Invited Talk 1 -- Type Processing by Constraint Reasoning -- Session 1 -- Principal Type Inference for GHC-Style Multi-parameter Type Classes -- Private Row Types: Abstracting the Unnamed -- Type and Effect System for Multi-staged Exceptions -- Session 2 -- Relational Reasoning for Recursive Types and References -- Proof Abstraction for Imperative Languages -- Reading, Writing and Relations -- Session 3 -- A Fine-Grained Join Point Model for More Reusable Aspects -- Automatic Testing of Higher Order Functions -- Invited Talk 2 -- Event Driven Software Quality -- Session 4 -- Widening Polyhedra with Landmarks -- Comparing Completeness Properties of Static Analyses and Their Logics -- Polymorphism, Subtyping, Whole Program Analysis and Accurate Data Types in Usage Analysis -- Session 5 -- A Modal Language for the Safety of Mobile Values -- An Analysis for Proving Temporal Properties of Biological Systems -- Computational Secrecy by Typing for the Pi Calculus -- Invited Tutorial -- Scheme with Classes, Mixins, and Traits -- Session 6 -- Using Metadata Transformations to Integrate Class Extensions in an Existing Class Hierarchy -- Combining Offline and Online Optimizations: Register Allocation and Method Inlining -- A Localized Tracing Scheme Applied to Garbage Collection -- Session 7 -- A Pushdown Machine for Recursive XML Processing -- XML Validation for Context-Free Grammars -- A Practical String Analyzer by the Widening Approach -- Session 8 -- A Bytecode Logic for JML and Types -- On Jones-Optimal Specializers: A Case Study Using Unmix.

This book constitutes the refereed proceedings of the 4th Asian Symposium on Programming Languages and Systems, APLAS 2006, held in Sydney, Australia in November 2006. The 22 revised full papers presented together with 2 invited talks and 1 tutorial were carefully reviewed and selected from 70 submissions. The symposium is devoted to both foundational and practical issues in programming languages and systems. Among the topics covered are semantics, logics, type theory, language design, program analysis, optimization and transformation, software security, verification, compiler systems, interpreters, abstract machines, domain-specific languages and systems, as well as programming tools and environments.