Decomposing recursions using algebras

Analogously to the decomposition of a datatype into a functor (which can be built from other functors to assemble a bigger language from smaller pieces of languages) and a single expression datatype with a sole constructor that builds an Expr from an F Expr (where F is the functor) -- analogously, a recursion can be decomposed into algebras and a fold function that applies the algebra throughout the datatype.