This volume contains the selected peer-reviewed revised articles that were p- st sented at the 21 International Symposium on Implementation and Application of Functional Languages (IFL 2009). IFL 2009 was held September 23-25, 2009 at Seton Hall University in South Orange, NJ, USA. This version of the IFL symposium marked a milestone by being the ?rst ever to be held in the United States. Ourgoalsareto makeIFLaregulareventheldintheUSAandinEurope byalternatingthehostcontinenteveryyearandtofostercollaborations,inter- tions, and friendships between researchers and practitioners on both continents and beyond. The IFL symposia bring together researchers and practitioners that are actively engaged in the implementation and the use of functional and functi- based programming languages. Every year IFL provides a venue for the p- sentation and discussion of new ideas and concepts, of work in progress, and of publication-riperesults. Participantsareinvitedtosubmiteitheradraftpaperor an extended abstract describing work to be presented at the symposium. These submissions are screened by the Program Committee Chair to make sure they are within the scope of IFL. The submissions accepted for presentation appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. After the sym- sium, authors are given the opportunity to consider the feedback received from discussions at the symposium and are invited to submit revised, full articles to the formalreviewprocess. Therevisedsubmissionsarereviewedbythe Program Committee using prevailing academic standards, and the best submissions are chosento appearin theformalproceedings. Thisvolumeisthe resultofthework done by the IFL 2009 Program Committee and the contributing authors.
