Part3.md

Part 3 - Features and Redesign

Chapter 8 - Evaluating a Portfolio

Money itself isn't lost or made. It's simply transfered from one perception to another. Like magic. - Gordon Gekko, Wall Street

Mixing Money

Heterogeneous combination of currencies demands that we create a new abstraction : conversion of money from one currency to another :

- Conversion always relates a pair of currencies
- Conversion is from one currency to another with a well-defined exchange rate
- The two exchange rates between a pair of currencies may or may not be arithmetical reciprocals of each other
    - Exchange Rate from EUR to USD may or may not be the mathematical reciprocal of USD to EUR
- It is possible for a currency to have no defined exchange rate to another currency
    - Inconvertible currencies : economical, political, ... reasons    

• Add our next test 5 USD + 10 EUR = 17 USD
  • What do we need for that ?
• Make the minimum to pass the tests
• Remove redundancy
  • How could we improve readability of our tests ?

Where we are

✅ 5 USD x 2 = 10 USD 
✅ 10 EUR x 2 = 20 EUR
✅ 4002 KRW / 4 = 1000.5 KRW
✅ 5 USD + 10 USD = 15 USD
✅ Separate test code from production code
✅ Remove redundant tests
✅ 5 USD + 10 EUR = 17 USD
1 USD + 1100 KRW = 2200 KRW
Determine exchange rate based on the currencies involved (from -> to)
Allow exchange rates to be modified

step by step guide

Chapter 9 - Currencies, Currencies, Everywhere

1 USD + 1100 KRW = 2200 KRW
Determine exchange rate based on the currencies involved (from -> to)

• We can use this table to determine exchange rates :

From	To	Rate
EUR	USD	1.2
USD	EUR	0.82
USD	KRW	1100
KRW	EUR	0.0009
EUR	KRW	1344
KRW	EUR	0.00073

• Let's write our next test 1 USD + 1100 KRW = 2200 KRW
• How could make it pass ?
• What happens if we try to evaluate in a currency without exchangeRates ?
  • How to improve it ?

Where we are

✅ 5 USD x 2 = 10 USD 
✅ 10 EUR x 2 = 20 EUR
✅ 4002 KRW / 4 = 1000.5 KRW
✅ 5 USD + 10 USD = 15 USD
✅ Separate test code from production code
✅ Remove redundant tests
✅ 5 USD + 10 EUR = 17 USD
✅ 1 USD + 1100 KRW = 2200 KRW
✅ Determine exchange rate based on the currencies involved (from -> to)
Improve error handling when exchange rates are unspecified
Allow exchange rates to be modified

step by step guide

Chapter 10 - Error Handling

What error drives our eyes and ears amiss ? - William Shakespeare

Error Wish List

- The Evaluate method should signal an explicit error when one or more necessary exchange rates ares missing
- The error message should be "greedy" - indicate all the missing exchange rates
- To prevent error from being ignored by the caller : no valid Money should be returned when an error happens due to missing exchange rates

• We will use exception here in case of failure
  • How could we implement it differently ?
• Refactor : what can be improved ?

Where we are

• We have added error handling
  • Portfolio evaluation is not simple anymore
    • Clumsy code to check if we have missing exchange rates
  • Let's add a new feature in our list

✅ 5 USD x 2 = 10 USD 
✅ 10 EUR x 2 = 20 EUR
✅ 4002 KRW / 4 = 1000.5 KRW
✅ 5 USD + 10 USD = 15 USD
✅ Separate test code from production code
✅ Remove redundant tests
✅ 5 USD + 10 EUR = 17 USD
✅ 1 USD + 1100 KRW = 2200 KRW
✅ Determine exchange rate based on the currencies involved (from -> to)
✅ Improve error handling when exchange rates are unspecified
Improve the implementation of exchange rates
Allow exchange rates to be modified

step by step guide

Chapter 11 - Banking on Redesign

On the whole it's worth evolving your design as your needs grow... - Martin Fowler

• Our Portfolio does too much work
  • Its primary job is to be a repository of Money entities
  • Not to manage Exchange Rates
• Our software program has grown with our needs
  • It is worth improving our design and looking for a better abstraction
• A principle of Domain Driven Design is continuous learning
  • We are missing a key entity
  • What is the name of the real-world institution that helps us exchange money ?

Bank concept

• What should be its responsibility ?
  • Hold exchange rates
  • Convert money between currencies

Dependency Injection

• We have identified a new entity
  • How should the dependencies between Bank and the other two existing entities look ?
  • The dependency of Portfolio on Bank is kept to a minimum
    • Bank is provided as a parameter to the Evaluate method
    • We do method injection

Putting It All Together

• Let's write a test to convert one Money object into another in the Bank entity
• We write the minimum to pass the test
• All our tests are green
  • We are ready to change our Portfolio.Evaluate method
    • What is the impact on our existing tests ?
• We now run the tests
  • Does it find any regression ?
  • If yes, add a test on it to ensure the regression will never come back
• Every tests are now green
  • Let's commit our work
• Refactor
  • Do you have any duplication ?
  • What can be improved ?

Use a more functional approach

• The rule : do not have try/catch for under control behaviors
• Add nuget package LanguageExt.Core for this purpose
  • Use the Either monadic structure to represent the failure
  • Adapt your tests then your production code
• Remove duplication and dead code (no more Exception required)

Where we are

• We changed internal organization of our code
• We used a combination of new tests and our existing suite of tests to ensure no features we harmed during this writing
• We cleaned up our tests as well
• We have only one feature remaining to implement :

✅ 5 USD x 2 = 10 USD 
✅ 10 EUR x 2 = 20 EUR
✅ 4002 KRW / 4 = 1000.5 KRW
✅ 5 USD + 10 USD = 15 USD
✅ Separate test code from production code
✅ Remove redundant tests
✅ 5 USD + 10 EUR = 17 USD
✅ 1 USD + 1100 KRW = 2200 KRW
✅ Determine exchange rate based on the currencies involved (from -> to)
✅ Improve error handling when exchange rates are unspecified
✅ Improve the implementation of exchange rates
Allow exchange rates to be modified

step by step guide