> On Dec 8, 2015, at 2:43 AM, Brent Royal-Gordon via swift-dev > <swift-dev@swift.org> wrote: > > My first question is simply this: is this the best place for the sort of > question I’m about to ask? Is there an IRC channel or Slack or something > where people who are working with Swift’s compiler internals are hanging out? > Are people on Twitter (hi, Joe Groff!) going to mind if I pester them with > random weird questions about compiler guts?
This is definitely the best place to ask implementation questions. 140 characters isn't great for deep dives on compiler internals. -Joe > With the idea in mind that the most important thing I want to find out is > “where do I ask things like this?”, here goes. > > ***** > > Based on this post > <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20151130/000518.html>, > I’m trying to make memberwise initializers give default values to parameters > which have are initialized in their declaration. This is basically just a > quick prototype to (a) learn about how the Swift compiler works, and (b) try > to figure out what the implementation issues are going to be like. (For > instance, I’ve already noticed that if two properties are declared in the > same tuple, it may be difficult to grab their default values.) > > My initial approach—which I’m well aware is probably all wrong—is to grab the > Expr from the property’s pattern binding and then attach it to the parameter > tuple as a default argument. I didn’t see any obvious way to clone an Expr, > so I’m basically just using the same instance and hoping for the best. > > --- a/lib/Sema/CodeSynthesis.cpp > +++ b/lib/Sema/CodeSynthesis.cpp > @@ -1952,13 +1952,27 @@ ConstructorDecl > *swift::createImplicitConstructor(TypeChecker &tc, > auto *arg = new (context) ParamDecl(/*IsLet*/true, Loc, var->getName(), > Loc, var->getName(), varType, decl); > arg->setImplicit(); > + > + auto initKind = DefaultArgumentKind::None; > + ExprHandle * initExpr = nullptr; > + > + // Is this property's default simple enough to copy? > + auto *varPatternBinding = var->getParentPatternBinding(); > + if (varPatternBinding && varPatternBinding->getNumPatternEntries() == > 1) { > + auto * init = varPatternBinding->getInit(0); > + if(init) { > + initExpr = ExprHandle::get(context, init); > + initKind = DefaultArgumentKind::Normal; > + } > + } > + > argNames.push_back(var->getName()); > Pattern *pattern = new (context) NamedPattern(arg); > pattern->setImplicit(); > TypeLoc tyLoc = TypeLoc::withoutLoc(varType); > pattern = new (context) TypedPattern(pattern, tyLoc); > patternElts.push_back(TuplePatternElt(var->getName(), SourceLoc(), > - pattern, false)); > + pattern, false, SourceLoc(), > initExpr, initKind)); > } > } > > This sort of works in that -dump-ast and -print-ast look right, but when I > let the rest of the compiler run, I get a crash half a dozen calls down from > SILGenModule::emitDefaultArgGenerator(). Apparently initExpr’s type is null, > causing an earth-shattering kaboom. > > My working theory is that, because createImplicitConstructor() is called > during type checking, some part of the type check, or some earlier pass in > the compiler, is not being performed which would normally infer the types of > the default values. But I’m really flying blind here, so it could very well > be that I’m abusing the AST in some horrible way or missing some step I > should obviously be performing. (I did notice, for instance, that the doc > comment on ExprHandle is vaguely gesturing at the idea that an expression > might be connected to the same AST in two different places.) > > So what I’d like to know is: > > 1) Again, what’s the best venue for these sorts of “I’ve just stumbled into a > maze of twisty little passages" questions? > 2) Is there any documentation on the AST design in general, or the > implementation of TuplePattern and default values in particular, that can > help me figure out this part of the code? > 3) Does anyone recognize what might be happening here? > > Thanks, > -- > Brent Royal-Gordon > Architechies > > _______________________________________________ > swift-dev mailing list > swift-dev@swift.org > https://lists.swift.org/mailman/listinfo/swift-dev _______________________________________________ swift-dev mailing list swift-dev@swift.org https://lists.swift.org/mailman/listinfo/swift-dev
