assumeSafeAppend and purity

[Vladimir Panteleev]

On Tuesday, 7 February 2012 at 01:47:12 UTC, Jonathan M Davis 
wrote:
> At present, assumeSafeAppend isn't pure - nor is capacity or 
> reserve. AFAIK, none of them access any global variables aside 
> from GC-related stuff (and new is already allowed in pure 
> functions). All it would take to make them pure is to mark the 
> declarations for the C functions that they call pure (and those 
> functions aren't part of the public API) and then mark them as 
> pure. Is there any reason why this would be a _bad_ idea?

pure void f(const(int)[] arr)
{
	debug /* bypass purity check to pretend assumeSafeAppend is pure 
*/
	{
		assumeSafeAppend(arr);
	}
	arr ~= 42;
}

void main()
{
	int[] arr = [0, 1, 2, 3, 4];
	f(arr[1..$-1]);
	assert(arr[4] == 4, "f has a side effect");
}