Monads explained simply

In short: A monad is a pattern for chaining computations that carry “extra behaviour” (no result, an error, a list, a side effect) cleanly one after another.

In more detail: It wraps a value in a container and offers an operation (bind, flatMap) that applies a function to the contents while handling the extra behaviour itself.

In Depth

• Option/Maybe: A value or “nothing”. flatMap runs the next function only if a value is there. This removes nested null checks.
• Result/Either: Success or error. The first error aborts the chain.
• List: flatMap applies a function to every element and flattens the results.
• Promise/Future: A value that arrives later. then corresponds to bind.
• Laws: Left identity, right identity and associativity make sure chains can be rearranged predictably.
• Mnemonic: A monad is an assembly line where each step already receives its input “unpacked”.

See also: Currying