Combinator

About 4 min

Also known as

Function Composition
Functional Combinator

Intent

Combine multiple smaller functions or operations into a single, more complex operation, allowing for flexible and reusable code.

Explanation

Real-world example

In the real world, the combinator design pattern can be likened to a meal preparation process in a kitchen. Imagine a chef who has a set of simple operations: chopping vegetables, boiling water, cooking rice, grilling chicken, and mixing ingredients. Each of these operations is a standalone function. The chef can combine these operations in various sequences to create different dishes. For example, to prepare a chicken rice bowl, the chef can combine the operations of cooking rice, grilling chicken, and mixing them with vegetables. By reusing these basic operations and combining them in different ways, the chef can efficiently prepare a wide variety of meals. Similarly, in software, the combinator pattern allows developers to combine simple, reusable functions to create more complex behaviors.

In plain words

The combinator design pattern combines simple, reusable functions to create more complex and flexible operations.

Wikipedia says

A combinator is a higher-order function that uses only function application and earlier defined combinators to define a result from its arguments.

Programmatic Example

In computer science, combinatory logic is used as a simplified model of computation, used in computability theory and proof theory. Despite its simplicity, combinatory logic captures many essential features of computation.

First of all, we have an interface consist of several methods contains, not, or, and .

// Functional interface to find lines in text.
public interface Finder {

    // The function to find lines in text.
    List<String> find(String text);

    // Simple implementation of function {@link #find(String)}.
    static Finder contains(String word) {
        return txt -> Stream.of(txt.split("\n"))
                .filter(line -> line.toLowerCase().contains(word.toLowerCase()))
                .collect(Collectors.toList());
    }

    // combinator not.
    default Finder not(Finder notFinder) {
        return txt -> {
            List<String> res = this.find(txt);
            res.removeAll(notFinder.find(txt));
            return res;
        };
    }

    // combinator or.
    default Finder or(Finder orFinder) {
        return txt -> {
            List<String> res = this.find(txt);
            res.addAll(orFinder.find(txt));
            return res;
        };
    }

    // combinator and.
    default Finder and(Finder andFinder) {
        return
                txt -> this
                        .find(txt)
                        .stream()
                        .flatMap(line -> andFinder.find(line).stream())
                        .collect(Collectors.toList());
    }
    // Other properties and methods...
}

Then we have also another combinator for some complex finders advancedFinder, filteredFinder, specializedFinder and expandedFinder.

// Complex finders consisting of simple finder.
public class Finders {

    private Finders() {
    }

    // Finder to find a complex query.
    public static Finder advancedFinder(String query, String orQuery, String notQuery) {
        return
                Finder.contains(query)
                        .or(Finder.contains(orQuery))
                        .not(Finder.contains(notQuery));
    }

    // Filtered finder looking a query with excluded queries as well.
    public static Finder filteredFinder(String query, String... excludeQueries) {
        var finder = Finder.contains(query);

        for (String q : excludeQueries) {
            finder = finder.not(Finder.contains(q));
        }
        return finder;
    }

    // Specialized query. Every next query is looked in previous result.
    public static Finder specializedFinder(String... queries) {
        var finder = identMult();

        for (String query : queries) {
            finder = finder.and(Finder.contains(query));
        }
        return finder;
    }

    // Expanded query. Looking for alternatives.
    public static Finder expandedFinder(String... queries) {
        var finder = identSum();

        for (String query : queries) {
            finder = finder.or(Finder.contains(query));
        }
        return finder;
    }
    // Other properties and methods...
}

Now that we have created the interface and methods for combinators, let's see how an application works with them.

public class CombinatorApp {

    private static final String TEXT = """
            It was many and many a year ago,
            In a kingdom by the sea,
            That a maiden there lived whom you may know
            By the name of ANNABEL LEE;
            And this maiden she lived with no other thought
            Than to love and be loved by me.
            I was a child and she was a child,
            In this kingdom by the sea;
            But we loved with a love that was more than love-
            I and my Annabel Lee;
            With a love that the winged seraphs of heaven
            Coveted her and me.""";

    public static void main(String[] args) {
        var queriesOr = new String[]{"many", "Annabel"};
        var finder = Finders.expandedFinder(queriesOr);
        var res = finder.find(text());
        LOGGER.info("the result of expanded(or) query[{}] is {}", queriesOr, res);

        var queriesAnd = new String[]{"Annabel", "my"};
        finder = Finders.specializedFinder(queriesAnd);
        res = finder.find(text());
        LOGGER.info("the result of specialized(and) query[{}] is {}", queriesAnd, res);

        finder = Finders.advancedFinder("it was", "kingdom", "sea");
        res = finder.find(text());
        LOGGER.info("the result of advanced query is {}", res);

        res = Finders.filteredFinder(" was ", "many", "child").find(text());
        LOGGER.info("the result of filtered query is {}", res);
    }

    private static String text() {
        return TEXT;
    }
}

Program output:

20:03:52.746 [main] INFO com.iluwatar.combinator.CombinatorApp -- the result of expanded(or) query[[many, Annabel]] is [It was many and many a year ago,, By the name of ANNABEL LEE;, I and my Annabel Lee;]
20:03:52.749 [main] INFO com.iluwatar.combinator.CombinatorApp -- the result of specialized(and) query[[Annabel, my]] is [I and my Annabel Lee;]
20:03:52.750 [main] INFO com.iluwatar.combinator.CombinatorApp -- the result of advanced query is [It was many and many a year ago,]
20:03:52.750 [main] INFO com.iluwatar.combinator.CombinatorApp -- the result of filtered query is [But we loved with a love that was more than love-]

Now we can design our app to with the queries finding feature expandedFinder, specializedFinder, advancedFinder, filteredFinder which are all derived from contains, or, not, and.

Applicability

This pattern is applicable in scenarios where:

The solution to a problem can be constructed from simple, reusable components.
There is a need for high modularity and reusability of functions.
The programming environment supports first-class functions and higher-order functions.

Known Uses

Functional programming languages like Haskell and Scala extensively use combinators for tasks ranging from parsing to UI construction.
In domain-specific languages, particularly those involved in parsing, such as parsing expression grammars.
In libraries for functional programming in languages like JavaScript, Python, and Ruby.
java.util.function.Function#compose
java.util.function.Function#andThen

Consequences

Benefits:

Enhances modularity and reusability by breaking down complex tasks into simpler, composable functions.
Promotes readability and maintainability by using a declarative style of programming.
Facilitates lazy evaluation and potentially more efficient execution through function composition.

Trade-offs:

Can lead to a steep learning curve for those unfamiliar with functional programming principles.
May result in performance overhead due to the creation of intermediate functions.
Debugging can be challenging due to the abstract nature of function compositions.

Chain of Responsibilityopen in new window: Relies on chaining objects, whereas Combinator chains functions.
Decoratoropen in new window: Similar to Combinator in enhancing functionality, but Decorator focuses on object augmentation.
Strategyopen in new window: Both involve selecting an algorithm at runtime, but Combinator uses composition of functions.