curryN with placeholder support (not optimized)

curryN.js

// this file is just so I can reference the idea behind placeholders - if you want a maintained version, check out:
//   - https://github.com/thisables/curry
//   - http://ramdajs.com

const __ = Symbol

const ARITIES = {}

const setArity = (arity, fn) => {
  if (!ARITIES[arity]) {
    const params = Array(arity + 1).join(', _').substr(2)
    ARITIES[arity] = new Function(
      'fn',
      `return function (${params}) { return fn.apply(this, arguments); }`
    )
  }
  return ARITIES[arity](fn)
}

const ignoredPlaceholderLen = (params) =>
  params.reduce((len, curr) => curr === __ ? len : len + 1, 0);

const hasPlaceholder = (args) => args.some(x => x === __)

const replacePlaceholders = (initialArgs, awaitedArgs) => {
  const replacements = awaitedArgs.slice(0)
  const newArgs = initialArgs.map((arg) => {
    if (arg === __ && replacements[0]) {
      arg = replacements.shift()
    }
    return arg
  })
  return [newArgs, replacements]
}

const ncurry = (arity, fn, ...initialArgs) => {
  const len = hasPlaceholder(initialArgs) ? initialArgs.length : arity
  const _curry = (...initialArgs) => {
    const f = (...awaitedArgs) => {
      [initialArgs, awaitedArgs] = replacePlaceholders(initialArgs, awaitedArgs)
      const allArgs = initialArgs.concat(awaitedArgs)
      if (ignoredPlaceholderLen(allArgs) >= len) {
        return fn(...allArgs.slice(0, len))
      }
      return _curry(...allArgs)
    }
    return setArity(len - ignoredPlaceholderLen(initialArgs), f)
  }
  return _curry(...initialArgs)()
}

If g is a curried ternary function, then the following are equivalent:

• g(1, 2, 3)
• g(__, 2, 3)(1)
• g(__, __, 3)(1)(2)
• g(__, __, 3)(1, 2)
• g(__, 2, __)(1, 3)
• g(__, 2)(1)(3)
• g(__, 2)(1, 3)
• g(__, 2)(__, 3)(1)