Improvement in pure functions specification

[Andrei Alexandrescu via Digitalmars-d]

On 12/21/2016 03:59 PM, John Colvin wrote:
> On Wednesday, 21 December 2016 at 15:40:42 UTC, Andrei Alexandrescu wrote:
>> On 12/20/2016 05:49 PM, Andrei Alexandrescu wrote:
>>> https://github.com/dlang/dlang.org/pull/1528 -- Andrei
>>
>> Dropped the void functions. On to the next scandal:
>>
>>> A function that accepts only parameters without mutable indirections and
>>> returns a result that has mutable indirections is called a $(I pure
>>> factory
>>> function). An implementation may assume that all mutable memory
>>> returned by
>>> the call is not referenced by any other part of the program, i.e. it is
>>> newly allocated by the function.
>>
>>
>> Andrei
>
> There are 3 levels:
> 1) no idea what's going on: e.g. the function returns a mutable
> reference and also reads from global mutable memory.

That would be not pure.

> 2) memory must be new: e.g. returns 2 mutable references, no accessing
> external mutable memory.

Yah, they could refer one another.

> 3) memory must be new and uniquely referenced: function returns 1
> mutable reference, does not access external mutable memory.

Yah.

> If I'm not mistaken only 3 enables anything useful like implicit casts
> to immutable.

The formulation is careful to not specify what can be done. For now "not 
referenced by any other part of the program" nicely covers 2 and 3.

> Also, "returned references" should be extended to include "out"
> parameters, because there's no difference as far as memory uniqueness is
> concerned.

Cool idea.


Andrei