This book constitutes the thoroughly refereed post-proceedings of the 21st International Workshop on Implementation and Applications of Functional Languages, IFL 2000, held in South Orange, NJ, USA, in September 2009. The 13 revised full papers presented were carefully reviewed and were selected from numerous submissions. The IFL symposia bring together researchers and practitioners that are actively engaged in the implementation and the use of functional and function based programming languages. Every year IFL provides a venue for the presentation and discussion of new ideas and concepts, of work in progress, and of publication-ripe results.