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Cats Code samples

My blog covers various Scala related topics mostly around functional programming. Here are some code examples illustrating the ideas I describe in my blog. Check out my blog to find out more:


Type classes are everywhere in the Scala ecosystem. If you want to learn advanced libraries like Scalaz, Cats or Shapeless you need to know about them. Even if you don't plan to use these libraries you can (and probably should) use type classes in your own applications. Type classes are borrowed from Haskell and are sometimes called "ad-hoc polymorphism"

Examples illustrating why type classes are so powerful can be found here

Check out the supporting blog post


You're probably familiar with the map method in the Scala standard library. Collections, Futures and Options all have a map method but unfortunately there's no base class for mappable types, making it hard to write generic code. Fortunately Cats includes a type class and various implementations which gives us this "base type"

Examples of Functors including composition can be found here

Check out the supporting blog post

Monads and Monad transformers

Functors compose, so long as we only use map() we can compose any combination of Functors e.g. List[Option[A]]. However Monads (types with flatMap()) don't necessarily compose without an additional data type known as a monad transformer

Examples of monad transformers (OptionT and EitherT) including workarounds for variance issues can be found here

Check out the supporting blog posts here and here


We can use a MonadTransformer to wrap Future[Either[L, R]] then map the left and right projections. But what happens if the underlying future fails? MonadError allows us to map the left projection if the underlying future holds a Left or the future fails

An example of MonadError usage can be found here

Check out the supporting blog post here


Monads are prevalent in the scala world. Think of Future's flatMap(). New developers often use Monads along with for comprehension to perform multiple independent operations but this is not what Monad's are designed for. Applicatives are the go to tool for performing multiple independent operations.

See an example of calling multiple external services in parallel using Applicatives here

An example of form style validation can be found here

Check out the supporting blog post here

Validated and ValidatedNel

Either doesn't really work when we want to accumulate errors. I explain why in my blog post. Validated is a much better option - it's usage alongside Applicative is almost identical to Either

Find an example here

The supporting blog post can be found here


Code samples to support the concepts on my blog. Mostly focussed on Functional programming with Cats







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