96. ◴[] No.45776601{3}[source]
97. bmacho ◴[] No.45776908{4}[source]
You don't have to use recursion, that is, you don't need language support for it. Having first class (named) functions is enough.

For example you can modify sum such that it doesn't depend on itself, but it depends on a function, which it will receive as argument (and it will be itself).

Something like:

  def sum_(f, l):
    if not l: return 0
    return l[0] + f(f, l[1:])

  def runreq(f, *args):
    return f(f, *args)

  print(runreq(sum_, [1,2,3]))
replies(1): >>45779627 #
98. hermitdev ◴[] No.45776939{4}[source]
While your example of `sum` is a nice, pure function, it'll unfortunately blow up in python on even moderately sized inputs (we're talking thousands of elements, not millions) due to lack of tail calls in Python (currently) and the restrictions on recursion depth. The CPython interpreter as of 3.14 [0] is now capable of using tail calls in the interpreter itself, but it's not yet in Python, proper.

[0]: https://docs.python.org/3/whatsnew/3.14.html#a-new-type-of-i...

replies(1): >>45777004 #
99. dragonwriter ◴[] No.45777004{5}[source]
Yeah, to actually use tail-recursive patterns (except for known-to-be-sharply-constrained problems) in Python (or, at least, CPython), you need to use a library like `tco`, because of the implementation limits. Of course the many common recursive patterns can be cast as map, filter, or reduce operations, and all three of those are available as functions in Python's core (the first two) or stdlib (reduce).

Updating one or more variables in a loop naturally maps to reduce with the updated variable(s) being (in the case of more than one being fields of) the accumulator object.

100. catlifeonmars ◴[] No.45777010{5}[source]
Agree in real life you can come up with meaningful names (and should when the names are used far away from the point of assignment), but it doesn’t make sense for GPs example, where the whole point was to talk about assignments in the abstract.
101. smrq ◴[] No.45777049{6}[source]
If you call a variable tau in production code then you're being overly cute. I know what it means, because I watch math YouTube for fun, but $future_maintainer in all likelihood won't.
replies(1): >>45780431 #
102. m463 ◴[] No.45777189[source]
I think a lot of this kind of stuff should have language support (like he mentions), even if it is not that functional and is just as a hint.

That said, utopias are not always a great idea. Making all your code functional might be philosophically satisfying, but sometimes there are good reasons to break the rules.

103. zerd ◴[] No.45777296[source]
It's funny that converting the first example to the second is a common thing a compiler does, Static single assignment [0], to make various optimizations easier to reason about.

[0] https://en.wikipedia.org/wiki/Static_single-assignment_form

104. raddan ◴[] No.45777402{6}[source]
> Only the stateful things need to remain stateful.

And I think it is worth noting that there is effectively no difference between “stateful” and “not stateful” in a purely functional programming environment. You are mostly talking about what a thing is and how you would like to transform it. Eg, this variable stores a set of A and I would like to compute a set of B and then C is their set difference. And so on.

Unless you have hybrid applications with mutable state (which is admittedly not uncommon, especially when using high performance libraries) you really don’t have to think about state, even at a global application level. A functional program is simply a sequence of transformations of data, often a recursive sequence of transformations. But even when working with mutable state, you can find ways to abstract away some of the mutable statefulness. Eg, a good, high performance dynamic programming solution or graph algorithm often needs to be stateful; but at some point you can “package it up” as a function and then the caller does not need to think about that part at all.

105. ErroneousBosh ◴[] No.45777557{5}[source]
> While the function is executing, some other thread adds two more values to the array.

This is not something that can happen.

replies(1): >>45779279 #
106. ErroneousBosh ◴[] No.45777571{5}[source]
But that's just it, why would a copy ever happen? Why would you want a correct and an incorrect version of your variable hanging about?
replies(1): >>45783489 #
107. ErroneousBosh ◴[] No.45777578{7}[source]
Okay, so this sounds like it's a method of programming that is entirely incompatible with anything I work on.

What sort of thing would it be useful for?

The kind of things I do tend to have maybe several hundred thousand floating point values that exist for maybe a couple of hundred thousandths of a second, get processed, get dealt with, and then are immediately overwritten with the next batch.

