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

yulkin
your book suggests to use Image.toByteData() to convert image to bytes, however I get the following error: "the getter ‘toByteData’ isn’t...
New
mikecargal
Title: Hands-on Rust: question about get_component (page 295) (feel free to respond. “You dug you’re own hole… good luck”) I have somet...
New
Mmm
Hi, build fails on: bracket-lib = “~0.8.1” when running on Mac Mini M1 Rust version 1.5.0: Compiling winit v0.22.2 error[E0308]: mi...
New
herminiotorres
Hi! I know not the intentions behind this narrative when called, on page XI: mount() |> handle_event() |> render() but the correc...
New
gilesdotcodes
In case this helps anyone, I’ve had issues setting up the rails source code. Here were the solutions: In Gemfile, change gem 'rails' t...
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
brunogirin
When trying to run tox in parallel as explained on page 151, I got the following error: tox: error: argument -p/–parallel: expected one...
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
Henrai
Hi, I’m working on the Chapter 8 of the book. After I add add the point_offset, I’m still able to see acne: In the image above, I re...
New
mcpierce
@mfazio23 I’ve applied the changes from Chapter 5 of the book and everything builds correctly and runs. But, when I try to start a game,...
New

Other popular topics Top

PragmaticBookshelf
Ruby, Io, Prolog, Scala, Erlang, Clojure, Haskell. With Seven Languages in Seven Weeks, by Bruce A. Tate, you’ll go beyond the syntax—and...
New
AstonJ
Or looking forward to? :nerd_face:
487 12657 259
New
AstonJ
I have seen the keycaps I want - they are due for a group-buy this week but won’t be delivered until October next year!!! :rofl: The Ser...
New
AstonJ
Do the test and post your score :nerd_face: :keyboard: If possible, please add info such as the keyboard you’re using, the layout (Qw...
New
PragmaticBookshelf
Tailwind CSS is an exciting new CSS framework that allows you to design your site by composing simple utility classes to create complex e...
New
AstonJ
We’ve talked about his book briefly here but it is quickly becoming obsolete - so he’s decided to create a series of 7 podcasts, the firs...
New
New
PragmaticBookshelf
Get the comprehensive, insider information you need for Rails 8 with the new edition of this award-winning classic. Sam Ruby @rubys ...
New
AstonJ
This is cool! DEEPSEEK-V3 ON M4 MAC: BLAZING FAST INFERENCE ON APPLE SILICON We just witnessed something incredible: the largest open-s...
New
RobertRichards
Hair Salon Games for Girls Fun Girls Hair Saloon game is mainly developed for kids. This game allows users to select virtual avatars to ...
New

Sub Categories: