--- title: Currying category: Functional language: en tag: - Functional decomposition --- ## Also known as * Partial Function Application ## Intent Currying decomposes a function that takes multiple arguments into a sequence of functions that each take a single argument. It helps in creating a higher-order function by partial application of its arguments. ## Explanation Real-world example > Consider a librarian who wants to populate their library with books. The librarian wants functions which can create books corresponding to specific genres and authors. Currying makes this possible by writing a curried book builder function and utilising partial application. In plain words > Decompose a function that take multiple arguments into multiple functions that take a single argument. Wikipedia says > In mathematics and computer science, currying is the technique of translating a function that takes multiple arguments into a sequence of families of functions, each taking a single argument. Programmatic example We have a `Book` class and `Genre` enum. ```java public class Book { private final Genre genre; private final String author; private final String title; private final LocalDate publicationDate; Book(Genre genre, String author, String title, LocalDate publicationDate) { this.genre = genre; this.author = author; this.title = title; this.publicationDate = publicationDate; } } public enum Genre { FANTASY, HORROR, SCI_FI; } ``` We could easily create a `Book` object with the following method: ```java Book createBook(Genre genre,String author,String title,LocalDate publicationDate){ return new Book(genre,author,title,publicationDate); } ``` However, what if we only wanted to create books from the `FANTASY` genre? We could pass in the `FANTASY` parameter on each method call; however, this is repetitive. We could define a new method specifically for creating `FANTASY` books; however, it is infeasible to create a new method for each book genre. The solution is to create a curried function. ```java static Function>>>book_creator =genre ->author ->title ->publicationDate ->new Book(genre,author,title,publicationDate); ``` Note that the order of the parameters is important. `genre` must come before `author`, `author` must come before `title` and so on. We must be considerate of this when writing curried functions to take full advantage of partial application. Using the above function, we can define a new function `fantasyBookFunc`, to generate `FANTASY` books as follows: ```java Function>>fantasyBookFunc=Book.book_creator.apply(Genre.FANTASY); ``` Unfortunately, the type signature of `BOOK_CREATOR` and `fantasyBookFunc` are difficult to read and understand. We can improve this by using the [builder pattern](https://java-design-patterns.com/patterns/builder/) and [functional interfaces](https://www.geeksforgeeks.org/functional-interfaces-java/#:~:text=A%20functional%20interface%20is%20an,any%20number%20of%20default%20methods). ```java public static AddGenre builder(){ return genre ->author ->title ->publicationDate ->new Book(genre,author,title,publicationDate); } public interface AddGenre { Book.AddAuthor withGenre(Genre genre); } public interface AddAuthor { Book.AddTitle withAuthor(String author); } public interface AddTitle { Book.AddPublicationDate withTitle(String title); } public interface AddPublicationDate { Book withPublicationDate(LocalDate publicationDate); } ``` The semantics of the `builder` function can easily be understood. The `builder` function returns a function `AddGenre`, which adds the genre to the book. Similarity, the `AddGenre` function returns another function `AddTitle`, which adds the title to the book and so on, until the `AddPublicationDate` function returns a `Book`. For example, we could create a `Book` as follows: ```java Book book=Book.builder().withGenre(Genre.FANTASY) .withAuthor("Author") .withTitle("Title") .withPublicationDate(LocalDate.of(2000,7,2)); ``` The below example demonstrates how partial application can be used with the `builder` function to create specialised book builder functions. ```java public static void main(String[]args){ LOGGER.info("Librarian begins their work."); // Defining genre book functions Book.AddAuthor fantasyBookFunc=Book.builder().withGenre(Genre.FANTASY); Book.AddAuthor horrorBookFunc=Book.builder().withGenre(Genre.HORROR); Book.AddAuthor scifiBookFunc=Book.builder().withGenre(Genre.SCI_FI); // Defining