I can't think of any reason why I'd ever need to know what they were a few iterations back. That's gone, maybe as much as a ten-thousandth of a second ago, which may as well be last year.

replies(1): >>45786497 #
108. ErroneousBosh ◴[] No.45777602{4}[source]
Yes, but you can do that without having loads of stale copies of incorrect data lying around, presumably.
109. emil0r ◴[] No.45778109{3}[source]
You are very right in that things need to change. If they don't, nothing interesting happens and we as programmers don't get paid :p. State changes are typically moved to the edges of a program. Functional Core, Imperative Shell is the name for that particular architecture style.

FCIS can be summed up as: R->L->W where R are all your reads, L is where all the logic happens and is done in the FP paradigm, and W are all your writes. Do all the Reads at the start, handle the Logic in the middle, Write at the end when all the results have been computed. Teasing these things apart can be a real pain to do, but the payoff can be quite significant. You can test all your logic without needing database or other services up and running. The logic in the middle becomes less brittle and allows for easier refactoring as there is a clear separation between R, L and W.

For your first question. Yes, and I might misunderstand the question, so give me some rope to hang myself with will ya ;). I would argue that what you really need to care about is the data that you are working with. That's the real program. Data comes in, you do some type of transformation of that data, and you write it somewhere in order to produce an effect (the interesting part). The part where FP becomes really powerful, is when you have data that always has a certain shape, and all your functions understands and can work with the shape of that data. When that happens, the functions starts to behave more like lego blocks. The data shape is the contract between the functions, and as long as they keep to that contract, you can switch out functions as needed. And so, in order to answer the question, yes, you do need to understand the entire program, but only as the programmer. The function doesn't, and that's the point. When the code that resides in the function doesn't need to worry about what the state of the rest of the program is, you as the programmer can reason about the logic inside, without having to worry about some other part of the program doing something that it should do that at the same time will mess up the code that is inside the function.

Debugging in FP typically involves knowing the data and the function that was called. You rarely need to know the entire state of the program.

Does it make sense?

110. combyn8tor ◴[] No.45778129{4}[source]
I did this accidentally the other day in Rust:

let x = some_function();

... A bunch of code

let x = some_function();

The values of x are the same. It was just an oversight on my part but wondered if I could set my linter to highlight multiple uses of the same variable name in the same function. Does anyone have any suggestions?

111. pxc ◴[] No.45778177{4}[source]
For me, well-written books are an enjoyable way to learn, and I'll admit I'm partial to that!

But of course you can learn in whatever way you like. Books are just a convenient example to point to as an indicator of how implementers, enthusiasts, and educators working with these techniques make sense of them and compare them to mutating variables. They're easy to refer to because they're notable public artifacts.

Fwiw, there's also an audiobook of the Red Book. To really follow the important parts, you'll want to be reading and writing and running code, but you can definitely get a sense of the more basic philosophical orientation just listening along while doing chores or whatever. :)

112. pxc ◴[] No.45778219{5}[source]
You might also think of it a bit like poetry: creativity emerging from the process of working within formal constraints. By asking how you can represent something familiar in a specially structured way, you can learn both about that structure and the thing you're trying to unite with it. Occasionally, you'll even create something beautiful or powerful, as well.

Maybe in that sense there's an "artificial" challenge involved, but it's artificial in the sense of being deliberate rather than merely arbitrary or absurd.

replies(1): >>45782526 #
113. amy_petrik ◴[] No.45778328[source]
Guys, guys, I don't think we're on the same page here.

The conversation I'm trying to have is "stop mutating all the dynamic self-modifying code, it's jamming things up". The concept of non-mutating code, only mutating variables, strikes me as extremely OCD and overly bureaucratic. Baby steps. Eventually I'll transition from my dynamic recompilation self-modifying code to just regular code with modifying variables. Only then can we talk about higher level transcendental OOP things such as singleton factory model-view-controller-singleton-const-factories and facade messenger const variable type design patterns. Surely those people are well reasoned and not fanatics like me

114. nayuki ◴[] No.45778564{4}[source]
In Rust, one way I use shadowing is to gather a bunch of examples into one function, but you can copy and paste any single example and it would work.

    fn do_demo() {
        let qr = QrCode::encode_text("foobar");
        print_qr(qr);
        
        let qr = QrCode::encode_text("1234", Ecc::LOW);
        print_qr(qr);
        
        let qr = QrCode::encode_text("the quick brown fox");
        print_qr(qr);
    }
In other languages that don't allow shadowing (e.g. C, Java), the first example would declare the variable and be syntactically correct to copy out, but the subsequent examples would cause a syntax error when copied out.
115. potetm ◴[] No.45778688{3}[source]
regardless of the mechanism, you still run into the exact same problem John had.
116. jimbokun ◴[] No.45779279{6}[source]
Why not? Which language?
replies(1): >>45780221 #
117. ◴[] No.45779458[source]
118. DemocracyFTW2 ◴[] No.45779627{5}[source]
> You don't have to use recursion

