readonly storage class

[Benjamin Thaut]

Am 08.04.2012 11:28, schrieb 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

I'll come up with a example.
But if what you say is true, why is immutable not implicitly convertible 
to const?

-- 
Kind Regards
Benjamin Thaut