const debacle

[Janice Caron]

On 26/03/2008, Davidson Corry <davidsoncorry at comcasst.net> wrote:
>  idem(x) foo(const T x, const U y);      // first promise
>  idem(y) bar(T x const, U y const);      // second promise
>
>  Does this make any sense? Or have I said something silly, or resurrected
>  an old and discarded idea, or otherwise taken a pratfall on my maiden sally?

It makes sense, and you're spot on. I think the problem is well
understood now. All we're lacking a good syntax to recommend. And it
seems to me that no one's come up with anything better than Walter's

    U bar(const(T) x, return const(U) y)

although actually I still think it should be

    const(U) bar(const(T) x, return const(U) y)

because inside the function body, every return statement is going to
be returning a const(U), not a U.

Your "bar" example doesn't make complete sense to me. If the function
can only return a U (as indicated by your idem(y) - which incidently
is similar to my earlier sliceof(y) suggestion), then why does x also
have the "returnable" parameter declaration syntax? Was that a typo or
am I missing something?

Walter's syntax is not as expressive as we might like, but it does
have the benefit of being simple. By contrast, making a distinction
between const-on-the-left and const-on-the-right is potentially
confusing - and also, no more expressive that Walter's idea, so
there's nothing to be gained by it.

For what it's worth, I have a new idea, largely inspired by Stephen,
which is to allow the keyword inout as a type constructor. inout would
basically make promise 2.

    inout(U) bar(const(T) x, inout(U) y)

I think it's fully expressive, and not too confusing, but on the down
side, it would mean that we'd then have three types of constancy
instead of just two