Generic code: @autoconst, @autopure, @autonothrow

[Peter Alexander]

On 29/08/10 2:29 AM, dsimcha wrote:
> An issue that comes up very frequently when trying to use const, pure or
> nothrow in generic code is lack of knowledge of whether the functions you're
> calling are const/pure/nothrow.  For example:
>
> T abs(T num) pure nothrow {
>      return (num<  0) ? -1 * num : num;
> }
>
> Looks pretty good.  Won't work with BigInt because opBinary!"*" isn't pure and
> can't practically be made pure.

I think the problem here is *not* that it's difficult to tell whether a 
function can be pure, but the fact that BigInt's binary product should 
be pure (because it is, in the mathematical sense).

This is just the whole const without mutable thing all over again i.e. 
there exist logically const member functions that change the state of 
their members (caching is the canonical example of this). This is why 
mutable exists. Similarly, there exist logically pure function that 
change the global state, or allocate memory (such as BigInt's op* here). 
There needs to be some way around that.

If there cannot be any way around it then pure should be dropped 
altogether because the language simply cannot support it.