Functional Principles - Primitive Obsession

 


In the below example, we see dishonest signatures as part of a primitive obsession.  The method signature suggests that you input a string and it returns a user, when in fact, only strings formatted in a particular way (ie. in email format) will return a user.  It makes the programmer think that the method works with primitive strings when in reality this string represents a domain concept with its own invariants.  

In this example, primitive obsession encourages you to repeat code when more than email is added to the organisation.  If a variable requires validation, maybe it is better off as a class.


To fix this issue we can create an Email class that holds all validation.  It returns the Result class which means we expect there may be failures.



























It's a good idea to have an abstract base class for all such classes






















When inherited by the email class it would look like this:



































































With above pattern this can be simplified to this:


























Email is definitely complex enough to create a type for and so is username, just about.  MoneyAmount with just one simple check probably isn't worth it.








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