Reducing type class boilerplate with Shapeless

If you followed our previous post on forging a DSL using type classes, you surely notice that writing type class instances is a rather repetitive task.

In today’s post we’re going to get rid of this by derivating type class instances automatically using Shapeless. We also use this post as an excuse to experiment with Shapeless and try to understand all the “magic” that’s happening. Continue reading “Reducing type class boilerplate with Shapeless”

Forging a DSL using Scala type classes

In this post we’re going to explore how to build a DSL (Domain Specific Language) with a user-friendly syntax while maintaining as much type-safety as possible. We want that any operations that is not allowed by the business rules fail at compile time. This would be really nice as it makes sure that no one writes such forbidden logic (even by mistake).

More over Scala provides really nice syntactic sugar that can make a DSL syntax pretty neat.

If you don’t know what type classes or don’t feel very comfortable with this concept, follow along as we’ll also explore how we can use them to dissociate data and behaviours (always a good practice). Continue reading “Forging a DSL using Scala type classes”

The free monad without the boilerplate

The free monad is really neat for creating DSL as it allows to completely separate the business logic from the implementation.

It leaves a great freedom for the implementation choices and should you change your implementation you just need to rewrite your interpreter without changing any of the business logic.

That’s great! However after playing a little around with the free monad I was a bit skeptical about the amount of boilerplate required to plug everything together. Writing smart constructors and injectors is not the most trivial thing (depending on how your team is familiar with functional programming).

To conclude this serie on the free monad we’ll have a look at some solutions to remove this boilerplate code, especially freasy and freek.
Continue reading “The free monad without the boilerplate”

The free monad in (almost) real life

Now that we have a fairly good understanding of what the free monad is and how it works. Let’s see what it looks like to write a real program based on this concept. This time we won’t implement Free ourselves but use the cats library for that. If you don’t know all the theory behind the free monad and just want to see how to use it then read on as this post builds an application from scratch.

In this post we’ll write simple DSLs, lift them into the free monad and even combine them into one program. There is some boiler plate around but I want to get a feeling of what it looks like to combine more DSLs and see how we can architecture a whole application around the free monad. Continue reading “The free monad in (almost) real life”

The Free monad explained – part 2

Remember in part 1 we implemented a little program to interact with the user. We nicely separated the business logic from the implementation. And as good as it is we now need to add more functionality to our application and this exactly what we’re going to cover here.

The idea is to define a new language for user authentication and then combine it with the user interaction language to build a more complex program.
Continue reading “The Free monad explained – part 2”

The Free monad explained – part 1

My next series of blog posts will be on the Free monad. ┬áDon’t be scared by the name, it’s not so difficult to grasp it (if explains correctly) and your code could greatly improve.

Why ? Because it allows a clear separation between the program definition (i.e. the business logic) and its implementation. That means if one day you decide to change your implementation (e.g change your persistent storage) you just need to re-write your interpreter – leaving the business logic untouched.
Continue reading “The Free monad explained – part 1”