Is this function pure?

[Nathan Reed]

Janice Caron wrote:
> Is this function pure?
> 
> int probablyNotPure(int x)
> {
> 	try
> 	{
> 		char[] a = new char[x];
> 		return a.length;
> 	}
> 	catch
> 	{
> 		return 0;
> 	}
> }
> 
> I must assume not, since it's not deterministic. But which part of it
> wrong? Should "new" be forbidden? Or is "try/catch"? Or both?

Pure is intended to mimic some of the capabilities of functional 
languages, yes?  Well, at the implementation level functional languages 
have to allocate memory just like anyone else, and those allocations can 
potentially fail just like always.  That doesn't keep functional 
language functions from being pure, so it shouldn't keep D functions 
from being pure either.

The function you've written is nondeterministic, but only due to the 
nondeterminism inherent in memory allocation.  It would be interesting 
to see if it's possible to write a function that acheives nondeterminism 
  *without* either memory allocation or reading global state (e.g. 
rand()).  I believe it's not.

If that is the case, then the only potential source of nondeterminism in 
pure functions is memory allocation, and I don't think we need to really 
worry about that too much.  After all, if we've run out of memory, we've 
likely got bigger problems than our pure functions becoming 
nondeterministic. :)

Thanks,
Nathan Reed