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GNU Smalltalk User's Guide

2.7.2 The Smalltalk-in-Smalltalk compiler

The Smalltalk-in-Smalltalk compiler is a nice compiler for Smalltalk code which is written in Smalltalk itself. Ideally, the C compiler would only serve to bootstrap the system, then a fully working Smalltalk compiler would start compiling methods.

The current status of the Smalltalk-in-Smalltalk compiler can be summarized thus: it does work, but it does not work well. This for many reasons: first of all it is slow (10-15 times slower than the C compiler), and it does not produce very optimized code. Anyway it is a good example of programming the GNU Smalltalk system, and its source code (found in the `compiler' directory) provides good insights into the Smalltalk virtual machine: so, after all, it is not that bad. If want to give it a try, just file in the Compiler package.

The compiler is built on a recursive descent parser which creates parse nodes in the form of instances of subclasses of STParseNode. Then the parser instantiates a compiler object which creates the actual method; more information on the inner workings of the compiler can be found in the comment for the STCompiler(6) class.

The parser's extreme flexibility can be exploited in three ways, all of which are demonstrated by source code available in the distribution:

• First, actions are not hard-coded in the parser itself: the parser creates a parse tree, then hands it to methods in STParser that can be overridden in different STParser subclasses. This is done by the compiler itself, in which a subclass of STParser (class STFileInParser) hands the parse trees to the STCompiler class.

  To build the documentation for the various class libraries, also, Smalltalk source code is interpreted and objects for the classes and methods being read in are created; then, polymorphism allows one to treat these exactly like usual classes. This machinery is contained in two files, `compiler/STLoader.st' (the parser) and `compiler/STLoaderObjs.st' (the classes that implement embryonic classes and namespaces read from the file).

• Second, an implementation of the "visitor" pattern is provided to help in dealing with parse trees created along the way; this approach is demonstrated by the Smalltalk code pretty-printer in class STFormatter.

• Third, just like all recursive descent parsers, it is pretty easy to figure out which part of the stream a method takes care of parsing, and you can override the parsing methods in a subclass to do "something interesting" even while a parse tree is created. This is demonstrated by the syntax highlighting engine included with the browser, implemented by the STPluggableParser and BCode classes.