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Kawa: Compiling Scheme to Java

1.12 Translation

The translation phase takes a top-level form (or body), and generates a ModuleExp, which is a top-level expression. This is done using a Translator, which keeps track of lexical bindings and other translation state.

class Translator { public Expression rewrite (Object exp) { ... } public Expression syntaxError (String message) { ... } ... }

The rewrite method converts a Scheme source form to an Expression. The syntaxError method is called when a syntax error is seen. It prints out the current source filename and line number with the given message.

A ModuleExp represents a top-level form.

class ModuleExp extends LambdaExp { public Object eval_module (Environment env) { if (body_is_simple) // Optimization return body.eval (env); Object v = eval (env); return ((ModuleBody) v).run (env); } ... }

ModuleExp is a sub-class of LambdaExp, since it is actually a dummy function created by wrapping the top-level forms in an implicit lambda. The eval_module method evaluates the top-level forms. It invokes the eval in LambdaExp (which invokes the compiler). The result of eval is a ModuleBody (see section 1.8 Procedures), which we can run. If the body is "simple" we don't bother actually evaluating the ModuleExp, since that entails compiling it to a bytecoded class; we just eval the body instead.