Ask

Ask[F, E] allows us to read a value of type E from a shared environment into a context F[_]. In practical terms, this means, that if we have an instance of Ask[F, E], we can now request a value of F[E] by using the ask method.

Simplified, it is defined like this:

trait Ask[F[_], E] {
  def ask: F[E]
}

This typeclass comes in handy whenever we want to be able to request a value from the environment, e.g. read from some external configuration.

Instances

A trivial instance for Ask can be just a plain function:

def functionAsk[E]: Ask[E => *, E] = new Ask[E => *, E] {
  def ask: E => E = identity
}

Other instances include Reader and ReaderT/Kleisli. In fact the implementation for ask is just ReaderT.ask, so it's a perfect fit.

Cats-mtl is able to lift Ask instances through a monad transformer stack, so for example, StateT[Reader[E, *], S, A] and EitherT[ReaderT[List, E, *], Error, A] will both have Ask instances whenever you import from cats.mtl.instances._.

In general every monad transformer stack where Reader appears once, will have an instance of Ask.