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Comment by amluto | Hacker News Reader

Also: avoidance of mutable state, even locally. A lot of functions can be more straightforwardly expressed using mutable variables and data structures, and most functional languages can handle local mutable state, but a lot of code in functional languages avoids it.

Conversely, a lot of code written in imperative languages would be clearer and/or less bug-prone if it avoided mutable state and used persistent data structures.

I wish there was a mainstream, high performance language that made both styles equally ergonomic.

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In my experience the main benefit of functional programming is function purity. I am completely fine with mutation inside of a function since the all of the mutation logic is self-contained in a single small block of text.

I think everyone should take a shot at writing a non-trivial functional program to see the benefit. Once you understand what makes it great, you can apply what you've learned to the majority of OOP/impure languages.

Does Rust not meet that description?

Rust is very good at making things immutable. But that doesn't mean it's particularly fun to, for example, append or prepend something to a list and retain a reference to both the old and the new list.

Compare to most "functional" languages, in which prepending an item to a list and ending up with immutable references to the old and new lists is almost the defining feature of the language.

Considering the performance implications of a full commitment to immutable data structures, I think Rust does a pretty good job here.

Dynamic arrays / vectors / slices or whatever you want to call them are probably the most important or fundamental non-plain-old-data data structure, and in a purely immutable environment these are essentially impossible (or have awful performance characteristics).

Yeah although I find FP languages' obsession with singly linked lists to be a real flaw because they have such awful performance characteristics. And also pretty bad ergonomics often. The number of times stuff comes out backwards...

Flix has "regions", which are a way to allow mutation locally while keeping the language purely functional otherwise: See Region-based Local Mutation in https://flix.dev/

> I wish there was a mainstream, high performance language that made both styles equally ergonomic.

Unironically, C++.

> > I wish there was a mainstream, high performance language that made both styles equally ergonomic.

> Unironically, C++.

At best C++ falls under "equally unergonomic".

Or depending on what "high performance" means (e.g. if Java or Go would be considered acceptable), Scala.

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