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Comment by danmur | Hacker News Reader

Scala has immutable types in the standard library too, but I don't think they make much sense without the language being statically typed. By which I mean adding immutable types to Python or whatever's stdlib would not be that hard, but it would most likely be confusing since you don't know for sure what you have until you look at it and see.

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Rust's deep immutability is different than most in that it allows setting a reference as recursively immutable even if the data type it's pointing to has mutable fields.

Scala gives you immutable data types but unless I'm mistaken there's no way to say "recursively disable mutating functions for all elements in this list"—you just have a list that cannot be mutated. If you stick a Java object in a Scala list, it's liable to mutate, so Scala has shallow immutability.

Yeah that's true, it is different. Neat :). With Scala you would put immutable types in your list if you wanted to, but I get what you mean, it's not the same.

Scala's immutable types are also "shallow". For example you can put mutable objects inside an immutable container and change them when you want. Rust avoids this using lifetimes.

It would be interesting if mutability was actually part of Scala's type system, for example if immutable Seq was defined as

    class Seq[T <: Immutable]

> I don't think they make much sense without the language being statically typed.

They absolutely do, in part because you son’t even have a type system you’d need to undermine in order to achieve mutation.

There are “immutability first” dynamically typed langages, and they work rather well (erlang, clojure).

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