Wikipedia purity example and discussion

[retard]

Sat, 06 Nov 2010 13:49:20 +0200, Vladimir Panteleev wrote:

> A while ago, someone added an example with pure functions to Wikipedia's
> D article:
> 
> http://en.wikipedia.org/wiki/D_(programming_language)#Functional
> 
> Someone on the talk page asked why does the program compile despite that
> mySum accesses a variable in its enclosing function:
> 
> http://en.wikipedia.org/wiki/Talk:D_(programming_language)
#Purity_of_mySum_function_in_the_.22Functional.22_section_.281.1.4.29
> 
> I replied with:
> 
>> The code indeed compiles. I think that the idea is that nested
>> functions have a hidden argument - a pointer to their enclosing scope
>> (main's local variables). However, that doesn't explain why the code
>> continues to compile when pivot is moved outside main(), or if you add
>> a call to a non-pure function in mySum - these sound like compiler
>> bugs.
> 
> I don't know much about purity, so I thought someone could shed some
> light on this?

The function isn't referentially transparent, at least. Well, it is in 
that particular example if the compiler can "infer" the constness of 
'pivot'. Otherwise the value requires a constness attribute. Depends on 
whether we are splitting hair again, right?


auto x = my_sum(a,b);
pivot = 42;
auto y = my_sum(a,b);

assert(x == y); // fails