He mentions in the preface that the reference programs are in Haskell. The formatting is LaTeX so it looks a bit odd.
I did some more digging and it turns out that the Mathematical notation used is called "Z Notation" after the Zermelo–Fraenkel set theory notation. Here is a guide, PDF is the second link in the list. The notation is part of a broader type of notation called Pidgin Code:
In computer programming, pidgin code is a mixture of several
programming languages in the same program, or pseudocode that is a
mixture of a programming language with natural language descriptions.
Hence the name: the mixture is a programming language analogous to a
pidgin in natural languages.
In numerical computation, mathematical
style pseudocode is sometimes called pidgin code, for example pidgin
ALGOL (the origin of the concept), pidgin Fortran, pidgin BASIC,
pidgin Pascal, and pidgin C. It is a compact and often informal
notation that blends syntax taken from a conventional programming
language with mathematical notation, typically using set theory and
matrix operations, and perhaps also natural language descriptions.
It can be understood by a wide range of mathematically trained people,
and is used as a way to describe algorithms where the control
structure is made explicit at a rather high level of detail, while
some data structures are still left at an abstract level, independent
of any specific programming language.
Normally non-ASCII typesetting
is used for the mathematical equations, for example by means of TeX or
MathML markup, or proprietary Formula editor formats.