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Comment by rtpg | Hacker News Reader

Experimentally, at least one major programming language (Rust) places mutability into the type system.

Whether or not something belongs into a type system is ultimately determined by the type system. We can choose whether or not mutability is considered a part of a type.

> When we speak of "type of a variable", we mean this variable can only be assigned (bound to) values of certain type. This has nothing to do with whether it can be reassigned (i.e. mutability).

This is a bit too simplistic IMO. You're talking about name bindings, the article is talking more about things like interior mutability.

Rebinding a name is ... generally not a type system concern by my understanding.

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You have a point; I am looking at it from quite functional programming perspective, because that's how type systems are typically understood. So from that perspective, interior mutability is a form of rebinding.

When you say "we can choose mutability as a part of a type", the question is, what kind of errors are we trying to prevent? What is the semantics we want to give? From that it should be obvious whether it can be subtype or not.

> So from that perspective, interior mutability is a form of rebinding

really? i honestly dont have _too_ much experience with functional languages, basically only elixir and consequently some amount of erlang in that vain... but i'd feel like thats not the same? but it may be that my point of view is too narrow.

from my experience with that functional language, the equivalent to this scenario would be a struct - and wherever i can mutate properties within it -- or need to reconstruct the struct from scratch.

both have technical consequences, eg if i passed the struct into a consumer somewhere which keeps it, it would get the "modified" version automatically when the property was changed

but on reconstruction, it'd have to introduce some kind of event listener to handle the reconstruction.

simple example for such a scenario would be eg a session within a SSE api. the mutated struct would trivially allow for an uninterrupted stream no matter how long the session is extended, the latter needs to pay attention so its not opening a memory leak to support that feature.

> it may be that my point of view is too narrow

Not sure if you're just being polite but, in case you're doubting, functional languages make that distinction too.

Haskell has `Data.Vector.Vector` vs `Data.Vector.Mutable.MVector`.

Clojure has `transient`.

OP is just mistaken.

A system programming language needs to match hardware where mutability is associated with memory addresses, something functional languages can afford to abstract away.

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