author book functions Book.AddTitle kingFantasyBooksFunc=fantasyBookFunc.withAuthor("Stephen King"); Book.AddTitle kingHorrorBooksFunc=horrorBookFunc.withAuthor("Stephen King"); Book.AddTitle rowlingFantasyBooksFunc=fantasyBookFunc.withAuthor("J.K. Rowling"); // Creates books by Stephen King (horror and fantasy genres) Book shining=kingHorrorBooksFunc.withTitle("The Shining") .withPublicationDate(LocalDate.of(1977,1,28)); Book darkTower=kingFantasyBooksFunc.withTitle("The Dark Tower: Gunslinger") .withPublicationDate(LocalDate.of(1982,6,10)); // Creates fantasy books by J.K. Rowling Book chamberOfSecrets=rowlingFantasyBooksFunc.withTitle("Harry Potter and the Chamber of Secrets") .withPublicationDate(LocalDate.of(1998,7,2)); // Create sci-fi books Book dune=scifiBookFunc.withAuthor("Frank Herbert") .withTitle("Dune") .withPublicationDate(LocalDate.of(1965,8,1)); Book foundation=scifiBookFunc.withAuthor("Isaac Asimov") .withTitle("Foundation") .withPublicationDate(LocalDate.of(1942,5,1)); LOGGER.info("Stephen King Books:"); LOGGER.info(shining.toString()); LOGGER.info(darkTower.toString()); LOGGER.info("J.K. Rowling Books:"); LOGGER.info(chamberOfSecrets.toString()); LOGGER.info("Sci-fi Books:"); LOGGER.info(dune.toString()); LOGGER.info(foundation.toString()); } ``` Program output: ``` Librarian begins their work. Stephen King Books: Book{genre=HORROR, author='Stephen King', title='The Shining', publicationDate=1977-01-28} Book{genre=FANTASY, author='Stephen King', title='The Dark Tower: Gunslinger', publicationDate=1982-06-10} J.K. Rowling Books: Book{genre=FANTASY, author='J.K. Rowling', title='Harry Potter and the Chamber of Secrets', publicationDate=1998-07-02} Sci-fi Books: Book{genre=SCI_FI, author='Frank Herbert', title='Dune', publicationDate=1965-08-01} Book{genre=SCI_FI, author='Isaac Asimov', title='Foundation', publicationDate=1942-05-01} ``` ## Class diagram ![currying-uml](./etc/currying.urm.png) ## Applicability * When functions need to be called with some arguments preset. * In functional programming languages or paradigms to simplify functions that take multiple arguments. * To improve code reusability and composability by breaking down functions into simpler, unary functions. ## Known uses * Functional programming languages like Haskell, Scala, and JavaScript. * Event handling in UIs where a function with specific parameters needs to be triggered upon an event. * APIs that require configuration with multiple parameters. ## Consequences Benefits: * Increases function reusability by allowing the creation of specialized functions from more generic ones. * Enhances code readability and maintainability by breaking complex functions into simpler, single-argument functions. * Facilitates function composition, leading to more declarative and concise code. Trade-offs: * Can lead to performance overhead due to the creation of additional closures. * May make debugging more challenging, as it introduces additional layers of function calls. * Can be less intuitive for developers unfamiliar with functional programming concepts. * As shown in the programmatic example above, curried functions with several parameters have a cumbersome type signature in Java. ## Related patterns * Function Composition: Currying is often used in conjunction with function composition to enable more readable and concise code. * [Decorator](https://java-design-patterns.com/patterns/decorator/): While not the same, currying shares the decorator pattern's concept of wrapping functionality. * [Factory](https://java-design-patterns.com/patterns/factory/): Currying can be used to create factory functions that produce variations of a function with certain arguments preset. ## Credits * [Java 8 in Action: Lambdas, Streams, and functional-style programming](https://amzn.to/3J6vEaW) * [Modern Java in Action: Lambdas, streams, functional and reactive programming](https://amzn.to/3J6vJLM) * [Functional Programming in Java: Harnessing the Power Of Java 8 Lambda Expressions](https://amzn.to/3TKeZPD) * [Currying in Java](https://www.baeldung.com/java-currying) * [What Is Currying in Programming](https://towardsdatascience.com/what-is-currying-in-programming-56fd57103431#:~:text=Currying%20is%20helpful%20when%20you,concise%2C%20and%20more%20readable%20solution.) * [Why the fudge should I use currying?](https://medium.com/dailyjs/why-the-fudge-should-i-use-currying-84e4000c8743)