Hands-On Design Patterns with React Native

Monad interface requirements

More formally, the monad interface should define two basic operators:

• Return (a -> M a), an operation that takes the a type and wraps it into a monad (M a)
• Bind (M a -> (a -> M b) -> M b), an operation that takes two arguments: a monad of the a type, and a function that operates on a and returns the M b (a -> M b) monad

In these terms, our constructor function is the return function. However, our map function is not compliant with the bind requirements. It takes a function that turns a into b (a -> b), then our map function automatically wraps b into M b.

Beside this, our monad needs to obey three monad laws:

• Left identity:

// for all x, fnMaybe(x).map(fn) == Maybe(fn(x))

• Right identity:

// for all xMaybe(x).map(x ...