The Type Astronaut's Guide to Shapeless

In Part I we introduce type class derivation, which allows us to create type class
instances for any algebraic data type using only a handful of generic rules. Part
I consists of four chapters: 

• In Chapter 2 we introduce generic representations. We also introduce
shapeless’ Generic type class, which can produce a generic encoding
for any case class or sealed trait. 

• In Chapter 3 we use Generic to derive instances of a custom type class.
We create an example type class to encode Scala data as Comma Separated Values (CSV), but the techniques we cover can be extended to
many situations. We also introduce shapeless’ Lazy type, which lets us
handle recursive data like lists and trees.

 • In Chapter 4 we introduce the theory and programming pa‚erns we
need to generalise the techniques from earlier chapters. Specifically we
6 CHAPTER 1. INTRODUCTION
look at dependent types, dependently typed functions, and type level
programming. This allows us to access more advanced applications of
shapeless. 

• In Chapter 5 we introduce LabelledGeneric, a variant of Generic
that exposes field and type names as part of its generic representations.
We also introduce additional theory: literal types, singleton types, phantom types, and type tagging. We demonstrate LabelledGeneric by
creating a JSON encoder that preserves field and type names in its output.

 In Part II we introduce the “ops type classes” provided in the shapeless.ops
package. Ops type classes form an extensive library of tools for manipulating
generic representations. Rather than discuss every op in detail, we provide a
theoretical primer in three chapters: 

• In Chapter 6 we discuss the general layout of the ops type classes and
provide an example that strings several simple ops together to form a
powerful “case class migration” tool.

 • In Chapter 7 we introduce polymorphic functions, also known as Polys,
and show how they are used in ops type classes for mapping, flat mapping, and folding over generic representations.

 • Finally, in Chapter 8 we introduce the Nat type that shapeless uses
to represent natural numbers at the type level. We introduce several
related ops type classes, and use Nat to develop our own version of
Scalacheck’s Arbitrary.