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Marco Cetica 2024-09-19 14:53:01 +02:00
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.gitea/workflows/LambdaTonic.yml Normal file
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name: LambdaTonic
on:
push:
branches:
- main
workflow_dispatch:
jobs:
build:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v3
- name: Execute unit tests
run: |
dnf update -y
dnf install maven -y
mvn test

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LICENSE Normal file
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MIT License
Copyright (c) 2024 Cetica Marco
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

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# λTonic 🥃 ![](https://github.com/ceticamarco/lambdatonic/actions/workflows/LambdaTonic.yml/badge.svg)
**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<LL>`
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:
```java
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
```java
<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:
```java
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
```java
<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
```java
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
```java
boolean isLeft();
```
`isLeft` returns true whether `Either<L, R>` is instantiated to the `Left<L>`, false otherwise
### Usage
```java
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
```java
boolean isRight();
```
`isRight` returns true whether `Either<L, R>` is instantiated to the `Right<L>`, false otherwise
### Usage
```java
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
```java
L fromLeft(L defaultValue);
```
`fromLeft` returns the content of the `Left<L>` value or a default value
### Usage
```java
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
```java
L fromLeft(L defaultValue);
```
`fromRight` returns the content of the `Right<R>` value or a default value
### Usage
```java
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
```java
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
```java
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
```java
Either<R, L> swap();
```
`swap` returns an `Either<R, L>` type with `Left<R>` and `Right<L>` swapped.
### Usage
```java
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](https://choosealicense.com/licenses/mit/).

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src/main/java/com/ceticamarco/lambdatonic/Either.java
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* <br /><br />
* This method
* applies a function(<i>fn</i>) to the values inside the data type,
* returning a new data type(i.e., a new functor) if and only if the the Either
* type is instantiated with the <i>Right</i> subtype. Otherwise it leaves the functor
* returning a new data type(i.e., a new functor) if and only if the Either
* type is instantiated to the <i>Right</i> subtype. Otherwise it leaves the functor
* unchanged.
* <br /><br />
* The type of the resulting functor is the return type specified on the <i>fn</i>