Advanced Currying with Placeholders in JavaScript

Currying is a functional programming technique that transforms a function with multiple arguments into a sequence of functions, each taking a single argument. This challenge extends the concept of currying by introducing placeholders, allowing for more flexible function application where arguments can be provided in any order or skipped. Mastering this will enhance your understanding of functional programming principles and JavaScript's higher-order functions.

Problem Description

Your task is to implement a JavaScript function curryWithPlaceholder that takes a function fn and returns a curried version of fn. This curried function should accept arguments and placeholders. A placeholder, represented by a special symbol (you can choose one, e.g., Symbol('placeholder') or a unique string like '__PLACEHOLDER__'), indicates that an argument is missing and should be filled in later.

The curried function should:

1. Collect arguments: It should accumulate all provided arguments and placeholders.
2. Handle placeholders: When the curried function is called again, it should attempt to fill in any missing arguments from the new set of arguments. If multiple placeholders exist, they should be filled in order from left to right.
3. Execute the original function: Once all arguments for the original function fn are available (i.e., no more placeholders are needed), the original function fn should be invoked with the collected and ordered arguments.
4. Return a new function (or value): If not all arguments are yet available, it should return a new function that continues the currying process. If all arguments are available, it should return the result of calling fn.
5. Argument ordering: Arguments provided to subsequent calls of the curried function should fill the placeholders in the order they appear. If there are more provided arguments than placeholders, the excess arguments should be appended to the end of the argument list for the next call.

Key Requirements:

• The curryWithPlaceholder function must accept the original function fn.
• The returned curried function must be able to accept any number of arguments, including placeholders.
• A mechanism for defining and recognizing placeholders is required.
• The curried function must correctly track and apply arguments to fulfill placeholders and remaining arguments.
• The original function fn should only be called when all its required arguments are present.

Edge Cases:

• Calling the curried function with no arguments.
• Calling the curried function with only placeholders.
• Calling the curried function with more arguments than initially needed for placeholders.
• Nested calls with a mix of arguments and placeholders.

Examples

Example 1:

const add = (a, b, c) => a + b + c;
const placeholder = Symbol('placeholder');

const curriedAdd = curryWithPlaceholder(add, placeholder);

// Call 1: Provide 'a' and a placeholder for 'b'
const step1 = curriedAdd(1, placeholder);
// step1 is a function

// Call 2: Provide 'c' and fill the placeholder for 'b'
const result1 = step1(3, 2); // 'b' gets 2, 'c' gets 3
// result1 should be 6 (1 + 2 + 3)

// Explanation:
// curriedAdd(1, placeholder) -> stores { args: [1], placeholders: 1 }
// step1(3, 2) -> sees a placeholder, fills it with 2. Sees 3, appends it.
// Now we have args: [1, 2, 3]. Calls add(1, 2, 3) which returns 6.

Example 2:

const greet = (greeting, name, punctuation) => `${greeting}, ${name}${punctuation}`;
const placeholder = Symbol('placeholder');

const curriedGreet = curryWithPlaceholder(greet, placeholder);

// Call 1: Provide greeting and placeholder for name
const stepA = curriedGreet('Hello', placeholder);
// stepA is a function

// Call 2: Provide punctuation, filling the placeholder for name
const result2 = stepA('!', 'Alice'); // 'name' gets 'Alice', 'punctuation' gets '!'
// result2 should be "Hello, Alice!"

// Explanation:
// curriedGreet('Hello', placeholder) -> stores { args: ['Hello'], placeholders: 1 }
// stepA('!', 'Alice') -> sees placeholder, fills with 'Alice'. Sees '!', appends it.
// Now we have args: ['Hello', 'Alice', '!']. Calls greet('Hello', 'Alice', '!') which returns "Hello, Alice!".

Example 3: (More complex placeholder usage)

const subtract = (x, y, z) => x - y - z;
const placeholder = Symbol('placeholder');

const curriedSubtract = curryWithPlaceholder(subtract, placeholder);

// Call 1: Provide only placeholders
const stepX = curriedSubtract(placeholder, placeholder);
// stepX is a function

// Call 2: Provide 'x' and 'z', filling the first placeholder ('y') with the second argument provided ('z')
const result3 = stepX(10, 5); // First placeholder ('y') gets 10, 'z' gets 5
// result3 should be -5 (10 - 10 - 5)

// Explanation:
// curriedSubtract(placeholder, placeholder) -> stores { args: [], placeholders: 2 }
// stepX(10, 5) -> Sees two placeholders. First placeholder ('y') is filled with 10. Second placeholder ('z') is filled with 5.
// Now we have args: [10, 10, 5]. Calls subtract(10, 10, 5) which returns -5.

Constraints

• The original function fn can have any number of arguments (arity).
• The placeholder value will be a single, consistently used symbol or string.
• The implementation should be efficient and avoid unnecessary overhead.
• Your curryWithPlaceholder function should be a standalone function.

Notes

• Consider how you will keep track of the arguments that have been provided and the placeholders that are still needed.
• Think about the order in which arguments should be applied when filling placeholders.
• You might find it helpful to first implement a basic currying function without placeholders, and then add placeholder functionality.
• The arguments object or rest parameters (...args) can be useful here.
• JavaScript's Function.prototype.apply() or spread syntax (...) can be used to call the original function with an array of arguments.
• When deciding on a placeholder, ensure it's a value that is unlikely to be passed as a regular argument to the original function. Using Symbol is a robust approach.