You're using recursion. `runreq()` calls `sum_()` which calls `sum()` in `return l[0] + f(f, l[1:])`, where `f` is `sum()`

replies(1): >>45782480 #
119. DemocracyFTW2 ◴[] No.45779719{6}[source]
I think JavaScript has a language / terminology problem here. It has to be explained constantly (see) to newcomers that `const a = []` does not imply you cannot say `a.push( x )` (mutation), it just keeps you from being able to say `a = x` further down (re-binding). Since in JavaScript objects always start life as mutable things, but primitives are inherently immutable, `const a = 4` does guarantee `a` will be `4` down the line, though. The same is true of `const a = Object.freeze( [] )` (`a` will always be the empty list), but, lo and behold, you can still add elements to `a` even after `const a = Object.freeze( new Set() )` which is, shall we say, unfortunate.

The vagaries don't end there. NodeJS' `assert` namespace has methods like `equal()`, `strictEqual()`, `deepEqual()`, `deepStrictEqual()`, and `partialDeepStrictEqual()`, which is both excessive and badly named (although there's good justification for what `partialDeepStrictEqual()` does); ideally, `equal()` should be both `strict` and `deep`. That this is also a terminology problem is borne out by explanations that oftentimes do not clearly differentiate between object value and object identity.

In a language with inherent immutability, object value and object identity may (conceptually at least) be conflated, like they are for JavaScript's primitive values. You can always assume that an `'abc'` over here has the same object identity (memory location) as that `'abc'` over there, because it couldn't possibly make a difference were it not the case. The same should be true of an immutable list: for all we know, and all we have to know, two immutable lists could be stored in the same memory when they share the same elements in the same order.

120. ErroneousBosh ◴[] No.45780221{7}[source]
Well, the stuff I'm writing is in C, but in general it would make no sense for anything to attempt to add items to a fixed-sized buffer.

If you have something so fundamentally broken as to attempt that, you'd probably want to look at mutexes.

Why one earth would you have something attempt to expand a fixed-sized buffer while something else is working on it?

replies(1): >>45781905 #
121. jstimpfle ◴[] No.45780295{4}[source]
Can you clarify?
replies(1): >>45781441 #
122. lock1 ◴[] No.45780431{7}[source]
Where do you draw the line then? Stopping at `tau` just because `$future_maintainer` might get confused feels like an arbitrary limit to me.

What about something like `gamma`? Lorentz factor? Luminance multiplier? Factorial generalization?

Why not just use the full sentence rather than assign it to an arbitrary name/symbol `gamma` and leave it dependent on the context?

And it's not that hard to add an inline comment to dispel the confusion

  const tau = 2*pi; // Alternate name for 2pi is "tau"
123. StopDisinfo910 ◴[] No.45780526{5}[source]
You don’t see what’s hilarious about recreating what you are pretending to remove only one abstraction level removed?

Anyway, I have great hopes for effect system as a way to approach this in a principled way. I really like what Ocaml is currently doing with concurrency. It’s clear to me that there is great value to unlock here.

replies(1): >>45782522 #
124. runeks ◴[] No.45780612[source]
One big problem with mutation is that it makes it too easy to violate many good design principles, e.g. modularity, encapsulation and separation of concerns.

Because any piece of code that holds a reference to a mutable variable is able to, at a distance, modify the behavior of a piece of code that uses this mutable variable.

Conversely, a piece of code that only uses immutable variables, and takes as argument the values that may need to vary between executions, is isolated against having its behavior changed at a distance at any time.

125. DemocracyFTW2 ◴[] No.45780631[source]
I'm really afraid that the weak point of the argument is really Scheme having a Lisp syntax. One might say syntax is the most superficial thing about a language but as a matter of fact it's the mud pool in front of the property where everybody's wheels get stuck and they feel their only option is to go into reverse and maybe try another day, or never. The same happens with APL; sure it's a genius who invented it and tic-tac-toe in a single short line of code is cool—doesn't mean many people get over the syntax.

