README

MonoidK<F> inherits SemigroupK<F>. The way to think of MonoidK<F> is a monoid for higher-kinds (K<F, A> types, rather than A types).

What that means is that any type that implements the MonoidK<F> trait gains an Empty() (a 'zero'/identity element ) value as well as the ability to Combine two K<F, A> structures together into one. Many implementations of MonoidK<F> use Combine to catch errors and propagate. So, if the first K<F, A> argument to Combine fails, it simply returns the second argument. If it succeeds, then the result of the first is returned. This works a bit like null propagation with the ?? operator. And while this isn't always how MonoidK is implemented, it's useful to know.

The MonoidK<F> trait combines with Applicative<F> and Monad<F> traits to provide the following default functionality:

• Filter | filter - if your type supports Monad<F> and MonoidK<F> you get free filtering and Where LINQ extension
• Choose | choose - if your type supports Monad<F> and MonoidK<F> then Choose does filtering and mapping
• OneOf | oneOf - takes a collection of K<F, A> structures, returns the first one to succeed.
• Some | some - evaluates a K<F, A> structure repeatedly, collecting the A values, until it fails (at least one must succeed). Returns the K<F, Seq<A>>
• Many | many - evaluates a K<F, A> structure repeatedly, collecting the A values, until it fails. Returns the K<F, Seq<A>>
• guard - conditional failure

Some of these you might recognise from the Parsec library. This completely generalises the concept of alternative structure coalescing.