You'd likely be more interested in the original paper [0] by John Hughes that Wadler built off of. It gets into a ton more detail about the algebraic structure of the documents they're defining:
> The question addressed in this paper is: how should libraries of combinators be designed? Our goal is to show how we can use formal specification of the combinators, and a study of their algebraic properties, to guide both the design and implementation of a combinator library. Our case study is a library for pretty-printing. But the methods we present are of wider applicability, and we also show how they can be used to develop a number of different monads.
Wadler's paper touches upon some of the same ideas, but the primary goal of his paper is to build a prettier printer, whereas Hughes aimed to demonstrate the power of algebraic properties.
> The question addressed in this paper is: how should libraries of combinators be designed? Our goal is to show how we can use formal specification of the combinators, and a study of their algebraic properties, to guide both the design and implementation of a combinator library. Our case study is a library for pretty-printing. But the methods we present are of wider applicability, and we also show how they can be used to develop a number of different monads.
Wadler's paper touches upon some of the same ideas, but the primary goal of his paper is to build a prettier printer, whereas Hughes aimed to demonstrate the power of algebraic properties.
[0] http://www.cse.chalmers.se/~rjmh/Papers/pretty.html