Sat Jul 1 12:18:12 EDT 2017

Code is data because of right fold

Any recursive datastructure has a generalized right fold by making
constructors abstract.

This can remove the need for an explicit representation as a data
structure.  A fold is enough.

( More generally: data's only reason for existence is to be passed as
input to code.  The only thing that is important, really, is the code
that produces real-world effect. )