Learning Module 11 — Type Classes and Monads

Why this is important: Type classes are Haskell’s solution to the “expression problem”: in an existing system, how do we add new functions (verbs) or new types (nouns)? In most languages it is dramatically easier to do one than the other. Type classes are an attempt to make both of these things easier.

The type classes we will focus on are the Functor (allowing for map to be written for any type), Applicative (like map for multiple-argument functions), and Monad (allowing us to be able to model state and other forms of computation in a safe way). They’ve been described as “programmable semicolons.”

Outcomes

• 11.1 – Implement the Eq or Ord type class for a given type. (2 points)
• 11.2 – Implement a Functor type class for a given type. (2 points)
• 11.3 – Implement an Applicative type class for a given type. (2 points)
• 11.4 – Implement a Monad type class for a given type. (4 points)