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

johnp
Hi Brian, Looks like the api for tinydb has changed a little. Noticed while working on chapter 7 that the .purge() call to the db throws...
New
jeffmcompsci
Title: Design and Build Great Web APIs - typo “https://company-atk.herokuapp.com/2258ie4t68jv” (page 19, third bullet in URL list) Typo:...
New
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
edruder
I thought that there might be interest in using the book with Rails 6.1 and Ruby 2.7.2. I’ll note what I needed to do differently here. ...
New
leba0495
Hello! Thanks for the great book. I was attempting the Trie (chap 17) exercises and for number 4 the solution provided for the autocorre...
New
digitalbias
Title: Build a Weather Station with Elixir and Nerves: Problem connecting to Postgres with Grafana on (page 64) If you follow the defau...
New
New
dsmith42
Hey there, I’m enjoying this book and have learned a few things alredayd. However, in Chapter 4 I believe we are meant to see the “>...
New
hazardco
On page 78 the following code appears: <%= link_to ‘Destroy’, product, class: ‘hover:underline’, method: :delete, data: { confirm...
New
tkhobbes
After some hassle, I was able to finally run bin/setup, now I have started the rails server but I get this error message right when I vis...
New

Other popular topics Top

Devtalk
Hello Devtalk World! Please let us know a little about who you are and where you’re from :nerd_face:
New
ohm
Which, if any, games do you play? On what platform? I just bought (and completed) Minecraft Dungeons for my Nintendo Switch. Other than ...
New
AstonJ
Or looking forward to? :nerd_face:
485 12328 258
New
dasdom
No chair. I have a standing desk. This post was split into a dedicated thread from our thread about chairs :slight_smile:
New
AstonJ
I’ve been hearing quite a lot of comments relating to the sound of a keyboard, with one of the most desirable of these called ‘thock’, he...
New
PragmaticBookshelf
Learn different ways of writing concurrent code in Elixir and increase your application's performance, without sacrificing scalability or...
New
mafinar
Crystal recently reached version 1. I had been following it for awhile but never got to really learn it. Most languages I picked up out o...
New
First poster: joeb
The File System Access API with Origin Private File System. WebKit supports new API that makes it possible for web apps to create, open,...
New
Help
I am trying to crate a game for the Nintendo switch, I wanted to use Java as I am comfortable with that programming language. Can you use...
New
PragmaticBookshelf
Author Spotlight Mike Riley @mriley This month, we turn the spotlight on Mike Riley, author of Portable Python Projects. Mike’s book ...
New

Sub Categories: