March 23, 2008
I have a small difficulty with the (this T) template specification. If the function returns void - /great/!

    class C
    {
        void f(this T)() { ... }
    }

becomes equivalent to

    class C
    {
        void f() { ... }
        const void f() { ... }
        invariant void f() { ... }
    }

If the function returns another C, /also/ great!

    class C
    {
        T g(this T)() { ... }
    }

gets you

    class C
    {
        C g() { ... }
        const const(C) g() { ... }
        invariant invariant(C) g() { ... }
    }

The problem is, I don't grok how to return a type whose constancy depends on the constancy of this, but isn't actually T.

I've heard rumors of a TransferConst!() template, but I can't find it documented, and don't know how to use it. I have a horrible suspicion it's not documented at all, because I Googled "TransferConst!", and the only hits Google found were previous discussions of it in the newsgroup archives. It found none in the actual digitalmars website itself.

Without the ability to take the constancy of one thing and apply it to another, (this T) is really very limited.

The irony is that if we went with my (const K) idea, instead of (this
T), it would all be so much simpler! Instead of

    class C
    {
        void f(this T)() { ... }
    }

we'd write

    class C
    {
        K void f(const K)() { ... }
    }

which again would expand to

    class C
    {
        void f() { ... }
        const void f() { ... }
        invariant void f() { ... }
    }

and instead of

    class C
    {
        T g(this T)() { ... }
    }

we'd write

    class C
    {
        K K(C) g(const K)() { ... }
    }

which again would expand to

    class C
    {
        C g() { ... }
        const const(C) g() { ... }
        invariant invariant(C) g() { ... }
    }

But we'd also have an added bonus. Because we have K (a placeholder for one of the keywords "auto", "const" or "invariant") readily to hand, we can do all sorts of other cool stuff with it, without all that mucking about with TransferConst!(). Even ignoring the fact that TransferConst! is poorly documented, (const K) gives you readily comprehensible code.

Any chance we could ditch (this T) and TransferConst!() in favor of (const K)?
March 23, 2008

Janice Caron wrote:
> The problem is, I don't grok how to return a type whose constancy
> depends on the constancy of this, but isn't actually T.

template ThisConst(TConst, TBase)
{
   static if (is (TConst == const))
   {
      alias const(TBase) ThisConst;
   }
   else static if (is (TConst == invariant))
   {
      alias invariant(TBase) ThisConst;
   }
   else
   {
      alias TBase ThisConst;
   }
}

class C
{
   ThisConst!(T) foo(this T)() { ... }
}

Completely untested, since I don't use dmd2 anymore.

> But we'd also have an added bonus. Because we have K (a placeholder
> for one of the keywords "auto", "const" or "invariant") readily to
> hand, we can do all sorts of other cool stuff with it, without all
> that mucking about with TransferConst!(). Even ignoring the fact that
> TransferConst! is poorly documented, (const K) gives you readily
> comprehensible code.
> 
> Any chance we could ditch (this T) and TransferConst!() in favor of (const K)?

That is certainly a cleaner solution.
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