pure member functions

[Steven Schveighoffer]

On Mon, 20 Sep 2010 15:45:10 -0400, Don <nospam at nospam.com> wrote:

> bearophile wrote:
>> Jonathan M Davis:
>>
>>> I assume that if you declare a member function as pure, then all of  
>>> its parameters - including the invisible this - are included in that.  
>>> That is, if all of them - including the invisible this - have the same  
>>> value, then the result will be the same.
>>  This D2 program runs with no errors, and here there isn't a D  
>> language/compiler bug:
>>  struct Foo {
>>     int x;
>>     this (int xx) { this.x = xx; }
>>     pure int bar() { return x; }
>> }
>> void main() {
>>     Foo f = Foo(1);
>>     assert(f.bar() == 1);
>>     f.x *= 2;
>>     assert(f.bar() == 2);
>> }
>>  Bye,
>> bearophile
>
> You do need to be careful about concluding how 'pure' works based on the  
> current behaviour of the compiler.
> There's a trap here. What if you use a hypothetical startTimer()  
> function which executes a delegate every few clock ticks?
>
> void main() {
>      Foo f = Foo(1);
>      startTimer( () { f.x++; });
>      scope(exit)
> 	killTimer();
>
>      assert(f.bar() == 1); // may fail!
>      f.x *= 2;
>      assert(f.bar() == 2);
> }

Wouldn't f have to be shared for this to be asynchronous?

> I actually think that 'pure' on a member function can only mean, it's  
> cacheably pure if and only if 'this' can be cast to immutable. Which  
> includes the important case where the call is made from a pure function  
> (this implies that the 'this' pointer is either a local variable of a  
> pure function, or an immutable object).
> Since pure functions cannot call impure functions, they can't do any of  
> this nasty asynchronous stuff.

I think it's ok for a function to be pure if all the arguments are  
unshared, regardless of immutability.  However, in order to cache the  
return value, the reference itself must not be used as the key, but the  
entire data of the reference.  Even if it's immutable, wouldn't you not  
want to cache the return values between two identical immutable objects?

-Steve