functional.es6.js

export const _ = Symbol();

export function curry(fn) {
  return function func(...args) {
    if (countRealArguments(_, args) < fn.length) {
      return function inner(...innerArgs) {
        return this::func(...merge(_, args, innerArgs))
      }
    } else {
      return this::fn(...args);
    }
  }
}

export function compose(...fns) {
  return function(value) {
    while (fns.length) {
      const fn = fns.pop();
      value = this::fn(value);
    }
    return value;
  }
}

function countRealArguments(unreal, args) {
  return args.filter( x => !Object.is(x, unreal) ).length;
}

function merge(unreal, args1, args2) {
  
  let [args, index] = args1.reduce(([args, index], current) => {
    return [
      Object.is(current, unreal) && index < args2.length
        ? [...args, args2[index++]] 
        : [...args, current], 
      index
    ];
  }, [[], 0]);

  return [...args, ...args2.slice(index)];
}