Currying with placeholder support is an extension of traditional currying that allows placeholders to be used for arguments. Placeholders are special markers that indicate where arguments should be supplied when invoking a curried function. This feature enhances the flexibility and readability of curried functions, enabling developers to create more expressive and versatile code.

What is currying with placeholder support?

Currying with placeholder support extends traditional currying by allowing developers to use special placeholders to indicate where arguments should be provided when invoking the curried function. Placeholders act as markers that reserve positions for arguments, enabling partial application and more flexible function composition.

Understanding placeholder currying practically

This implementation of currying with placeholder support enables the creation of flexible and reusable functions by allowing partial application of arguments with placeholders. It enhances code readability and promotes composability by enabling functions to be composed dynamically with missing arguments. The use of placeholders facilitates the creation of specialized functions tailored to specific use cases, enhancing the overall flexibility and expressiveness of the codebase.

// Implementing currying with placeholder support
const curry = (fn) => {
    return function curried(...args) {
        // If enough arguments are provided, 
        // call the original function
        if (args.length >= fn.length &&
            !args.includes(curry.placeholder)) {
            return fn.apply(this, args);
        } else {
            // Otherwise, return a curried function 
            // with placeholder support
            return function (...nextArgs) {
                const combinedArgs = args.map(
                    arg => arg === curry.placeholder && 
                    nextArgs.length ? nextArgs.shift() : arg).
                    concat(nextArgs);
                return curried(...combinedArgs);
            };
        }
    };
};

// Placeholder symbol for 
// missing arguments
curry.placeholder = Symbol();

// Example: Curried function to 
// concatenate three strings
const concat3 = curry((a, b, c) => 
    `${a} ${b} ${c}`);

// Partial application using placeholders
const concatHello = concat3('Hello,', 
    curry.placeholder, 'World!');
console.log(concatHello('Welcome'));
console.log(concatHello('Greetings'));

Hello, Welcome World!
Hello, Greetings World!

Difference between currying and currying with placeholder support

The main difference between traditional currying and currying with placeholder support lies in the ability to use placeholders for arguments. Traditional currying requires all arguments to be provided sequentially, whereas currying with placeholder support allows for partial application by specifying placeholders for certain arguments.