Let's define a mapping F to act as a "signature-step" of an algebraic data type with operations op as a map (functor) from X to Sum_op X^arity(op). This involv

Let's define a mapping F to act as a "signature-step" of an algebraic data type with operations op as a map (functor) from X to Sum_op X^arity(op). This involves a sum of products (or coproduct of products). See "Recursive types for free" https://homepages.inf.ed.ac.uk/wadler/papers/free-rectypes/free-rectypes.txt https://homepages.inf.ed.ac.uk/wadler/papers/free-rectypes/f...