On 2/16/12 3:48 PM, SiegeLord wrote:
Firstly, let me preface this... if you use templates to get around the
const system's imperfections, you are admitting that the const system is
broken. Now, on with the program.
Hold them horses. I disagree. You're just saying it, but what's your basis?
My unique experience in using D2 without Phobos lead me to encounter two
cases that show how the D2 const system is just a pain in the behind for
some really reasonable tasks.
First case:
You want to sort an array of strings using a function. Strings can be
all of these types: char[], const(char)[] and immutable(char)[]. What
would be the signature of such a function?
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. Qualified types are distinct,
and obey the classic subtyping and layout rules known since the dawn of
humankind: const is a supertype of mutable and immutable, and they all
have the same layout. Complaining about that equates to complaining
about subtyping.
It can't be sort(const(char)[][]) because it's unsafe to case char[][]
and immutable(char)[][] to that argument type (see
http://d.puremagic.com/issues/show_bug.cgi?id=4251 ).
It can't be sort(const(char[])[]) because you can't actually sort that!
The only good way I found is to use a cast inside the function with the
second signature. Obviously I'm glad there's a workabout, but surely a
cast isn't a good thing.
You'd do the same if you wanted to sort arrays of base and arrays of
derived with the same routine.
Second case:
inout was meant to solve issues with functions that return slices of
inputs. What about a class that is dedicated to the same functionality?
E.g. this works fine:
inout(char)[] half(inout(char)[]);
But what about this:
struct Slicer
{
char[] a;
char[] half();
}
Note that the type of the input (the member 'a') must be the same as the
output of the half method. I don't know how to accomplish this without
templates.
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.
But as I said in the preface, you shouldn't need templates
for such a simple task.
char and const char are different types. The embedded presupposition is
that they are somewhat similar, the qualifier being some sort of
attribute of the type. That's not the case.
Note that doing this isn't satisfactory:
struct Slicer
{
char[] a;
inout(char)[] half() inout;
}
because there may be other members inside that struct that may need to
remain mutable.
Agreed.
This is very relevant, incidentally, for the planned library
implementation of associative arrays. How would this be implemented when
an associative array is a struct?
inout(char)[] test(inout(char)[])
{
inout(char)[][int] a;
}
It doesn't even compile now (in dmd 2.058).
Associative arrays must be templates.
I don't have any solutions to these problems, incidentally... I think
they are complex, but definitely worthy of having a reasonable solution
that doesn't involve needless (in this case) templates.
Again, we must make templates a part of the setup and discuss what's
going on.
Andrei