FWIW I believe that JS for one would greatly benefit from much better support for immutable data, including time- and space-efficient ways to produce modified copies of structured data (like you don't think twice when you do `string.replace(...)` where you do in fact produce a copy; `list.push(...)` could conceivable operate similarly).

replies(1): >>45785576 #
126. nvarsj ◴[] No.45780778[source]
> I think it may be one of those things you have to see in order to understand.

Or the person doesn't understand, then declares the language to be too difficult to use. This probably happens more than the former, sadly.

ex. I've heard people argue for rewriting perfectly working Erlang services in C++ or Java, because they find Erlang "too difficult". Despite it being a simpler language than either of those.

127. ndr ◴[] No.45780834{3}[source]
In python there's no let, var nor const. So yes.
128. a4isms ◴[] No.45781098{6}[source]
There is no exception for ANY data structure that includes references to other data structures or primitives. Not only can you add or remove elements from an array, you can change them in place.

A const variable that refers to an array is a const variable. The array is still mutable. That's not an exception, its also how a plain-old JavaScript object works: You can add and remove properties at will. You can change its prototype to point to something else and completely change its inheritance chain. And it could be a const variable to an unfrozen POJO all along.

That is not an exception to how things work, its how every reference works.

replies(1): >>45785602 #
129. bjoli ◴[] No.45781186[source]
I spent some time discussing in another thread discussing why the foreach loop is so bad in many languages. Most of the bugs I write come from me managing state, yet if I want to do much more than going start to end of a collection I have to either use methods that are slower than a proper loop or I have to manage all the state myself.

In common lisp you have the loop macro (or better: iterate), in racket you have the for loops. I wrote a thing for guile scheme [0]. Other than that I dont know if many nice looping facilities. In many languages you can achieve all that with conbinatoes and what not, but always at the cost of performance.

I think this is an opportunity for languages to become safer and easier to use without changing performance.

0:https://rikspucko.koketteriet.se/bjoli/goof-loop

130. Thorrez ◴[] No.45781429{5}[source]
Oh, good point. I misunderstood your previous question.

Is there a name that refers to the broader group that includes both constants and variables? In practice, and in e.g. C++, "variable" is used to refer to both constants and actual variables, due to there not being a different common name that can be used to refer to both.

131. repstosb ◴[] No.45781441{5}[source]
Modern CPUs do out-of-order execution, which means they need to identify and resolve register sharing dependencies between instructions. This turns the notional linear model of random-access registers into a DAG in practice, where different instructions that might be in flight at once actually read from or write to different "versions" of a named register. Additionally, pretty much every modern CPU uses a register renaming scheme, where the register file at microarchitecture level is larger than that described in the software-level architecture reference, i.e. one instruction's "r7" has no relationship at all to another's r7".

Caches aren't quite as mix-and-match, but they can still internally manage different temporal versions of a cache line, as well as (hopefully) mask the fact that a write to DRAM from one core isn't an atomic operation instantly visible to all other cores.

Practice is always more complicated than theory.

replies(1): >>45785832 #
132. Thorrez ◴[] No.45781637{5}[source]
Depends on the language. In C++

  const std::vector<int>& foo = bar.GetVector();
foo is a constant object reference cannot have its properties changed (and also cannot be changed to refer to a new object).

  std::vector<int>& foo = bar.GetVector();
Is an object reference that can have its properties changed (but cannot be changed to refer to a new object).
133. filoeleven ◴[] No.45781656{3}[source]
Javascript only enforces reassignments to const. So this,

  const arr = []
  arr.push(“grape nuts”]
is just peachy in JS and requires the programmer to avoid using it.

More importantly, because working immutably in JS is not enforced, trying to use it consistently either limits which libraries you can use and/or requires you to wrap them to isolate their side effects. ImmerJS can help a lot here, since immutability is its whole jam. I’d rather work in a language where I get these basic benefits by default, though.

134. filoeleven ◴[] No.45781905{8}[source]
There’s a mismatch between your assumptions coming from C and GP’s assumptions coming from a language where arrays are not fixed-length. Having a garbage collector manage memory for you is pretty fundamental to immutable-first languages.

Rich Hickey asked once in a talk, “who here misses working with mutable strings?” If you would answer “I do,” or if you haven’t worked much in languages where strings are always immutable and treated as values, it makes describing the benefits of immutability more challenging.

