A generalization of NoStrings:

'''The Smell''': Primitive Obsession is using primitive data types to represent domain ideas. For example, we use a String to represent a message, an Integer to represent an amount of money, or a Struct/Dictionary/Hash to represent a specific object.

'''The Fix''': Typically, we introduce a ValueObject in place of the primitive data, then watch like magic as code from all over the system shows FeatureEnvySmell and wants to be on the new ValueObject. We move those methods, and everything becomes right with the world.

When I introduce a ValueObject, I expect it to become "attractive code", meaning literally a class/module that attracts behavior towards it as new methods/functions. For example, instead of scattering all the parsing/formatting behavior for dates stored as text, introduce a DateFormat class which attracts that behavior as methods called parse() and format(), almost like magic.  -- JbRainsberger

'''The Tools''': Some languages make this easier or harder on you. For example, in Ruby, it's easy to start with a String and then upgrade it to a proper Object, since everything is DuckType''''''d. However, even with its flexibility, it can be a good idea to AvoidHashySyntaxInRuby.
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CategoryCodeSmell, CategoryTypingDebate