GHC exercises 🚀

For solutions, please see the Exercises.hs modules inside the answers
branch.

OK, you know your monoids from your monads. You know how to write a
terminal application or two. What's next? What are these language extensions
that keep being mentioned? How "type-safe" can you really be?

This repository, I hope, will provide some stepping stones. We'll go
through extensions one at a time, and build up a richer vocabulary for talking
about Haskell programs, and look to move our assertions up into types, where
they can be verified at compile time.

---

What this isn't.

This is a deep dive into GHC extensions, the power that each one gives us, and
how we can combine extensions to achieve very strong guarantees at
compile-time. This is not based around concepts; there won't be sections on
"dependently-typed programming", or "generic programming", though these
concepts will turn up throughout as we dig deeper into the extensions.

If you're interested in something more project-based, I absolutely, 1000%
recommend Thinking with Types,
written by Sandy Maguire. It is a
fantastic resource, and one on which I already rely when explaining
concepts to others.

Contents

1. GADTs
2. FlexibleInstances
3. KindSignatures
4. DataKinds
5. RankNTypes
6. TypeFamilies
7. ConstraintKinds
8. PolyKinds
9. MultiParamTypeClasses
10. FunctionalDependencies

Setup

Assuming you have Cabal or
Stack setup, you should be
able to navigate to any of the exercise* directories, and run your usual
commands:

Repl

$ stack repl
$ cabal repl

Build

$ stack build
$ cabal build

It's going to make it a lot easier to iterate through the exercises if you
cabal install ghcid or stack install ghcid. Just as above, once this is
done, you can navigate to the exercise directory and run it with your preferred
repl command:

$ ghcid -c "stack repl"
$ ghcid -c "cabal repl"