In rust, mutable parameters are a separate type than immutable parameters. I wonder if this would be useful in languages even *without* a borrow checker. ie I write the function fn add_one(x: mut int) The compiler would *prevent* passing an immutable integer in (which would be the default type) - ThreadSky

hillelwayne.com • 167 days ago

In rust, mutable parameters are a separate type than immutable parameters. I wonder if this would be useful in languages even *without* a borrow checker.

ie I write the function

fn add_one(x: mut int)

The compiler would *prevent* passing an immutable integer in (which would be the default type)

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pinealservo.bsky.social•167 days ago

That takes an int by value and *copies* it to the destination (rather than moving it from the calling context, as would occur for types that are not `Copy` ); the local binding within the function is allowed to be mutated though.

You would want a mutable reference for your example, I think.

pinealservo.bsky.social•167 days ago

I guess your hypothetical language may well pass everything by reference, but my point is that Rust is (more, but not completely) explicit about more than just mutability and the aspects tend to interact in interesting ways.

hillelwayne.com•167 days ago

Then you know that if a mutable function was called, your variable is mutated.

Then again, what if you're looking at an alias to a mutable var, how do you know it's updated

Wait fuck I just reinvented the borrow checker

blen.blinsay.com•167 days ago

fun context!

https://bsky.app/profile/steveklabnik.com/post/3ldh7gbbles2w

blen.blinsay.com•167 days ago

but yeah tl;dr my experience has also been that borrow checking catches things where mut just reminds me the borrow checker is good at catching things

r.whal.ing•167 days ago

This also maps pretty closely to passing pointers vs passing values in C. Passing a pointer implies that the called function has the ability to mutate whatever it received.

Standard ML can do things like this with passing values vs ref cells containing values, if I remember right.

jonahplusplus.dev•167 days ago

You mean something like `&mut int`? In Rust, adding mut to a function parameter (e.g. `fn foo(mut x: i32)`) just means that you can mutate x in that function (since the parameter will be copied). `&mut i32` means that the reference is mutable (as opposed to `&i32`).

tofoobar.bsky.social•166 days ago

from my understanding &mut does not make the reference mutable, but the value behind the reference.

if you pass &i32 you can only "see" the value behind the reference but not change it (this makes more sense with structs j think)

mempler.de•167 days ago

That's what the const keyword is for in most languages. Rust just does the opposite of const by default

leftnutsociety.bsky.social•167 days ago

sounds like a borrow checkers with extra steps

steveklabnik.com•167 days ago

have you spent much time with mutable value semantics yet? worth looking at if you're thinking about stuff like this

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