Practical Aspects of Declarative Languages

This book constitutes the refereed proceedings of the 21^st International Conference on Practical Aspects of Declarative Languages, PADL 2019, held in Lisbon, Portugal, in January 2019. The 14 revised full papers were carefully reviewed and selected from 35 submissions. The papers present original work emphasizing novel applications and implementation techniques for all forms of declarative concepts, including logic, constraint, and functional languages.