[phobos] is*Range + Qualified Types

[David Simcha]

Ok, now I get why using Unqual to get tail const wouldn't work.  My silly
oversight:

// The following doesn't work, though the natural tail
// const for const(Cycle!(int[])) is Cycle!(const(int)[]),
// because Unqual!(const(Cycle!(int[]))) == Cycle!(int[]).
import std.range, std.traits;

void main() {
    const myRange = cycle([1,2,3,4]);
    pragma(msg, typeof(myRange));

    Unqual!(typeof(myRange)) myRangeMutable = myRange;
    pragma(msg, typeof(myRange));
}


On Thu, Aug 12, 2010 at 11:37 AM, Steve Schveighoffer
<schveiguy at yahoo.com>wrote:

> Tail-const is doable, but does not enjoy the implicit conversion that T[]
> has.
>
> Without the implicit conversion, tail-const relies on unsafe casting, and
> that
> sucks.
>
> I agree with Andrei we need a language solution.  The question really is
> not
> what the solution should do, that much is clear -- apply const to a subset
> of
> the members (either all references or members designated by some
> identifier).
> The question is, what does the syntax look like.  That was the major
> stumbling
> block that flipped Walter's switch to "tail-const doesn't work".
>
> I think we should concentrate on structs and not class references, since
> class
> references have no syntax that separates the reference from the data.  At
> least
> with structs, you can identify the parts to apply const to.  We have a
> somewhat
> clunky solution in Rebindable for classes, so it would be nice to include
> them,
> but past experience has shown that to be a big rat hole.
>
> -Steve
>
> >
> >From: David Simcha <dsimcha at gmail.com>
> >To: Discuss the phobos library for D <phobos at puremagic.com>
> >Sent: Thu, August 12, 2010 11:28:24 AM
> >Subject: Re: [phobos] is*Range + Qualified Types
> >
> >
> >
> >
> >On Thu, Aug 12, 2010 at 1:43 AM, Andrei Alexandrescu <andrei at erdani.com>
> wrote:
> >
> >
> >>
> I think the main argument is that currently most of std.algorithm doesn't
> work
> with const arrays, which have a simple "tail-const" version. const(T[]) is
> implicitly convertible to const(T)[].
> >>
> >>That doesn't apply to most other ranges, which don't have an obvious
> >>"tail-const" version.
> >>
> >>David, I think we need to think through a bit more before proceeding. The
> way I
> >
> >>assume you want to proceed is to essentially add a special function
> signature
> >>for each algorithm and have it forward to the peeled version. Perhaps we
> could
>
> >>look at a simpler solution, e.g. one that would involve a language
> change.
> >>
>
> Fair enough.  If you think this might be better solved at the language
> level,
> that's a worthwhile discussion to have.  I do believe, though, that most
> ranges
> besides T[] do have an obvious "tail-const" version, since in practice most
> ranges are structs that have the iteration state stored inline and only use
> indirection to store the payload, if anywhere.
>
> At any rate, I think this is a must-solve problem.  Despite its theoretical
> beauty, I find D's const/immutable system to be utterly useless (and I've
> made a
>
> serious attempt to use it in a real multithreaded program) for all but the
> simplest cases in practice, for three reasons:
>
> 1.   It's too hard to create non-trivial immutable data structures,
> especially
> without relying on unchecked casts during construction.
>
> 2.  So many things in Phobos (even things as simple as std.math.pow()
> before I
> recently fixed it) behave incorrectly when given const/immutable data.
>  This
> also applies to other libraries I use, including ones that I'm the main
> author
> of, so I'm just as guilty of it as anyone.  Given that noone, including me,
> seems to be able to get this right in generic code, perhaps this does point
> to
> the need for a language-level solution.
>
> 3.  inout is currently so bug-ridden it's not even funny.
>
>
>
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