[Athas]

I'm not going to claim that mutability is never useful for performance, but many large scale simulations can be expressed quite elegantly using bulk operations on arrays or other structures, with no mutability in sight. Both particle simulations a la n-body and stencil operations are in this category. An efficient low-level implementation of such bulk operations involves mutable updates, just like any functional language is compiled to "impure" assembly code, but the programming model used for application programming can remain pure.

[vram22]

Interesting. Can you explain, with a somewhat simple example, how this can be efficiently implemented, or at all? I mean preserving the appearance of immutability at the source language level, while mutating the original structure under the hood for performance.

[Athas]

Any vectorised operation in Numpy is an example of this. The pure subset of Numpy can be used to write useful programs, but the Numpy functions/methods are mostly implemented in impure C.

Another example is completely pure array programming such as in Accelerate[0] or Futhark[1].

[0]: http://www.acceleratehs.org/

[1]: https://futhark-lang.org

[senkora]

A somewhat related idea is called "benign effects." The idea is that you write code with an immutable interface that uses mutation in its implementation.

So there are "effects" (non-functional state changes) that are encapsulated ("benign").

I learned this term in reference to Standard ML at CMU.

This is different from what you're asking because it isn't a compiler optimization and it isn't actually checked by the language at all, but it works pretty well in practice.

It's like unsafe in Rust: you write most of your code assuming a useful property that you then break in the small percentage of code that needs to break it.

[kdmccormick]

Not very knowledgable on this myself, unfortunately, but I believe that in graphics programming, shaders written in GLSL often take the form of a series of functional, mathematical transformations of vertices. Those transforms are run in the GPU as highly parallelized array operations, probably using a lot of mutable state. But those details are mostly hidden from the shader programmer.

[zabzonk]

C++ supports this via the mutable keyword https://stackoverflow.com/questions/105014/does-the-mutable-... though not particularly for performance purposes.

[vram22]

Thanks to all who replied.

[leephillips]

I concur, I should have been more precise in my comment.