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Kawa: Compiling Scheme to Java

1.13 Syntax and Macros

class Syntax { public abstract Expression rewrite (Object obj, Translator tr); ... }

The rewrite method in Translator checks for syntactic keywords and macros. If the car of a "call" is a Syntax or if it is a Symbol that is bound to a Syntax, then its rewrite method is called.

As an example, this trivial class implements quote.

class quote extends Syntax { public Expression rewrite (Object obj, Translator tr) { // Error-checking is left out. Object value = ((Pair)obj).car; return new QuoteExp(value); } ... }

Much more complicated is the Syntax that implements define-syntax.

class define_syntax extends Syntax { public Expression rewrite (Object obj, Translator tr) { enter (new SyntaxRules (...)); } ... }

The result is a SyntaxRules object, which contains an encoded representation of the patterns and templates in the syntax-rules. This is in its own right a Syntax object.

class SyntaxRules extends Syntax { SyntaxRule[] rules; public Expression rewrite (Object obj, Translator tr) { Object[] v = new Object[maxVars]; for (int i = 0; i < rules.length;) { SyntaxRule r = rules[i++]; if (r.match (obj, v)) return r.execute_template(v, tr); } return tr.syntaxError ("no matching syntax-rule"); } ... }

Contrast evaluating a procedure definition (lambda), which causes a new sub-class of Procedure to be created and compiled, while evaluating a define-syntax only causes a new instance of SyntaxRules to be created. This is because the syntax-rules can be represented using relatively simple and compact data structures. A traditional low-level macro facility specifies the transformations using executable code, and that probably would need a new Procedure sub-class.