Haskell Design Patterns

Types, pattern matching and polymorphism

Algebraic types give us a very concise way to model composite types, even recursive ones. Pattern matching makes it easy to work with algebraic types. Type classes enable both the fundamental types of polymorphism: parametric and ad-hoc.

Together, these capabilities allow us to easily express many of the Gang of Four patterns.

Algebraic types and pattern matching

Algebraic data types can express a combination of types, for example:

type Name = String
type Age = Int
data Person = P String Int -- combination

They can also express a composite of alternatives:

data MaybeInt = NoInt | JustInt Int

Here, each alternative represents a valid constructor of the algebraic type:

maybeInts = [JustInt 2, JustInt 3, JustInt 5, ...