In the interests of helping to understand what a monad is, can someone provide an example using java ? Are they possible ?

Lambda expressions are possible using java if you download the pre-release lambda compatible JDK8 from here http://jdk8.java.net/lambda/

An example of a lambda using this JDK is shown below, can someone provide a comparably simple monad ?

public interface TransformService { int[] transform(List<Integer> inputs); } public static void main(String ars[]) { TransformService transformService = (inputs) -> { int[] ints = new int[inputs.size()]; int i = 0; for (Integer element : inputs) { ints[i] = element; } return ints; }; List<Integer> inputs = new ArrayList<Integer>(5) {{ add(10); add(10); }}; int[] results = transformService.transform(inputs); }

Just FYI:

The proposed JDK8 Optional class **does satisfy** the three Monad laws. Here’s a gist demonstrating that.

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

The two functions:

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`

- Haskell’s Maybe:
Apply a

*function in monadic context*- Haskell’s Maybe:
`>>=`

(aka`bind`

) - Scala’s Option:
`flatMap`

- Functional Java’s Option:
`flatMap`

- JDK8’s Optional:
`flatMap`

- Haskell’s Maybe:

Please see the above gist for a java demonstration of the three laws.

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 an instance of `Optional<Integer>`

, the functions you can pass to its `flatMap`

method 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.

Hope this is helpful!

**Update:** Blogged about this here.

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