I speak of the property rewriting where an expression like
foo.prop++;
is rewritten as
auto t = foo.prop();
t++;
foo.prop(t);
So, Walter or Andrei or someone on the planning behind the scenes,
please lend me your thoughts:
How much time is left to make this sort of thing happen?
If a working patch for this showed up, would it have a reasonable chance
of acceptance at this point?
I really want to make this happen, even if I have to pay someone to do
it or finish it off. It's very close but I have almost nil free time
for this stuff.
Note that I have made it work and seen it in action. There'd be a patch
two months ago if I hadn't decided to rebel against the way DMD did things*.
Now I'll try and remind you why this patch is important:
- I see no other way to make properties behave like lvalues in an
elegant manner. Explicit property syntax does not help this. This is
quite a semantics problem, and the syntax is completely orthogonal.
- Having property rewrites allows the special case for "array.length +=
foo;" to be removed. Property rewriting is the more general solution
that will work for all properties and in arbitrary expressions.
- By treating opIndex and opIndexAssign as properties then that pair
alone will make cases like "a[i]++;" work correctly without the need for
opIndexUnary overloads. Also "a[i] += foo" will work too, as well as
anything else you haven't thought of yet.
- It has implications for purity. We may want to figure this out before
its too late. Consider "foo.prop + bar;", where prop returns a struct
with opBinary!("+") defined. It ends up being "foo.prop.opAdd(bar)".
In general this is "foo.property.someFunction(args...)". In the general
case you want it to call the setter function for the property so that
any changes in state get propagated back to foo, as we would expect from
an lvalue. If there is no setter, it's an error. However, in the
(usual) case of opAdd, this is an unnecessary restriction since opAdd is
likely pure and just returns a value -- there is no need to require foo
to have a setter.
It makes sense to me to allow the purity of member functions to
determine whether or not any parent property needs its setter called.
Alternatively we could just make member function calls not count as side
effects for properties. That would be more dangerous though, because
sometimes they ARE side effects. Things like += and -= are not as
troubling because they are ALWAYS side effects, so we know what to do
with them. I want +=, -=, ++, etc. to be covered even if this purity
issue is completely ignored.
* So I made it work. Just not well enough for production quality,
unfortunately. The problem had nothing to do with the rewrite being
difficult or impossible to do. Instead, the problem was me rebelling
against the way DMD does things, followed by failing on rare instances
of valid code. Calling Expression->semantic() twice on the same
expression will do that.** This was all a little over two months ago,
back when I was still unemployed. I've hardly touched it since then.
** If you're in the know, you might wonder why I would do a fool thing
like call Expression->semantic() twice on the same expression. When I
originally wrote the function for doing this rewrite, I wrote it to
climb the expression tree with recursive calls. So I tried to keep that
quality. I'd let semantic run, then I'd recursive over the tree, then
run semantic again. I had an idea in mind to optimize away the second
pass for expressions devoid of properties, or to even minimize the
scanning to only the branches that had both side effects and properties.
I was hoping this would at least make it work. But it doesn't.
What I need to do then is rewrite my recursive function for doing the
rewrite into a flat and non-recursive function that tracks its state
using some combination of variables in Scope and variables in
Expression. I'd be done already if I had just written it that way in
the first place.
