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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// Disclaimer: Some of the functions in this module implement APIs from
+// Jeremy Ashkenas's http://underscorejs.org/ library and all credits for
+// those goes to him.
+
+"use strict";
+
+module.metadata = {
+  "stability": "unstable"
+}
+const { arity, name, derive, invoke } = require("./helpers");
+
+/**
+ * Takes variadic numeber of functions and returns composed one.
+ * Returned function pushes `this` pseudo-variable to the head
+ * of the passed arguments and invokes all the functions from
+ * left to right passing same arguments to them. Composite function
+ * returns return value of the right most funciton.
+ */
+const method = (...lambdas) => {
+  return function method(...args) {
+    args.unshift(this);
+    return lambdas.reduce((_, lambda) => lambda.apply(this, args),
+                          void(0));
+  };
+};
+exports.method = method;
+
+/**
+ * Invokes `callee` by passing `params` as an arguments and `self` as `this`
+ * pseudo-variable. Returns value that is returned by a callee.
+ * @param {Function} callee
+ *    Function to invoke.
+ * @param {Array} params
+ *    Arguments to invoke function with.
+ * @param {Object} self
+ *    Object to be passed as a `this` pseudo variable.
+ */
+exports.invoke = invoke;
+
+/**
+ * Takes a function and bind values to one or more arguments, returning a new
+ * function of smaller arity.
+ *
+ * @param {Function} fn
+ *    The function to partial
+ *
+ * @returns The new function with binded values
+ */
+const partial = (f, ...curried) => {
+  if (typeof(f) !== "function")
+    throw new TypeError(String(f) + " is not a function");
+
+  let fn = derive(function(...args) {
+    return f.apply(this, curried.concat(args));
+  }, f);
+  fn.arity = arity(f) - curried.length;
+  return fn;
+};
+exports.partial = partial;
+
+/**
+ * Returns function with implicit currying, which will continue currying until
+ * expected number of argument is collected. Expected number of arguments is
+ * determined by `fn.length`. Using this with variadic functions is stupid,
+ * so don't do it.
+ *
+ * @examples
+ *
+ * var sum = curry(function(a, b) {
+ *   return a + b
+ * })
+ * console.log(sum(2, 2)) // 4
+ * console.log(sum(2)(4)) // 6
+ */
+const curry = new function() {
+  const currier = (fn, arity, params) => {
+    // Function either continues to curry arguments or executes function
+    // if desired arguments have being collected.
+    const curried = function(...input) {
+      // Prepend all curried arguments to the given arguments.
+      if (params) input.unshift.apply(input, params);
+      // If expected number of arguments has being collected invoke fn,
+      // othrewise return curried version Otherwise continue curried.
+      return (input.length >= arity) ? fn.apply(this, input) :
+             currier(fn, arity, input);
+    };
+    curried.arity = arity - (params ? params.length : 0);
+
+    return curried;
+  };
+
+  return fn => currier(fn, arity(fn));
+};
+exports.curry = curry;
+
+/**
+ * Returns the composition of a list of functions, where each function consumes
+ * the return value of the function that follows. In math terms, composing the
+ * functions `f()`, `g()`, and `h()` produces `f(g(h()))`.
+ * @example
+ *
+ *   var greet = function(name) { return "hi: " + name; };
+ *   var exclaim = function(statement) { return statement + "!"; };
+ *   var welcome = compose(exclaim, greet);
+ *
+ *   welcome('moe');    // => 'hi: moe!'
+ */
+function compose(...lambdas) {
+  return function composed(...args) {
+    let index = lambdas.length;
+    while (0 <= --index)
+      args = [lambdas[index].apply(this, args)];
+
+    return args[0];
+  };
+}
+exports.compose = compose;
+
+/*
+ * Returns the first function passed as an argument to the second,
+ * allowing you to adjust arguments, run code before and after, and
+ * conditionally execute the original function.
+ * @example
+ *
+ *  var hello = function(name) { return "hello: " + name; };
+ *  hello = wrap(hello, function(f) {
+ *    return "before, " + f("moe") + ", after";
+ *  });
+ *
+ *  hello();    // => 'before, hello: moe, after'
+ */
+const wrap = (f, wrapper) => derive(function wrapped(...args) {
+  return wrapper.apply(this, [f].concat(args));
+}, f);
+exports.wrap = wrap;
+
+/**
+ * Returns the same value that is used as the argument. In math: f(x) = x
+ */
+const identity = value => value;
+exports.identity = identity;
+
+/**
+ * Memoizes a given function by caching the computed result. Useful for
+ * speeding up slow-running computations. If passed an optional hashFunction,
+ * it will be used to compute the hash key for storing the result, based on
+ * the arguments to the original function. The default hashFunction just uses
+ * the first argument to the memoized function as the key.
