This paper describes a generic method to derive an animation from a recursive definition, with the objective of debugging and understanding this definition by expliciting its control structure. This method is based on a well known algorithm of factorizing a recursive function into the composition of the producer and the consumer of its call tree. We developed a systematic method to transform both the resulting functions in order to draw the tree step by step. The theory of data types as fixed points of functors, generic recursion patterns, and monads, are fundamental to our work and are brie y presented. Using polytypic implementations of monadic recursion patterns and an application to manipulate and generate graph layouts we developed a prototype that, given a recursive function written in a subset of Haskell, returns a function whose execution yields the desired animation
