Programming Languages and Systems

This book constitutes the refereed proceedings of the 21st Asian SymposiumonProgramming Languages and Systems, APLAS 2023, held in Taipei, Taiwan,duringNovember 26-29, 2023. The 15 full papers included in this book are carefully reviewed andselected from32submissions. They were organized in topical sections as follows: semantics, logics, and foundational theory; design of languages, type systems, and foundational calculi; domain-specific languages; compilers, interpreters, and abstract machines; program derivation, synthesis, and transformation; program analysis, verification, and model-checking; logic, constraint, probabilistic, and quantum programming; software security; concurrency and parallelism; tools and environments for programming and implementation; and applications of SAT/SMT to programming and implementation.