Welcome to the “Domain Modelling with Haskell” workshop, based off the screencast series from Haskell at Work. This document contains the instructions, exercises, and references for you to get started with Haskell and domain modelling, in the example domain of project management.
The properly rendered version of this document is hosted at:
https://owickstrom.github.io/domain-modelling-with-haskell-workshop/
Do not read directly at GitHub.
We will model a simple project management system, and implement reporting functionality based on our core data structures. The sections in this workshop explore increasingly abstract techniques for working with Haskell data structures. We begin with regular Haskell data structures, monoids, and explicit recursion.
If you don’t have any strong preferences on your Haskell installation, and just want to get going, it is recommended to use Stack to install the Haskell compiler, the required libraries, and compiling the code. The website has the installation instructions on the home page.
If you do install GHC and Cabal in another way, do consider using Cabal new-build, a sandbox, or some other means of isolating the build.
To get you up and running, begin by cloning this repository to your local machine:
$ git clone https://github.com/owickstrom/domain-modelling-with-haskell-workshop.git
...
$ cd domain-modelling-with-haskell-workshopThen, install all dependencies required, and build the project:
$ stack buildThe Cabal file for this project has a number of library dependencies already, so that you can have them installed beforehand, and complete the workshop without an (or with a flaky) internet connection.
All right! If all went well, you are ready to get started. You may edit Haskell files in any text editor you like. If you don’t have any strong preference, both VS Code and Atom are easy to get started with.
Setting up Haskell tooling is way out of the scope of this workshop, and you will not need anything fancy. We will load and test the code in GHCi (a REPL) anyway.
We will begin by laying the groundwork for our system. The concepts and features that we need in the project management system are described informally below. The "things" should be modelled as data types, and "operations" as functions (possibly with IO.)
Project
The core concept in the system is a project. A project can be either a single project or a project group. Both single projects and project groups have names, and single projects also have project IDs, which are natural numbers. A project group has a list of child projects.
- Note: project IDs can be wrapped in a newtype instead of
using a "raw" numeric type, making it more informative and safer to pass around in the code.
Budget
A budget describes the expected income and expenditure for a project, both being monetary values.
Transaction
A transaction is a very simplified concept describing a sale or a purchase. Each type of transaction has an amount (a monetary value.)
Get budget by project ID (query)
Given a project ID, we need to be able to retrieve a budget for the corresponding project.
- Note: To save time, hard-code or generate a random result,
instead of using a real persistent database.
Get transactions by project ID (query)
Given a project ID, we need to be able to retrieve a list of transactions for the corresponding project.
- Note: To save time, hard-code or generate a random result,
instead of using a real persistent database or querying an external system.
Report
A report represents the result of the report calculation. It has:
- a budget profit, where budget profit = income − expenditure
- a net profit, where net profit = sales − purchases, and
- a difference, where difference = net profit − budget profit.
This repository contains the source code for “Domain Modelling with Haskell,” a series of screencasts on Haskell at Work. You will find each episode’s result among these branches:
data-structuresfoldable-traversablewritertfixplate(you are here)
React
Typescript
Django
Laravel
Facebook
Microsoft
Google
Alibaba
D3
Tencent