On Mon, 19 May 2014 13:44:59 -0400, Ola Fosheim Grøstad
<[email protected]> wrote:
On Monday, 19 May 2014 at 17:35:34 UTC, Steven Schveighoffer wrote:
Returning the same immutable object, when called with the same
immutable parameters, should never cause a break in code, pure or not.
This isn't at all obvious to me. Also I think the "coin flip trick"
represent a class of algorithms that depend on imposing a sort order on
objects.
Anything that uses the order of unrelated addresses is incorrect outside
of the heap code itself.
I can easily see an algorithm break if you sometimes receive the same
object and sometimes receive a new object with the same values as
parameters.
Then you should have no problem producing an example, right?
That's how an optimizer works, sometimes it optimizes, sometimes not. If
your algorithm depends on running something twice and then comparing the
two results then it could easily break.
The whole POINT of pure functions is that it will return the same thing.
The fact that it lives in a different piece of memory or not is not
important. We have to accept that. Any code that DEPENDS on that being in
a different address is broken.
For instance, I would consider it fully possible and reasonable, and to
only break already-broken code (except for testing implementation, which
would have to change anyway), to make idup just return the argument if the
argument is immutable.
-Steve
