monadgist.md

Monad

All it takes to be a Monad is to provide two functions which conform to three laws.

The two functions:

1. Place a value into monadic context

• Haskell's Maybe: return / Just
• Scala's Option: Some
• Functional Java's Option: Option.some
• JDK8's Optional: Optional.of

2. Apply a function in monadic context

• Haskell's Maybe: >>= (aka bind)
• Scala's Option: flatMap
• Functional Java's Option: flatMap
• JDK8's Optional: flatMap

NOTE: One of the key things to understand is the signature of the function to apply in monadic context: it takes the raw value type, and returns the monadic type.

In other words, if you have Optional<Integer>, the functions you can pass to flatMap will have the signature (Integer) -> Optional<U>, where U is a value type which does not have to be Integer, for example String:

Optional<Integer> maybeInteger = Optional.of(1);

// Function that takes Integer and returns Optional<Integer>
Optional<Integer> maybePlusOne = maybeInteger.flatMap(n -> Optional.of(n + 1));

// Function that takes Integer and returns Optional<String>
Optional<String> maybeString = maybePlusOne.flatMap(n -> Optional.of(n.toString));

You don't need any sort of Monad Interface to code this way, or to think this way. In Scala, you don't code to a Monad Interface (unless you are using Scalaz library...). It appears that JDK8 will empower Java folks to use this style of chained monadic computations as well.