On 2/16/12 6:49 PM, SiegeLord wrote:
On Thursday, 16 February 2012 at 23:14:54 UTC, Andrei Alexandrescu wrote:
Hold them horses. I disagree. You're just saying it, but what's your
basis?
Because some cases (as shown below) trivially work within the const
system, while some closely related ones don't. You're not going to be
able to convince me of a demarcation that requires some const issues to
require templates, and some not.
Some cases also work trivially with subtyping, while some closely
related ones don't.
This boils down to: "You want to sort an array of T[], U[], or V[],
where the three types are loosely-related, except U is a supertype of
both T and V and the three have the same layout. What would be the
signature of such a function?"
The answer is (to a reasonable approximation) simple:
sort(X)(X[] data) if (is(X : U) && X.sizeof == U.sizeof);
This has nothing to do with qualifiers.
Because you removed them. While I agree with the type argument to a
point, qualifiers have more meaning than just arbitrary type creation:
they talk about mutability.
That's a given. But that doesn't confer them infinite powers otherwise
inaccessible; you seem to require any flexibility that seems reasonable
within a context, and that's simply put impossible. There is a point
where inout's powers stop (inout can be considered a special case
designed for a few common cases).
I should confess that the subtyping relation (const(T) is a supertype of
both T and immutable(T)) has from day 1 been a guiding design principle
for us, so that shouldn't be taken lightly. There is a relation between
types that is somewhere in between simple subtyping and qualified types:
subtype with layout conservation, i.e. you know that T is a supertype of
U and both T and U have identical layout. For such types we could
accommodate special capabilities in the type system; they'd be
applicable beyond qualified types.
The desired function signature states that
the contents of the strings that are in the array will not be modified,
your generic version does not have that stipulation.
I can make up a
body for that sort function which will work for types which fit your
description, but fail for const(char)[], char[] and immutable(char)[].
I think it's well worth trying this exercise. Given
class Base {}
class D1 : Base {}
class D2 : Base {}
define a non-template function that sorts Base[], D1[] and D2[] without
casts.
I don't know how to swim with a hand tied to my back, either. The
correct approach is to integrate templates in the discussion and
analyze _that_ context, not the artificial context that precludes
templates. D is not Go.
As much as you might prefer D to be 100% about templates,
(let's stay on topic and not make this ad hominem, thanks)
it is not, and
there is a subset of it which is usable without them.
This calls for the obvious answer that there's a subset of D that's
usable without const.
This subset is the
subject of this thread. There is no a priori reason why the first case
should work and second should not within the confines of the const system.
I understand your complaint, but I don't know how to design a type
system that is at the same time reasonably small and simple and allows
all of your examples and some related ones. We have "inout" which is
helpful but at the end of the day a special case for a category of
situations. We can't expect it to do miracles.
Andrei
