λTonic 🥃

λTonic(LambdaTonic) is functional library designed for modern Java(+21).

This library introduces a new algebraic data type called Either<L, R>; that is, an immutable sum type that discriminates between two values, Left<L> and Right<R>, representing the failure and the success values, respectively.

The Either<L, R> data type is implemented using a sealed interface, while the Left<L> and the Right<R> are record classes that adopt the Either<L, R> protocol. Both the Left<L> and the Right<R> data types can be used inside a switch statement using Java pattern matching.

Overview

The Either<L, R> algebraic data type can be used orthogonally over exceptions to propagate an error from a function. Consider the following scenario:

public class Main {
    public static Either<Error, Double> division(double dividend, double divisor) {
        // Return an error whether the divisor is zero
        if(divisor == 0) {
            return new Left<>(new Error("Cannot divide by zero"));
        }

        // Otherwise return the result of the division
        return new Right<>(dividend / divisor);
    }

    public static void main(String[] args) {
        // Try to divide 15 by 3
        Either<Error, Double> divResult = division(15, 3);
        switch (divResult) {
            case Left<Error, Double> err -> System.err.println(err.value().getMessage());
            case Right<Error, Double> val -> System.out.printf("15 / 3 = %f\n", val.value());
        }

        // Try to divide 2 by 0
        var div2Result = division(2, 0);
        switch (div2Result) {
            case Left<Error, Double> err -> System.err.println(err.value().getMessage());
            case Right<Error, Double> val -> System.out.printf("2 / 0 = %f\n", val.value());
        }
    }
}

In this example we have defined a division method that takes two arguments and perform a division on them. To handle the case when the divisor parameter is equal to zero, we return a new Left<L> instance of the Either<L, R> type, while in any other case, we return a new Right<R> instance of the Either<L, R> type. In the caller method(i.e., the main) we can then execute a custom statement using Java's builtin pattern matching.

API Usage

The Either<L, R> data type supports a broad spectrum of features, below there is a list of all supported functionalities.

• map

Description

<T> Either<L, T> map(Function<R, T> fn);

The map method applies a function(fn) to the values inside the data type, returning a new data type if and only if the Either<L, R> type is instantiated to the Right<R> type. The map method adheres to the functor laws(identity and composition of morphisms), which allows the Either<L, R> data type to be classified as a functor.

Usage

The map method can be used to apply a computation to the value inside a functor:

public class Main {
    // ...
    public static void main(String[] args) {
        var resDivision = division(15, 3);
        var resSquared = resDivision.map(x -> x * x);

        switch (resSquared) {
            case Left<Error, Double> err -> System.err.println(err.value().getMessage());
            case Right<Error, Double> val -> System.out.println(val.value()); // prints 25.0
        }
    }
}

• bimap

Description

<T, K> Either<T, K> bimap(Function<L, T> onLeft, Function<R, K> onRight);

The bimap method applies the onLeft method to the Left<L> subtype or the onRight to the Right<R>.

Usage

public class Main {
    public static void main(String[] args) {
        var sc = new Scanner(System.in);

        // Read from stdin
        System.out.print("Enter a divisor: ");
        var input = sc.nextInt();

        // Divide a fixed dividend by user input divisor
        var divRes = division(15, input);

        // Apply a function regardless of the type of Either
        // On the left we uppercase the error message
        // On the right we square the result
        var bimapRes = divRes.bimap(
            err -> new Error(err.toString().toUpperCase()),
            val -> val * val
        );

        switch (bimapRes) {
            case Left<Error, Double> err -> System.err.println(err.value().getMessage());
            case Right<Error, Double> val -> System.out.println(val.value());
        }
    }
}

• isLeft

Description

boolean isLeft();

isLeft returns true whether Either<L, R> is instantiated to the Left<L>, false otherwise

Usage

public class Main {
    public static void main(String[] args) {
        var sc = new Scanner(System.in);

        // Read from stdin
        System.out.print("Enter a divisor: ");
        var input = sc.nextInt();

        // Divide a fixed dividend by user input divisor
        var divRes = division(15, input);
        
        if(divRes.isLeft()) {
            System.out.println("Cannot divide by zero");
        } else {
            System.out.println(divRes.fromRight(-1.0));
        }
    }
}

• isRight

Description

boolean isRight();

isRight returns true whether Either<L, R> is instantiated to the Right<L>, false otherwise

Usage

public class Main {
    public static void main(String[] args) {
        var sc = new Scanner(System.in);

        // Read from stdin
        System.out.print("Enter a divisor: ");
        var input = sc.nextInt();

        // Divide a fixed dividend by user input divisor
        var divRes = division(15, input);

        if(divRes.isRight()) {
            System.out.println(divRes.fromRight(-1.0));
        } else {
            System.out.println("Cannot divide by zero");
        }
    }
}

• fromLeft

Description

L fromLeft(L defaultValue);

fromLeft returns the content of the Left<L> value or a default value

Usage

public class Main {
    public static void main(String[] args) {
        var sc = new Scanner(System.in);

        // Read from stdin
        System.out.print("Enter a divisor: ");
        var input = sc.nextInt();

        // Divide a fixed dividend by user input divisor
        var divRes = division(15, input);
        
        // Prints out the error message or nothing  
        System.out.println(divRes.fromLeft(new Error("")).getMessage());
    }
}

• fromRight

Description

L fromLeft(L defaultValue);

fromRight returns the content of the Right<R> value or a default value

Usage

public class Main {
    public static void main(String[] args) {
        var sc = new Scanner(System.in);

        // Read from stdin
        System.out.print("Enter a divisor: ");
        var input = sc.nextInt();

        // Divide a fixed dividend by user input divisor
        var divRes = division(15, input);

        // Prints out the actual value or nothing
        System.out.println(divRes.fromRight(0.0));
    }
}

• toOptional

Description

Optional<R> toOptional();

toOptional converts an Either<L, R> data type to a java.util.Optional, where the Right<R> becomes a non-null Optional<R> and the Left<L> becomes a null Optional.

Usage

public class Main {
    public static void main(String[] args) {
        var sc = new Scanner(System.in);

        // Read from stdin
        System.out.print("Enter a divisor: ");
        var input = sc.nextInt();

        // Divide a fixed dividend by user input divisor
        var divRes = division(15, input).toOptional();

        // Prints out the actual value or nothing
        divRes.ifPresent(System.out::println);
    }
}

• swap

Description

Either<R, L> swap();

swap returns an Either<R, L> type with Left<> and Right<> swapped.

Usage

public class Main {
    public static void main(String[] args) {
        Either<String, Integer> val = new Left<>("generic error");
        Either<Integer, String> res = val.swap();

        System.out.println(res.isLeft()); // Prints false
        System.out.println(res.isRight()); // Prints true
    }
}

License

This software is released under the MIT license. You can find a copy of the license with this repository or by visiting the following page.