Summary

This was quite a section. Let's recount the things we've learned.

We discussed several ways to handle errors in Haskell:

1. Encoding errors as a data type and using the Either type to encode "a value or an error". Useful approach for uneffectful code
2. Using ExceptT when we want to combine the approach in (1) on top of an existing type with monadic capabilities
3. Using exceptions for IO code

We've also learned a few new abstractions and techniques:

1. The Traversable type class, for data structures that can be traversed from left to right, such as linked lists, binary trees and Maps. Pretty useful when combined with another applicative functor type like Either or IO
2. The Monad type class extends the Applicative type class with the join :: m (m a) -> m a function. We learned that Either implements this type class interface, and so does IO
3. The MonadTrans type class for monad transformers for types that take other monads as inputs and provide a monadic interface (>>=, do notation, etc.) while combining both their capabilities. We saw how to stack an Either-like monad transformer, ExceptT, on top of IO

We are almost done – only a couple more things left to do with this project. Let's go!

You can view the git commit of the changes we've made and the code up until now.