bmccutchon/let-expressions.js

## let-expressions.js
/**
 * Workaround for the lack of Scheme-style let expressions in JavaScript.
 *
 * Usage:
 * letexp((var1 = <expr1>, var2 = <expr2>, [...] varN = <exprN>) =>
 *   <body>)
 *
 * Executes and returns the result of the expression defined in <body>. Said
 * expression can make use of the variables defined above (var1, var2, etc.). In
 * general, the variables follow the the rules of ECMAScript 6 default function
 * parameters, which they are. This function allows the programmer to build
 * complex expressions.
 */
const letexp = f => f();

/**
 * For example, here is a function to compute the length of the hypotenuse of a
 * triangle given the lengths of its legs. It uses letexp to define a helper
 * function for squaring a number while keeping the body down to one expression.
 */
const hypot = (x, y) =>
  letexp((sq = n => n*n) =>
    Math.sqrt(sq(x) + sq(y)));

/**
 * Example 2: factorial function. This emulates a Scheme idiom where a
 * tail-recursive function with an accumulator is hidden within a function that
 * supplies the correct value for the accumulator. (This is a bit contrived,
 * since it would be better to write factAcc at the top level, then do `export
 * fact = n => factAcc(n, 1)`.)
 */
const fact = n =>
  letexp((factAcc = (n, acc) =>
            n === 1 ? acc : factAcc(n - 1, acc * n)) =>
    factAcc(n, 1));
	/**
	* Workaround for the lack of Scheme-style let expressions in JavaScript.
	*
	* Usage:
	* letexp((var1 = <expr1>, var2 = <expr2>, [...] varN = <exprN>) =>
	* <body>)
	*
	* Executes and returns the result of the expression defined in <body>. Said
	* expression can make use of the variables defined above (var1, var2, etc.). In
	* general, the variables follow the the rules of ECMAScript 6 default function
	* parameters, which they are. This function allows the programmer to build
	* complex expressions.
	*/
	const letexp = f => f();

	/**
	* For example, here is a function to compute the length of the hypotenuse of a
	* triangle given the lengths of its legs. It uses letexp to define a helper
	* function for squaring a number while keeping the body down to one expression.
	*/
	const hypot = (x, y) =>
	letexp((sq = n => n*n) =>
	Math.sqrt(sq(x) + sq(y)));

	/**
	* Example 2: factorial function. This emulates a Scheme idiom where a
	* tail-recursive function with an accumulator is hidden within a function that
	* supplies the correct value for the accumulator. (This is a bit contrived,
	* since it would be better to write factAcc at the top level, then do `export
	* fact = n => factAcc(n, 1)`.)
	*/
	const fact = n =>
	letexp((factAcc = (n, acc) =>
	n === 1 ? acc : factAcc(n - 1, acc * n)) =>
	factAcc(n, 1));