java-design-patterns/currying/README.md

---
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<Genre, Function<String, Function<String, Function<LocalDate, Book>>>>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<String, Function<String, Function<LocalDate, Book>>>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)