Von Neumann famously thought Assembly and higher-level language compilers were a waste of time. How much that opinion was based on his facility with machine code I don’t know, but compilers certainly helped other programmers to write more closely to the problem they want to solve instead of tracking registers in their heads. Immutable state is a similar offloading-of-incidental-complexity to the machine.

replies(1): >>45784734 #
135. bmacho ◴[] No.45782480{6}[source]
> You're using recursion.

No, see GP.

> `runreq()` calls `sum_()` which calls `sum()` in `return l[0] + f(f, l[1:])`, where `f` is `sum()`

Also no, see GP.

replies(1): >>45785308 #
136. eyelidlessness ◴[] No.45782522{6}[source]
I don’t agree with your characterization that anyone is “pretending”. The whole point of abstraction is convenience of reasoning. No one is fooling themselves or anyone else, nor trying to. It’s a conscious choice, for clear purposes. That’s precisely as hilarious as using another abstraction you might favor more, such as an effect system.
137. eyelidlessness ◴[] No.45782526{6}[source]
This is a fantastic way to put it, thank you for adding it!
138. kubanczyk ◴[] No.45783489{6}[source]
Taking your point of view: you assigned a value1 to a name. Then you assigned a value2 to the same name.

You say that value2 is correct. It logically follows that value1 was incorrect. Why did you assign it then?

The names are free, you can just use a correct name every single time.

139. ErroneousBosh ◴[] No.45784734{9}[source]
I must admit I do regard assembly language with some suspicion, because the assembler can make some quite surprising choices. Ultra-high-level languages like C are worse, though, because they can often end up doing things like allocating really wacky bits of memory for variables and then having to get up to all sorts of stunts to index into your array.
140. DemocracyFTW2 ◴[] No.45785308{7}[source]
I am too stupid to understand this. This:

    def sum_(f, l):
      if not l: return 0
      return l[0] + f(f, l[1:])

    def runreq(f, *args):
      return f(f, *args)

    print(995,runreq(sum_, range(1,995)))
    print(1000,runreq(sum_, range(1,1000)))
when run with python3.11 gives me this output:

    995 494515
    Traceback (most recent call last):
      File "/tmp/sum.py", line 9, in <module>
        print(1000,runreq(sum_, range(1,1000)))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^
      File "/tmp/sum.py", line 6, in runreq
        return f(f, *args)
               ^^^^^^^^^^^
      File "/tmp/sum.py", line 3, in sum_
        return l[0] + f(f, l[1:])
                      ^^^^^^^^^^^
      File "/tmp/sum.py", line 3, in sum_
        return l[0] + f(f, l[1:])
                      ^^^^^^^^^^^
      File "/tmp/sum.py", line 3, in sum_
        return l[0] + f(f, l[1:])
                      ^^^^^^^^^^^
      [Previous line repeated 995 more times]
    RecursionError: maximum recursion depth exceeded in comparison
A RecursionError seems to indicate there must have been recursion, no?
141. zelphirkalt ◴[] No.45785576{3}[source]
Doesn't even have to be true copies. Structural sharing is a thing, that enables many or most functional data structures and avoids excessive memory usage. I agree with your point, and it would put JS higher in my liked languages list.
142. everforward ◴[] No.45785602{7}[source]
I know, and I do agree it's consistent, but then it doesn't make any sense to me as a keyword in a language where non-primitives are always by-reference.

You can't mutate the reference, but you _can_ copy the values from one array into the data under an immutable reference, so const doesn't prevent basically any of the things you'd want to prevent.

The distinction makes way more sense to me in languages that let you pass by value. Passing a const array says don't change the data, passing a const reference says change the data but keep the reference the same.

143. FooBarBizBazz ◴[] No.45785832{6}[source]
Realistically, the compiler is building a DAG called SSA; and then the CPU builds a DAG to do out of order execution, so at a fine grain -- the basic block -- it seems to me that the immutable way of thinking about things is actually closer to the hardware.
144. stickfigure ◴[] No.45786497{8}[source]
It is useful for the vast majority of business processing. And, if John Carmack is to be believed, video game development.

Carmamack's post explains it - if you make a series of immutable "variables" instead of reassigning one, it is much easier to debug. This is a microcosm of time travel debugging; it lets you look at the state of those variables several steps back.

In don't know anything about your specific field but I am confident that getting to the point where you deeply understand this perspective will improve your programming, even if you don't always use it.

145. waynesonfire ◴[] No.45786721[source]
Python is like a mutation wet-dream. The language is so broken in modern times.
146. ◴[] No.45787597[source]