StevenNunez

StevenNunez

Real-World Event Sourcing: Command Tense (page 6)

In v2 of the calculator, does it make sens for the command tenses to be present?

defmodule EventSourcedCalculator.V2 do                                              
  def handle_command(%{value: _val}, %{cmd: :add, value: v}) do                     
    %{event_type: :value_added, value: v}                                           
  end                                                                               
                                                                                    
  def handle_command(%{value: _val}, %{cmd: :sub, value: v}) do                     
    %{event_type: :value_subtracted, value: v}                                      
  end                                                                               
                                                                                    
  def handle_command(%{value: _val}, %{cmd: :mul, value: v}) do                     
    %{event_type: :value_multiplied, value: v}                                      
  end                                                                               
                                                                                    
  def handle_command(%{value: _val}, %{cmd: :div, value: v}) do                     
    %{event_type: :value_divided, value: v}                                         
  end                                                                               
                                                                                    
  def handle_event(%{value: val},                                                   
                   %{event_type: :value_added, value: v}) do                        
    %{value: val + v}                                                               
  end                                                                               
                                                                                    
  def handle_event(%{value: val},                                                   
                   %{event_type: :value_subtracted, value: v}) do                   
    %{value: val - v}                                                               
  end                                                                               
                                                                                    
  def handle_event(%{value: val},                                                   
                   %{event_type: :value_multiplied, value: v}) do                   
    %{value: val * v}                                                               
  end                                                                               
                                                                                    
  def handle_event(%{value: val},                                                   
                   %{event_type: :value_divided, value: v}) do                      
    %{value: val / v}                                                               
  end                                                                               
end 

For instance the command to add a value is value_added, however, making it add_value does 2 things. Communicates that the value has not been added yet and also allows consumers to more naturally respond to a command to add a value. It makes sense for handlers upon adding the numbers to emit value_added as an event.

Am I misunderstanding something?

First Post!

StevenNunez

StevenNunez

I wonder if a better example would have been to model the actions from a calculator.

commands = [
  %{action: "press button", value: 1},
  %{action: "press button", value: :plus},
  %{action: "press button", value: 1},
  %{action: "press button", value: :equal},
]

When you run through these commands, the event handler for a button press with the value of equal would attempt to run the calculation. All other events would build a buffer.

This isn’t perfect either though since you’d need to introduce taking input into the example with complicates things in addition to holding state.

Where Next?

Popular Pragmatic Bookshelf topics Top

telemachus
Python Testing With Pytest - Chapter 2, warnings for “unregistered custom marks” While running the smoke tests in Chapter 2, I get these...
New
jesse050717
Title: Web Development with Clojure, Third Edition, pg 116 Hi - I just started chapter 5 and I am stuck on page 116 while trying to star...
New
Alexandr
Hi everyone! There is an error on the page 71 in the book “Programming machine learning from coding to depp learning” P. Perrotta. You c...
New
sdmoralesma
Title: Web Development with Clojure, Third Edition - migrations/create not working: p159 When I execute the command: user=> (create-...
New
JohnS
I can’t setup the Rails source code. This happens in a working directory containing multiple (postgres) Rails apps. With: ruby-3.0.0 s...
New
rmurray10127
Title: Intuitive Python: docker run… denied error (page 2) Attempted to run the docker command in both CLI and Powershell PS C:\Users\r...
New
jskubick
I’m under the impression that when the reader gets to page 136 (“View Data with the Database Inspector”), the code SHOULD be able to buil...
New
nicoatridge
Hi, I have just acquired Michael Fazio’s “Kotlin and Android Development” to learn about game programming for Android. I have a game in p...
New
adamwoolhether
Is there any place where we can discuss the solutions to some of the exercises? I can figure most of them out, but am having trouble with...
New
AufHe
I’m a newbie to Rails 7 and have hit an issue with the bin/Dev script mentioned on pages 112-113. Iteration A1 - Seeing the list of prod...
New

Other popular topics Top

DevotionGeo
I know that these benchmarks might not be the exact picture of real-world scenario, but still I expect a Rust web framework performing a ...
New
PragmaticBookshelf
Design and develop sophisticated 2D games that are as much fun to make as they are to play. From particle effects and pathfinding to soci...
New
New
Margaret
Hello everyone! This thread is to tell you about what authors from The Pragmatic Bookshelf are writing on Medium.
1147 29994 760
New
mafinar
This is going to be a long an frequently posted thread. While talking to a friend of mine who has taken data structure and algorithm cou...
New
AstonJ
Was just curious to see if any were around, found this one: I got 51/100: Not sure if it was meant to buy I am sure at times the b...
New
PragmaticBookshelf
Develop, deploy, and debug BEAM applications using BEAMOps: a new paradigm that focuses on scalability, fault tolerance, and owning each ...
New
CommunityNews
A Brief Review of the Minisforum V3 AMD Tablet. Update: I have created an awesome-minisforum-v3 GitHub repository to list information fo...
New
PragmaticBookshelf
Fight complexity and reclaim the original spirit of agility by learning to simplify how you develop software. The result: a more humane a...
New
PragmaticBookshelf
A concise guide to MySQL 9 database administration, covering fundamental concepts, techniques, and best practices. Neil Smyth MySQL...
New

Sub Categories: