Introduction

This application demonstrates an approach to building a business domain agnostic command interpreter that has been specialised for the energy comparison business domain.

Architecture

The application is made up of a driver, a parser, a repl and commands and formats. These components are intentionally decoupled from each other. This provides the benefits listed below:

• the driver can decide what commands and formats should form the vocabulary of the parser
• the parser can be used across different business domains with no code change
• the repl provides interactive access for the User to the parser
• the commands are simple to reason about as they are stateless and single-purpose
• the formats can be easily reused between commands to allow for different possible outputs

Driver

The purpose of the driver is to provide an entry point into the application. It creates a repl and parser and finds suitable commands and formats. The commands and formats are injected into the parser and the parser is injected into the repl to build the final application.

Parser

The purpose of the parser is to respond to queries by invoking a handler. The parser composes command-format pairs into handlers. If a query is made that can be processed by a handler, that handler delegates the processing of the query to its command and the formatting of that command's output to its format.

Repl

The purpose of the REPL is to provide the User with interactive access to the parser. It is coupled to stdin from which it consumes commands and stdout on which it produces results. It is not integrated with the parser as I want the parser to be able to be used in different ways e.g. as a command interpreter for commands received over the wire in the form of RESTful API calls.

Command

The purpose of the command is to calculate the answer to a business domain question. It is highly coupled to the business domain and its specifics. By designing it as a stateless, functional component, it becomes simple to think about, change and test.

Format

The purpose of the format is to decide how a value should be displayed. Like the command, it is a stateless, functional component that is single-purpose in nature. This makes it simple to understand, modify and write tests for.

Install Steps

• Install Node 7.8.0

Build Steps

Open up a command line

$

Install the project dependencies

$ npm install

Run Steps

Confirm application works correctly as an executable

$ chmod u+x ./bin/comparison
$ diff ./data/expected_output <(./bin/comparison ./data/plans.json < ./data/inputs)

Confirm application works correctly as a REPL

$ npm start ./data/plans.json

Test Steps

Run the tests

$ npm test

View the test results in your browser

$ open coverage/lcov-report/index.html

Technology Stack

• Node - used to write the energy plan comparison engine