Programming Languages for Scalable Software Extension and Composition

class Exp { Type type; abstract Exp accept(Visitor v); } class Abs extends Exp { String x; Exp e; // λx.e Exp accept(Visitor v) { e = e.accept(v); return v.visitAbs(this); } } class Visitor { Exp visitAbs(Abs a) { return a; } } class TypeChecker extends Visitor { Exp visitAbs(Abs a) { ... } } class Emitter extends Visitor { Exp visitAbs(Abs a) { print(...); return a; } } class Compiler { void main() { ... } Exp parse() { ... } } Figure 3.2: Lambda calculus compiler