Infer purity of functions too?

[Steven Schveighoffer]

On Mon, 18 Jul 2011 16:01:08 -0400, bearophile <bearophileHUGS at lycos.com>  
wrote:

> Maybe someone has already answered this question, but I don't remember  
> it.
>
> DMD 2.054 is able to infer if a template function is pure. Isn't it a  
> good idea to use the same machinery to infer the unmarked functions too?  
> A template may call a second function that's not annotated with 'pure'  
> despite being pure. If the compiler is able to infer the purity of the  
> second function, then both the template and the second function can be  
> pure. This may offer optimization opportunities. Is doing this too much  
> slow?

It might not be possible.  For example, if the target function has no  
public implementation.  This is not the case for templates -- the  
implementation must be available.

In theory, it's possible for the compiler to mark a function whose source  
is available as pure, and indeed, most could be.  It would be a nice  
solution to the issue we have now where so much is not pure.  At some  
point though, optional may not be what you want.  In fact, you may want  
the compiler to complain that a function you marked as pure isn't actually  
pure.  Relying on the compiler to determine purity has drawbacks...

-Steve