Static functions in immutable types...

[Steven Schveighoffer]

On Tue, 08 Dec 2009 11:53:03 -0500, Michal Minich  
<michal.minich at gmail.com> wrote:

> Hello Steven,
>
> I'm sorry, but have big trouble to understand what you want to say. So I  
> will comment to what I can, and try to rephrase my post.
>
>> On Mon, 07 Dec 2009 05:34:52 -0500, Michal Minich
>> <michal.minich at gmail.com> wrote:
>>
>>> Hello Tomek,
>>>
>>>> ... don't make sense. So is there a reason why they aren't excluded
>>>> from  the "everything in an immutable is immutable" rule?
>>>> Tomek
>>> Immutable static member function could be useful...
>>>  marking struct or class as const or immutable is the same as marking
>>> every its member so. The problem I see is in definition of what const
>>> or  immutable member function means:
>>>  from D specs: "Immutable member functions are guaranteed that the
>>> object  and anything referred to by the this reference is immutable."
>>>  Which applies both to instance and s static functions. I think the
>>> above  definition should apply only to instance functions. Static
>>> const or  immutable functions should  be defined as:
>>>  Immutable static member functions are guaranteed that the static
>>> variables of object and anything referred to by these variables is
>>> immutable.
>>>
>> Let's say there was an immutable static member function which treated
>> the  static data members as immutable.  This means nobody else can
>> change  them.
>
> Couldn't they be changed from outside of the type?

That would be a violation of immutability.  If they can change, then they  
are not immutable.  Note they cannot be "temporarily" immutable, they are  
immutable from the moment that something casts it to immutable.  It is up  
to you, the programmer, to ensure that no mutable references exist after  
you perform the cast.  Using a mutable reference to immutable data results  
in undefined behavior.

To illustrate:

int x;
immutable(int) *y;

void foo()
{
    y = cast(immutable(int)*)&y;  // no longer allowed to use x!
}

void bar()
{
    foo();
    x++; // undefined behavior!
}

>
>> This means that there cannot be any static member
>> functions that  are not immutable.  This means that the data members
>> cannot be marked as  public unless they are also marked as immutable.
>>  In this case, the equivalent thing is to mark all static data members
>> as  immutable, and leave the static function as a normal function.
>>  This is why the OP said they "don't make sense"  Although his
>> statement  really should be "immutable static functions don't make
>> sense", not  "static functions in immutable types don't make sense,"
>> as they do make  sense, they should just not be marked as immutable.
>>  The bug filed is absolutely a bug, it should be possible to have a
>> normal  static function inside an immutable type.
>
> Currently everything is marked as immutable in immutable type. Do you  
> think it is good to have exception for static function?

Either you make an exception for a static function -or- applying immutable  
to a static function is a no-op.  There is no reason to restrict immutable  
types from having static functions.

> So to rephrase:
> 1. from D specs: " A struct declaration can have a storage class of  
> const, immutable or shared. It has an equivalent effect as declaring  
> each member of the struct as const, immutable or shared." -- I'm ok with  
> this, it is simple.
>
> 2. from D specs: "Immutable member functions are guaranteed that the  
> object and anything referred to by the this reference is immutable." --  
> this rule is wrong, because it mentions *this* and at the same time it  
> applies to static member functions, which obviously doesn't have *this*  
> reference.
>
> 3. I propose changing rule in point 2 to: "Immutable *non-static* member  
> functions are guaranteed that the object and anything referred to by the  
> this reference is immutable." and adding this one: "Immutable static  
> member functions are guaranteed that the static variables of object and  
> anything referred to by these variables is immutable"
>
> And I'm asking if the change is reasonable - Or how should be defined  
> semantics of static immutable member function?

static immutable member functions are either illegal in the case where all  
static member data is not immutable or equivalent to static member  
functions in the case where all the member data is immutable.

I agree the rule needs to change, here is how I would change it:

A struct declaration can have a storage class of const, immutable or  
shared. It has an equivalent effect as declaring each member of the struct  
except static functions as const, immutable or shared.

Immutable/const/shared non-static member functions are guaranteed that the  
object and anything referred to by the this reference is  
immutable/const/shared.  Declaring a static member function is  
immutable/const/shared is considered an error.

-Steve