Proposal: Relax rules for 'pure'

[Steven Schveighoffer]

On Wed, 22 Sep 2010 16:42:09 -0400, sclytrack <sclytrack at fake.com> wrote:

>
>> weaklypure void reverse(int[] x)
>> {
>>     for(int i = 0; i * 2 < x.length; i++)
>>         swap(x[i], x[$-1-i]);
>> }
>> pure int foo(const(int)[] x)
>> {
>>      auto x2 = x.dup;
>>      reverse(x2);
>>      // do some calculation on x2
>>      ...
>>      return calculation;
>> }
>
> noglobal void reverse(MyClass x)
> {
>   x.text = "";
> }
>
> So weakly-pure should not access global stuff. But that is harder
> to track by the compiler with mutable parameters.

No, it just means they cannot access data except through their arguments.   
Global stuff is perfectly valid to pass in.  However, any references must  
not be shared, because those could be actively changed by other threads.

> Or weakly-pure is only noglobal when called in pure routines, because of  
> the
> immutable barrier.

pure, weak or strong, means you cannot access global variables.  e.g.:

int x;
shared int y;

pure foo(int *n)
{
    x = 5; // illegal
    *n = 5; // fine
}

foo(&x); // fine, x is TLS, it is guaranteed not to change from another  
thread.
foo(&y); // nope

The difference between weak and strong is that weak pure functions cannot  
be more optimized than normal functions.  Strong pure functions can enjoy  
the optimizations that functional languages have.

-Steve