Box

Monads

In programming; a Monad is a Design Pattern. It’s a structure, a wrapper which “enriches” a value by giving it a context.

Famous examples of Monads are:

• Option/Maybe monad (it can represent a missing/null value)
• Either monad (it can represent a successful operation or a failure)
• IO/Effect monad (it can represent side-effects)
• Task monad (it can represent asynchronous side-effects)

 

A Monad has 2 particular operations:

• a unit (often called return or pure) which takes a value and puts it into the container (yielding a so called “monadic value”)
• a chain (often called bind or flatMap) which concatenates Monads.

For example, when flatMap is introduced in JavaScript Arrays, we will be able to think about it as a Monad:

• Array.of(1) // Puts 1 into the array
• Array.map(n => n + 1) // Maps the values into the array, this makes it a Functor
• Array.flatMap(n => [n,n]) // Flattens the generated arrays into one array, this makes it a Monad
   

For these reasons, a Monad is often referred to as “a Functor which can flatten”.