[swift-dev] SR-3423 Starter questions

Brent Royal-Gordon brent at architechies.com
Mon Nov 13 08:00:28 CST 2017

> On Nov 11, 2017, at 2:59 PM, Mohammed Ennabah via swift-dev <swift-dev at swift.org> wrote:
> I dug into the codebase as stated in the steps you mentioned, and tried to change one of the errors to see if it really change when I use Swift REPL, but nothing changed. Is it possible that I change something and directly affect the Swift compiler? (maybe I need to do a build first? Or maybe related to xcode-select?)

If you just type "swift" at the command line, you'll run the version of Swift built into Xcode. You will need to build Swift first ("utils/build-script" is the easiest way, but not the fastest), and you will also need to run the version of the Swift interpreter you just built. You can do that with a command like:


(The path will be a little different if you use the "-x" flag to build-script, which generates an Xcode project, albeit one that's a pain to use.)

> Each type of diagnostics has 4 parts, ERROR(ID,Options,Text,Signature). Diagnostics use the first 3 parts and pass parentheses to the signature. What is meant by the signature and why it’s used?

Error messages actually use a sort of format string language similar to printf(); for those messages which take parameters, the signature gives their types. For example, DiagnosticsParse.def includes this error:

	      "expected identifier in %0 declaration", (StringRef))

So code which emits that error must include a string to put in place of the "%0" token:

	// When we write "import" followed by a keyword that isn't "class", "struct", "protocol", etc.:
	diagnose(Tok, diag::expected_identifier_in_decl, "import");

	// When we try to parse an identifier after e.g. "typealias" and don't find one:
	P.diagnose(P.Tok, diag::expected_identifier_in_decl, DeclKindName);

	// When we try to parse an identifier after "case" and find some punctuation or a literal:
	diagnose(CaseLoc, diag::expected_identifier_in_decl, "enum 'case'");

Strings are the most common kind of parameter, but you can also pass other types and choose between different wordings based on them. For instance, here's a fix-it note I added for multiline string literals which takes a boolean to indicate singular or plural:

	      "change indentation of %select{this line|these lines}0 to match closing delimiter", (bool))

This error, which is shown when there's a non-digit in an integer literal, takes both a string containing the bad character, and a number (0 to 3) indicating which literal format it was parsing:

	      "'%0' is not a valid %select{binary digit (0 or 1)|octal digit (0-7)|"
	      "digit|hexadecimal digit (0-9, A-F)}1 in integer literal",
	      (StringRef, unsigned))

If you look around that file and the other Diagnostics*.def files, you'll see some other interesting examples. Have fun poking around!

Brent Royal-Gordon

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