2017-lazy-evaluate

Lazy Evaluate

Installation and testing

Run: npm install to install testing dependences and npm test to run the code against the test spec.

Introduction

We'd like you to complete a short programming exercise in JavaScript. It shouldn't take longer than an hour to complete. Hopefully you will find the challenge fun!

• We are interested in seeing your ability to solve an algorithmic problem.

• It may be tempting to Google the answer for it if you're getting stuck. Please don't. We know there are solutions out there on the internet. Keep calm, you may find it helps to work through the data structures and how they flow through the execution of the algorithm in a simple example case.

If we like your submission and ask you in for a face-to-face interview, there will be another short pair programming exercise.

• It will be obvious if you have looked up the answer as we will ask you to extend or modify your solution in someway.

• You will get the opportunity to demonstrate your skills on a different, gentler problem. (We recognise that problem-solving under the pressure of a face-to-face interview can be significantly more challenging.)

Good Luck!

Instructions

Lazy evaluation is an evaluation strategy which delays the evaluation of an expression until its value is needed.

The challenge is for you to implement Lazy. The requirements of Lazy and the specification of its interface follows.

Lazy()

Constructor. Creates a new instance of a Lazy which models a lazy computation. A Lazy instance has two methods detailed below.

add(fn[, arg1, arg2, ...])

Adds a function to the chain of functions to be evaluated at a later stage.

• The add function can receive an arbitrary number of arguments.
• The first argument to add is a function.
• When the function is called it will be called with the remaining arguments supplied to add (if any) followed by a single argument that will be an item from the target array supplied to evaluate.
• You should be able to chain together calls to add and evaluate (see complete example below)
• Don't be defensive about the degenerate cases (E.g. bad / missing arguments)

Example usage with a simple function with no extra arguments:

lazy.add(Math.sqrt);

Example usage with function expecting several arguments and some arguments to provide when called:

lazy.add(function plus(a, b) { return a + b; }, 1);

evaluate(target)

• Returns an array containing the result of applying the chain of functions to the array target.
• The functions should be applied in the order they were added to the Lazy instance.
• Assume that add was called at least once (I.e. don't account for the case where the Lazy instance had no functions added to it).
• Don't be defensive about the degenerate cases (E.g. bad / missing arguments)

Example usage:

lazy.evaluate([ 1, 2, 3 ]);

Putting it altogether: A complete example of usage

const computation = new Lazy();

computation
  .add(function timesTwo(a) { return a * 2; })   // simple function
  .add(function plus(a, b) { return a + b; }, 1) // a plus function that will be given 1 as its first argument
  .evaluate([1, 2, 3]);  
// returns [3, 5, 7]