head const (again), but for free?

[sighoya]

On Wednesday, 13 January 2021 at 19:22:07 UTC, Ola Fosheim 
Grøstad wrote:
> On Wednesday, 13 January 2021 at 18:37:02 UTC, sighoya wrote:
>> Yep, head const is usually readonly or better final, readonly 
>> should be transitive.
>
> Why? "immutable" is transitive in D. readonly memory or 
> registers are not transitive.

The difference is readonly memory didn't know about the type of 
data saved in them. A pointer or raw data is in the end the same 
thing, just bytes.

Immutable var in high level languages denote a value even if this 
value consists of pointers.
Most languages simply don't follow the model.

> No, readonly in technical specifications usually means that it 
> is isn't possible to write to it at all. e.g. readonly hardware 
> registers. They can still point to writable memory.

Usually, readonly is only a view of a variable, not a part of 
type, so you can't change the value of a readonly variable though 
the value can be changed from outside.
So we still have a variable whereas for immutable we have what we 
call a value.


> "readonly" in Typescript and C# is immutable, not const.

But what happens if I initialize a readonly variable with an 
object which I mutate from the outside afterwards?
This shouldn't be possible with immutable because of being 
globally unchangeable.
In D you can only pass immutable parts into the initialization of 
an immutable aggregate to prohibit any side effect.
But is this the same behavior with readonly in Typescript, I 
don't think so.

The proposed readonly in C# would be likewise non immutable as 
shown from 
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/readonly:

>Because reference types contain a reference to their data, a 
>field that is a readonly reference type must always refer to the 
>same object. That object isn't immutable.


> Which languages are you thinking of?

C#, Typescript, Rust, Swift.

I think what D offers with immutable is unique, I can't even 
count other languages with my fingers providing the same feature 
except pure languages.