[swift-dev] SR-3423 Starter questions

[Mohammed Ennabah]

> On Nov 13, 2017, at 4:00 PM, Brent Royal-Gordon <brent at architechies.com> wrote:
> 
>> On Nov 11, 2017, at 2:59 PM, Mohammed Ennabah via swift-dev <swift-dev at swift.org <mailto: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:
> 
> 	../build/Ninja-DebugAssert/swift-macosx-x86_64/bin/swift
> 
> (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:
> 
> 	ERROR(expected_identifier_in_decl,none,
> 	      "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:
> 
> 	NOTE(lex_multiline_string_indent_change_line,none,
> 	      "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:
> 
> 	ERROR(lex_invalid_digit_in_int_literal,none,
> 	      "'%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!
> 

Good work. I believe that these are pretty good examples to understand how the Swift diagnosis work in practice.

> -- 
> Brent Royal-Gordon
> Architechies
> 

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