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Comment by PerfectElement | Hacker News Reader

> If you're still doing mutable programming for some god-forsaken reason

What is mutable programming? Using mutable objects is something I do everyday. Am I doing things wrong?

> Am I doing things wrong?

No. Mutating state that is shared between different parts of a program is often a bad idea. The functional programming community learned the first half of that, and now goes round preaching the mistaken idea that all mutation is bad.

Not everyone, or even most of the FP community does that. There is even a famous paper on how lambda is the ultimate imperative.

Is useful in single threaded programs too, an example is to avoid having to do deep copies everywhere (which is less performant than using persistent data structures) as to have multiple versions in time of some data that you can hang on to.

I don't think they're referring to multiple threads when they say different part of the program, but instead that you should be able to reason about mutation locally.

For example, if I pass an argument into a function, it may be 'unexpected' that the argument is mutated - I can not reason about that mutation locally (unless it's very explicit or a known idiom such as push).

However, within a function, avoiding mutation seems pointless as you should have no trouble reasoning about it. At some point you really are just throwing away performance with significantly diminishing benefits.

Shared mutability across threads is definitely a huge pain in the ass though.

In the end I think we're all just trying to reduce the state space we have to manage in our heads when we read and write code, and removing mutability reduces that space.

Ok, but here you are doing all the manual work of creating a copy as to avoid mutating the arg/returning a new one and, it may be less peformant because of whole copy, knowing your programming language automtically defaults and does this for you in a performant way is a big win for reducing cognitive overhead in large programs.

I don't disagree, but the way Erlang (and thus Elixir) does it is that you get a small stack space to contain your immutable data -- and when you unwind the function the stack just gets thrown away instead of involving the GC which I'd argue is still plenty fast.

I do agree that copying stuff around is generally expensive. I am just unsure how expensive it is in smaller functions that aren't called 10_000 times a second.

Immutability is a common part of functional programming approaches. In Javascript, it's especially common in the React+Redux community. Redux expects you will update data immutably, and React works best when you do immutable updates as well.

Here's some good overviews of why and how to do immutable updates in JS:

https://redux.js.org/faq/immutable-data

https://redux.js.org/recipes/structuring-reducers/immutable-...

https://daveceddia.com/react-redux-immutability-guide/

The idea of "editing" stuff instead of creating new versions of them.

Whilst mutable programming is faster on write, it is much more difficult to figure out if something has changed, so any function that needs to only do work when stuff has changed (e.g. a React component), it is much much better to use immutable style programming because you only have to see if the memory address has changed as opposed to deeply compare current and previous objects.

If you are doing deep structure compare to check if an object was "changed", you're doing "mutable programming" wrong.

IMHO, if you're doing deep compare on anything for any reason, it's usually a sign that the data model is on a shaky ground.

I disagree that mutable code is inherently faster to write. Like any paradigm you can adopt, it probably feels that way igfyou start injecting immutability into an existing project, but sooner or later you settle into different design patterns which support it better, and it's not faster or slower to write. Probably faster to debug though.

Until hardware changes away from being an intrinsically imperative machines, immutable approaches will be slower simply because the hardware doesn’t really support that.

We saw FP hardware leading to performance boosts kind of happen with GPUs (pixel and vertex shaders are just transformers), but then they got back to imperative again with GPGPU.

It is not much better. It is something different. It's popular because of the way React works. It is not always correct because it is easier to understand how state changes. Using array push over concat shouldn't make understanding more difficult and using the slower function for this reason is missing the point.

Only if some hardcore functional purist is around.

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