+ */
+const memoize = (f, hasher) => {
+  let memo = Object.create(null);
+  let cache = new WeakMap();
+  hasher = hasher || identity;
+  return derive(function memoizer(...args) {
+    const key = hasher.apply(this, args);
+    const type = typeof(key);
+    if (key && (type === "object" || type === "function")) {
+      if (!cache.has(key))
+        cache.set(key, f.apply(this, args));
+      return cache.get(key);
+    }
+    else {
+      if (!(key in memo))
+        memo[key] = f.apply(this, args);
+      return memo[key];
+    }
+  }, f);
+};
+exports.memoize = memoize;
+
+/*
+ * Creates a version of the function that can only be called one time. Repeated
+ * calls to the modified function will have no effect, returning the value from
+ * the original call. Useful for initialization functions, instead of having to
+ * set a boolean flag and then check it later.
+ */
+const once = f => {
+  let ran = false, cache;
+  return derive(function(...args) {
+    return ran ? cache : (ran = true, cache = f.apply(this, args));
+  }, f);
+};
+exports.once = once;
+// export cache as once will may be conflicting with event once a lot.
+exports.cache = once;
+
+// Takes a `f` function and returns a function that takes the same
+// arguments as `f`, has the same effects, if any, and returns the
+// opposite truth value.
+const complement = f => derive(function(...args) {
+  return args.length < arity(f) ? complement(partial(f, ...args)) :
+         !f.apply(this, args);
+}, f);
+exports.complement = complement;
+
+// Constructs function that returns `x` no matter what is it
+// invoked with.
+const constant = x => _ => x;
+exports.constant = constant;
+
+// Takes `p` predicate, `consequent` function and an optional
+// `alternate` function and composes function that returns
+// application of arguments over `consequent` if application over
+// `p` is `true` otherwise returns application over `alternate`.
+// If `alternate` is not a function returns `undefined`.
+const when = (p, consequent, alternate) => {
+  if (typeof(alternate) !== "function" && alternate !== void(0))
+    throw TypeError("alternate must be a function");
+  if (typeof(consequent) !== "function")
+    throw TypeError("consequent must be a function");
+
+  return function(...args) {
+    return p.apply(this, args) ?
+           consequent.apply(this, args) :
+           alternate && alternate.apply(this, args);
+  };
+};
+exports.when = when;
+
+// Apply function that behaves as `apply` does in lisp:
+// apply(f, x, [y, z]) => f.apply(f, [x, y, z])
+// apply(f, x) => f.apply(f, [x])
+const apply = (f, ...rest) => f.apply(f, rest.concat(rest.pop()));
+exports.apply = apply;
+
+// Returns function identical to given `f` but with flipped order
+// of arguments.
+const flip = f => derive(function(...args) {
+  return f.apply(this, args.reverse());
+}, f);
+exports.flip = flip;
+
+// Takes field `name` and `target` and returns value of that field.
+// If `target` is `null` or `undefined` it would be returned back
+// instead of attempt to access it's field. Function is implicitly
+// curried, this allows accessor function generation by calling it
+// with only `name` argument.
+const field = curry((name, target) =>
+  // Note: Permisive `==` is intentional.
+  target == null ? target : target[name]);
+exports.field = field;
+
+// Takes `.` delimited string representing `path` to a nested field
+// and a `target` to get it from. For convinience function is
+// implicitly curried, there for accessors can be created by invoking
+// it with just a `path` argument.
+const query = curry((path, target) => {
+  const names = path.split(".");
+  const count = names.length;
+  let index = 0;
+  let result = target;
+  // Note: Permisive `!=` is intentional.
+  while (result != null && index < count) {
+    result = result[names[index]];
+    index = index + 1;
+  }
+  return result;
+});
+exports.query = query;
+
+// Takes `Type` (constructor function) and a `value` and returns
+// `true` if `value` is instance of the given `Type`. Function is
+// implicitly curried this allows predicate generation by calling
+// function with just first argument.
+const isInstance = curry((Type, value) => value instanceof Type);
+exports.isInstance = isInstance;
+
+/*
+ * Takes a funtion and returns a wrapped function that returns `this`
+ */
+const chainable = f => derive(function(...args) {
+  f.apply(this, args);
+  return this;
+}, f);
+exports.chainable = chainable;
+
+// Functions takes `expected` and `actual` values and returns `true` if
+// `expected === actual`. Returns curried function if called with less then
+// two arguments.
+//
+// [ 1, 0, 1, 0, 1 ].map(is(1)) // => [ true, false, true, false, true ]
+const is = curry((expected, actual) => actual === expected);
+exports.is = is;
+
+const isnt = complement(is);
+exports.isnt = isnt;