On Tue, Mar 12, 2013 at 12:28:07PM -0400, Nick Sabalausky wrote:
> On Tue, 12 Mar 2013 16:02:24 +0100
> "TommiT" <tommitiss...@hotmail.com> wrote:
>
> > On Tuesday, 12 March 2013 at 13:44:35 UTC, Nick Sabalausky wrote:
> > > On Tue, 12 Mar 2013 12:51:03 +0100
> > > "TommiT" <tommitiss...@hotmail.com> wrote:
> > >
> > >> On Tuesday, 12 March 2013 at 02:39:06 UTC, TommiT wrote:
> > >> > struct S1 implements A2 {
> > >> > void foo() { }
> > >> > void bar() { }
> > >> > }
> > >>
> > >> That's not good. Types shouldn't have to explicitly say that
> > >> they implement a concept.
> > >
> > > I *strongly* disagree. A much as I love ranges (for example),
> > > their duckiness is the one thing I consider to be a huge mistake.
> >
> > The problem with having to explicitl specify that a type
> > implements a certain concept, is the resulting strong coupling
> > between the concept definition and the type. This prevents "happy
> > accidents" like the following from happening:
> >
> > Alice and Bob write libraries without knowing anything about each
> > other or each other's code. Alice implements the following in her
> > library:
> >
> > concept IntegerLike {
> > ...
> > }
> >
> > void foo(IntegerLike N)(N n) { }
> >
> > Bob implements the following in his library:
> >
> > struct SafeInt {
> > ...
> > }
> >
> > Later Bob realizes that Alice has written this cool function foo
> > which accepts his type SafeInt as an argument because SafeInt
> > just so happens to fulfill the requirements of the IntegerLike
> > concept defined in Alice's library.
> >
> > Although, the majority of concepts should come from the standard
> > library.
>
> "Happy accidents" is nothing more than another way of saying "Shit
> fucked up, but by pure dumb luck there was no damage". It's an
> absolutely *terrible* thing to encourage and design for. You may as
> well just go dynamic all the way, a la ActionScript 2 or Python - it's
> all the same "let random things happen by accident and blindly hope it
> just happens to turn out correct" philosophy.
>
> Design-by-accident is an anti-pattern.
>
> To me more specific, the problem with duck typing is that it falsely
> assumes that name+signature uniquely defines semantics (which is
> clearly not a valid assumption). Avoiding accidental screwups under
> duck typing *is* feasible if there's only a few well-known duck types
> that are ever in play (ex: our entire list of available duck types is
> a handful of phobos-defined ranges and basically nothing else). But it
> does not scale: The likelihood of accidental fuckups is multiplied
> with each additional duck type in existence, with non-stdlib duck
> types carrying a greater "accidental fuck up" weight.
I see it from another perspective: I've had to deal with proprietary
libraries that defined types that couldn't be used with a particular
container type, just because the container type expects the item type to
implement a particular interface, but it doesn't. But actually, it
*does* have the requisite members to implement that interface; it just
didn't *say* it implemented that interface. So there's the need for Yet
Another Useless Java-style Wrapper Class just to work around this
nonsense. Duck-typing solves this problem.
Of course, full-out ducktyping has its own problems, like you said; but
there needs to be a way of rewriting APIs such that you could say "type
T doesn't implement interface I right now, but actually, if you rewrite
T.A to T.X and T.B to T.Y, then T implements interface I just fine".
Though with D, I suspect this may actually be possible via alias this:
struct RewireInterface(T, Interface, Tuple!(string,string)... rewrites)
{
T t;
alias t this;
foreach (rewrite; rewrites) {
alias rewrite[0] = rewrite[1];
}
}
OK, maybe not. But the foreach could be replace with a suitable
recursive template so that the generated aliases are at the top-level in
RewireInterface. Probably some other hack is needed to work around the
need for Tuple in the compile-time parameters, which I'm pretty sure DMD
rejects right now. But assuming all this can be worked around, you could
do something like this:
struct StraitJacketedProprietaryItem {
int propX() { ... }
int propY() { ... }
}
concept MyInterface {
int myX();
int myY();
}
alias NonStraitJacketedItem = RewireInterface!(
StraitJacketedProprietaryItem, MyInterface,
"myX", "propX",
"myY", "propY"
);
assert(NonStraitJacketedItem implements MyInterface);
OK, lots of pseudo-code going on here, but you get the idea.
T
--
It is widely believed that reinventing the wheel is a waste of time; but I
disagree: without wheel reinventers, we would be still be stuck with wooden
horse-cart wheels.
