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

New
brian-m-ops
#book-python-testing-with-pytest-second-edition Hi. Thanks for writing the book. I am just learning so this might just of been an issue ...
New
jskubick
I’m running Android Studio “Arctic Fox” 2020.3.1 Patch 2, and I’m embarrassed to admit that I only made it to page 8 before running into ...
New
jgchristopher
“The ProductLive.Index template calls a helper function, live_component/3, that in turn calls on the modal component. ” Excerpt From: Br...
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
mert
AWDWR 7, page 152, page 153: Hello everyone, I’m a little bit lost on the hotwire part. I didn’t fully understand it. On page 152 @rub...
New
jwandekoken
Book: Programming Phoenix LiveView, page 142 (157/378), file lib/pento_web/live/product_live/form_component.ex, in the function below: d...
New
Keton
When running the program in chapter 8, “Implementing Combat”, the printout Health before attack was never printed so I assumed something ...
New
dtonhofer
@parrt In the context of Chapter 4.3, the grammar Java.g4, meant to parse Java 6 compilation units, no longer passes ANTLR (currently 4....
New
redconfetti
Docker-Machine became part of the Docker Toolbox, which was deprecated in 2020, long after Docker Desktop supported Docker Engine nativel...
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
New
AstonJ
This looks like a stunning keycap set :orange_heart: A LEGENDARY KEYBOARD LIVES ON When you bought an Apple Macintosh computer in the e...
New
PragmaticBookshelf
Learn different ways of writing concurrent code in Elixir and increase your application's performance, without sacrificing scalability or...
New
PragmaticBookshelf
Use WebRTC to build web applications that stream media and data in real time directly from one user to another, all in the browser. ...
New
PragmaticBookshelf
Author Spotlight: Peter Ullrich @PJUllrich Data is at the core of every business, but it is useless if nobody can access and analyze ...
New
PragmaticBookshelf
Build modern server-driven web applications using htmx. Whatever programming language you use, you’ll write less (and cleaner) code. ...
New
NewsBot
Node.js v22.14.0 has been released. Link: Release 2025-02-11, Version 22.14.0 'Jod' (LTS), @aduh95 · nodejs/node · GitHub
New
Fl4m3Ph03n1x
Background Lately I am in a quest to find a good quality TTS ai generation tool to run locally in order to create audio for some videos I...
New
mindriot
Ok, well here are some thoughts and opinions on some of the ergonomic keyboards I have, I guess like mini review of each that I use enoug...
New

Sub Categories: