readonly storage class

[Jonathan M Davis]

On Sunday, April 08, 2012 11:16:40 Benjamin Thaut wrote:
> While typing D code I usually come across the problem that neither const
> nor immutable describe the usage pattern of the memory I'm currently
> working on 100%. Sometimes I have immutable data that has been shared
> among threads that I want to pass to a function. Then I have some const
> data that I want to pass to the same function. Currently you don't have
> any other choice but to write that function two times. But the function
> itself does not need the "extended" properties of const or immutable:
> 
> const: can be casted back to mutable
> immutable: can be implicitly shared among threads
> 
> The only thing the function cares about is, that it will not change the
> data passed to it. It would be kind of nice to have a thrid storage
> class "readonly". It can not be casted back to mutable and it can not be
> implicitly shared among threads, but both const and immutable implicitly
> convert to readonly, because both of these storage classes lose one of
> their properties during conversion. That way you only have to write the
> function once and can pass both const and immutable data to it.
> 
> Just an idea, comments and critics welcome.

I would point out that casting const to mutable and then altering the variable 
is subverting the type system. The compiler does not support casting away 
either const or immutable to alter _anything_. So, as far as the type system 
is concerned, if you want a function that takes both const and immutable, it 
should take const.

Now, you _can_ cast away const and alter a variable if you're careful, but 
you're subverting the type system when you do so and throwing away any 
guarantees that the compiler gives you. It's far from safe.

Given that casting away const on a variable and then mutating is subverting 
the type system and thate therefore the compiler is free to assume that you 
will never do it, I don't see what your idea of readonly would buy us. It's 
the same as const.

- Jonathan M Davis