[dmd-internals] Requesting some DMD hacking advice for property lowering.

[Don Clugston]

Hi Chad, I've only just found this email. Some comments below.

On 28 June 2010 01:56, Chad Joan <chadjoan at gmail.com> wrote:
> Alright it's been a number of months since I've dabbled in D, but my
> spare time is slowly increasing... for now.  Fingers crossed.
>
> So I'm going to attack this property lowering thing again.
>
> Here is a reminder of what I'm talking about:
> http://prowiki.org/wiki4d/wiki.cgi?DocComments/Property
>
> When I did this first I got 95% of the way there and hit a corner-case
> in DMD's code.  This chicken-and-egg problem is what happened:
>
> (1) e->semantic(sc) is called.  (e is an Expression)
> -   This will call e's children's semantic.
>
> (2) resolveProperties must come after e->semantic(sc).
> -   It needs e->semantic to get the type info and
> -   to know if e contains properties (recursively).
>
> (3) If e has side effects and the mutated value(s) contain properties,
> then resolveProperties turns the mutated value(s) into multiple
> expressions within a comma expression such that no properties are in a
> side-effectful expression.
>
> (4) The resulting CommaExp's (and other generated Exps) must have
> semantic(sc) run on them to create type information.
> -   This will call e's children's semantic(sc), again.
>
> (5) semantic(sc) must not be run on an Expression more than once.
> -   This is why my 95%-of-the-way-there code would work perfectly most
> of the time and then fail in very rare instances.
>
> 4 and 5 contradict each other.
>
> I'm wondering if there is some canonically accepted way of dealing with
> this kind of stuff in DMD.  I'd hate to write this and have it go to
> waste because I missed an idiom or coding convention.
>
> I've considered these possibilities:
>
> - I could guard the execution of semantic to make sure it is never done
> more than once:
>
> Expression *FooExp::semantic( Scope *sc )
> {
>     if ( semanticAlreadyCalledOnThis )
>         return this;
>
>     ... etc
> }

FuncDeclaration has semanticRun which basically does that.

BTW, I have found that CommaExp often needs a special case. Maybe it
just needs a non-recursive semantic().
Personally I have almost never made intrusive patches. (My opDollar
patch is the only one I can think of, and it still isn't included in
the compiler ).  If you can't do it without an intrusive change, it
might be better to just get it to 95% complete, with a clear
explanation of what else needs to be done.

- Don.