Invited Papers -- Answer Set Programming in 2010: A Personal Perspective -- An Introduction to Maude and Some of Its Applications -- Non-Monotonic Reasoning - Answer Set Programming -- Efficient Application of Answer Set Programming for Advanced Data Integration -- Implementing Query Answering for Hybrid MKNF Knowledge Bases -- An ASP-Based System for Team-Building in the Gioia-Tauro Seaport -- Types -- Explicitly Typed Exceptions for Haskell -- Conversion by Evaluation -- Parallelism and Distribution -- Skeleton Composition Using Remote Data -- Netlog, a Rule-Based Language for Distributed Programming -- Code Quality Assurance -- Similar Code Detection and Elimination for Erlang Programs -- Static Detection of Race Conditions in Erlang -- Automating Mathematical Program Transformations -- ActionScript In-Lined Reference Monitoring in Prolog -- Domain Specific Languages -- An Ode to Arrows -- Lazy Combinators for Executable Specifications of General Attribute Grammars -- A Domain-Specific Language Approach to Protocol Stack Implementation -- Programming Aids -- First-Order Interactive Programming -- An ER-Based Framework for Declarative Web Programming -- Constraints -- Lazy Explanations for Constraint Propagators -- On the Implementation of the CLP( ) Language -- Tabling - Agents -- Compact Lists for Tabled Evaluation -- A Simple and Efficient Implementation of Concurrent Local Tabling -- An Efficient Implementation of Linear Tabling Based on Dynamic Reordering of Alternatives -- Prospective Storytelling Agents.

This book constitutes the refereed proceedings of the 12th International Symposium on Practical Aspects of Declarative Languages, PADL 2010, held in Madrid, Spain, in January 2010, colocated with POPL 2010, the Symposium on Principles of Programming Languages. The 22 revised full papers presented together with 2 invited talks were carefully reviewed and selected from 58 submissions. The volume features original work emphasizing novel applications and implementation techniques for all forms of clarative concepts, including functions, relations, logic, and constraints. The papers address all current aspects of declarative programming; they are organized in topical sections on non-monotonic reasoning - answer set programming, types, parallelism and distribution, code quality assurance, domain specific languages, programming aids, constraints, and tabling - agents.