Maki

A Java type library.

Maki provides discriminate unions of up to 8 types (Variant1<T1>, Variant2<T1, T2> ... Variant8<T1, T2, ...>), Unit and Never, an Either<TLeft, TRight> union and an Error<T> monad.

Samples

Variant
Either
Error
Never

Variant

VariantN<...> represents a discriminate union type of up to 8 types. Unlike using a base type and inheritance, a Variant represents a closed set of types and no relationship between them is needed.

Variant3<String, Integer, Double> variant = new Variant3<>("Hello world!");

// Use is(T.class) to test for inhabiting type, <T>get() to retrieve it
assertTrue(variant.is(String.class));
System.out.println(variant.<String>get());

// Can reassign another type
variant.set(42);

assertTrue(variant.is(Integer.class));

// Use Apply and supply it with an action or function for each possible type,
// the one corresponding to the inhabiting type will get called
variant.apply(
    s -> { System.out.println("It's a string!"); },
    i -> { System.out.println("It's an int!"); },
    d -> { System.out.println("It's a double!"); });

Either

Either<TLeft, TRight> can hold a value of either TLeft or TRight type.

Either<String, Integer> either = Either.makeLeft("Hello world!");

assertTrue(either.isLeft());
assertFalse(either.isRight());

System.out.println(either.getLeft());

either.setRight(42);

assertFalse(either.isLeft());
assertTrue(either.isRight());

System.out.println(2 * either.getRight());

Error

Error<T> represents a value of type T or an Exception. This allows packaging exceptions as part of the return type and handling them at any point in the code.

// Initialize with a function that may throw
Error<String> error = Error.make(() ->
    {
        Random random = new Random();

        if (random.nextBoolean())
            throw new Exception();

        return "Success";
    });

if (error.hasValue())
    System.out.println(error.getValue());
else
    System.out.println("Exception was thrown: " + error.getError());

Never

Using Never as a return type explicitly shows the function cannot return.