UDAs - Restrict to User Defined Types?

[Nick Sabalausky]

On Thu, 08 Nov 2012 21:24:49 -0800
Jonathan M Davis <jmdavisProg at gmx.com> wrote:

> On Thursday, November 08, 2012 21:10:55 Walter Bright wrote:
> > Many algorithms (at least the ones in Phobos do) already do a check
> > to ensure the inputs are the correct kind of range. I don't think
> > you'll get very far trying to use a range that isn't a range.
> > 
> > Of course, you can always still have bugs in your range
> > implementation.
> 
> Given that a range requires a very specific set of functions, I find
> it highly unlikely that anything which isn't a range will qualify as
> one. It's far more likely that you screw up and a range isn't the
> right kind of range because one of the functions wasn't quite right.
> 
> There is some danger in a type being incorrectly used with a function
> when that function requires and tests for only one function, or maybe
> when it requires two functions. But I would expect that as more is
> required by a template constraint, it very quickly becomes the case
> that there's no way that any type would ever pass it with similarly
> named functions that didn't do the same thing as what they were
> expected to do. It's just too unlikely that the exact same set of
> function names would be used for different things, especially as that
> list grows. And given that ranges are a core part of D's standard
> library, I don't think that there's much excuse for having a type
> that has the range functions but isn't supposed to be a range. So, I
> really don't see this as a problem.
> 

Looking at one set of interfaces in isolation, sure the chances might
be low. (Just like the chances of name collisions when hygeine is
lacking, and yet we thankfully have a real module system, instead of C's
clumsy "Well, it should usually work ok!" garbage.) But it's a terrible
precedent. Scale things up, use ducks as common practice, and all of a
sudden you're right back into the same old land of "no-hygeine". Bad,
sloppy, lazy precedent. AND the presumed benefit of the duckness is
minimal at best. Just not a good design, it makes all the wrong
tradeoffs.