Summary

They say Elm doesn't support typeclasses. What they actually mean is that it doesn't support implicit values. Typeclasses can be defined as records with their instances provided as explicit values. This package does exactly that.

Status

This library is in a stage of active development and not yet well covered with tests, which is why it is not yet advised to rely production systems on it. To change the status sooner you're welcome to PR with tests, benchmarks and missing instance declarations for basic types.

FAQ

Why no Functors, Monads and alike?

Unfortunately Elm's type system has two limitations:

• it doesn't support forall quantification;
• it doesn't support higher-kinded polymorphism.

Having such features we would be able to provide definitions such as the following:

type alias Functor f =
  {
    map : forall a b. (a -> b) -> f a -> f b
  }

list : Functor List
list = { map = List.map }

However until we get these features the best we can do is something like this:

type alias Functor a b fa fb =
  {
    map : (a -> b) -> fa -> fb
  }

list : Functor a b (List a) (List b)
list = { map = List.map }

Because such type signatures seem likely to breed terrible APIs, we haven't yet dived into studying this opportunity deeper and limited this library to classes which at least don't require higher-kinded polymorphism.