[capableweb]

> Whether it's JavaScript's `const`

const is JS is referring to the reference and has nothing to do with the value, it's not even shallow immutability, there is none at all.

    const arr = []
    arr.push(1)

is valid JavaScript.

Bit nitpicky maybe, but wouldn't want people to get the impression const gives you any sort of immutability.

[masklinn]

> wouldn't want people to get the impression const gives you any sort of immutability.

const makes the binding immutable but not the object. Object.freeze makes the object immutable but not it’s children.

[cormacrelf]

Accidentally mutating globals was a big problem with the language. Const didn't solve it completely: you can still implicitly create globals by forgetting to declare the binding locally. But at least if you have a global you can make it an immutable binding and get warned about mutating it some of the time (ie assignments). As far as I'm concerned that is all it is useful for. For a local binding you don't really get much benefit since it's not deeply immutable, and it isn't an improvement to the ambiguity of local/global scopes, so you're frankly better off not using it as it will trick newcomers.

The same argument doesn't apply to Data's shallow immutability. It will give you errors when mutating at least some of the fields. If your code can catch you mutating a number, then you can notice the bug and be reminded to make deep copies etc. It's an improvement, just like Object.freeze.

[rtpg]

It's really the same problem extended to attributes of Data. The "immutability" in Data _also refers to the reference_.

To take Python tuples (which are morally pretty close to Data)

  my_data = ([],false)
  my_data[0].push(1)

tuples are immutable, but this happens, because the "immutability" is in the same vein as JS's const: "we are always pointing to the same object".

[lolinder]

This is exactly the same problem that OP was drawing out with Ruby's Data: the references inside the Data object are immutable and cannot be changed to point to other objects, but once you've dereferenced the (immutable) pointers there are no immutability guarantees. Hence, shallow immutability.

The only conceptual difference between this and JS's `const` is that you can't use `const` to declare object properties immutable; for that you need Object.freeze().