This book constitutes revised selected papers from the 21st International Symposium on Trends in Functional Programming, TFP 2020, which was held in Krakow, Poland, during February 13-14, 2020. The 11 full papers presented in this volume were carefully reviewed and selected from 22 submissions.

Domain-specific languages.- PaSe: An Extensible and Inspectable DSL for Micro-Animations.- BinderAnn: Automated Rei cation of Source Annotations for Monadic EDSLs.- Generating next step hints for task oriented programs using symbolic execution.- Debugging and testing.- Scaling Up Delta Debugging of Type Errors.- Flexible Formality: Practical Experience with Agile Formal Methods.- White-Box Path Generation in Recursive Programs.- Reasoning and effects.- A Proof Assistant Based Formalisation of a Subset of Sequential Core Erlang.- One-shot Algebraic Effects as Coroutines.- An equational modeling of asynchronous concurrent programming.- State will do.- Parallelism.- Placement Strategies: Structured Skeleton Composition with Location-Aware Remote Data.

This book constitutes revised selected papers from the 21st International Symposium on Trends in Functional Programming, TFP 2020, which was held in Krakow, Poland, during February 13-14, 2020.The 11 full papers presented in this volume were carefully reviewed and selected from 22 submissions. They were organized in topical sections named: domain-specific languages; debugging and testing; reasoning and effects; and parallelism.The following chapters are available open access under a Creative Commons Attribution 4.0 International License via link.springer.com:Chapter 3: Generating next step hints for task oriented programs using symbolic execution by Nico Naus and Tim Steenvoorden, and Chapter 5: Flexible Formality Practical Experience with Agile Formal Methods by Philipp Kant et al