In C#, callback is frequently used. For example, a very simple Add function, without asynchrony:

Unlike the Reader< , > monad, the Writer< , > monad output contents with a sequence of functions:

Sometimes there are functions work with a shared environment. Typical examples are:

represents a abstract machine with one state or a number of state. C# use state machine a lot. For example:

As mentioned in a previous part, in purely functional programming, functions cannot have side effects. For example, when defining LINQ queries, laziness and purity are expected. So, how should the imp

Theoretically, Tuple<> should be counted as the Id<> monad. However, it is lack of laziness. In the context of C# and LINQ, it is only monad-like.

Again, Lazy<> is the simplest monad, it is just the lazy version of Tuple<>, and should be considered as the Id<> monad. This is the implementation of its SelectMany:

Previous part introduced SelectMany for monad IEnumerable<>. Actually SelectMany is more than meets the eye, and can be used to implement other LINQ queries.