The abstract Expression class represents partially processed expressions. These are in principle independent of the source language, though there are still some Scheme assumptions wired in.

class Expression
{ ...;
  public abstract Object eval
    (Environment e);
  public abstract void compile
    (Compilation comp, Target targ);

The eval method evaluates the Expression in the given Environment. The compile method is called when we are compiling the body of a procedure. It is responsible for generating bytecodes that evaluate the expression, and leave the result in a result specified by the Target parameter. This is usually the Java evaluation stack, but we will go into more detail later.

class QuoteExp extends Expression
{ ...;
  Object value;
  public QuoteExp(Object val)
  { value = val; }
  public Object eval(Environment env)
  { return value; }
  public void compile
    (Compilation comp, Target target)
  { comp.compileConstant (value, target); }

A QuoteExp represents a literal (self-evaluating form), or a quoted form.

class ReferenceExp extends Expression
{ ...;
  Symbol symbol;
  Declaration binding;

A ReferenceExp is a reference to a named variable. The symbol is the source form identifier. If binding is non-null, it is the lexical binding of the identifier.

class ApplyExp extends Expression
{ ...;
  Expression func;
  Expression[] args;

An ApplyExp is an application of a procedure func to an argument list args.

class ScopeExp extends Expression
{ ...;
  ScopeExp outer;  // Surrounding scope.
  public Declaration add_decl(Symbol name)
  { ...Create new local variable... }

A ScopeExp is a abstract class that represents a lexical scoping construct. Concrete sub-classes are LetExp (used for a let binding form) and LambdaExp.

class LambdaExp extends ScopeExp
{ ...;
  Symbol name; // Optional.
  Expression body;
  int min_args;
  int max_args;

The Scheme primitive syntax lambda is translated into a LambdaExp, which represents anonymous procedures. Each LambdaExp is compiled into a different bytecoded class. Invoking eval causes the LambdaExp to be compiled into a class, the class to be loaded, an instance of the class to be created, and the result coerced to a Procedure.

Other sub-classes of Expression are IfExp (used for conditional expressions); BeginExp (used for compound expressions); SetExp (used for assignments); and ErrorExp (used where a syntax error was found);