Basically it's advocating removing if/switch statements and replacing them with polymorphic method calls. I understand that polymorphism has its value, but think that it's only valuable when, for lack of a better phrase, the thing that's being polymorphosed is "big and varied" enough that it makes sense to impose this extra level of indirection. I think that it being hard to explain when something is worth it, is enough justification that principles shouldn't be arbitrarily decided based on that.
However, if you already have a class type for each key
I think that's precisely what I was trying to say - polymorphism makes sense when you already have a bunch of classes to do it with, and classes which already contain lots of other fields and methods; it doesn't make sense to create a bunch of classes just to use polymorphism.
Often, though, a series of complicated if statements is hinting at a type system for your objects that hasn't yet materialized in your code. I find it's a good idea to always look at cascading if statements and switch statements and ask, "Would this be cleaner if I reified these concepts as types?"
This is the single most important trick for factoring out shitty code. I cannot believe how many times reification collapsed complexity in our code base, or how not using it was the source of bugs.
If you have cascading ifs, there is a good chance there is a huge set of ifs for every place this type system is missing. Meaning, if you wanted to add another "case" to a feature, you are modifying cascading ifs in 5-10 places, not even just one.
Wrapping up all of this code into an implementation of an interface, that is "hooked in" at each contact point allows you to add a cohesive new use case by generating a new implementation of an interface, instead of "forgetting 2 places in the code" and causing massive bugs or issues because of it.
If it is sensible to call the same function with them as the same parameter, the objects are not unrelated. There is a very real and relevant-to-the-application sense in which they share a common type. (Now, if they aren't sharing implementation -- which they clearly aren't in at least one relevant way if you are avoiding an "if" statement by putting them in a class heirarchy -- then it probably makes more sense for them to implement a common interface rather than being subclasses of the same class, in a language which distinguishes those rather than using multiple inheritance class heirarchies plus method overloading to serve the role of interfaces.)
That reminds me of this, not sure if it's satirical or not: http://www.antiifcampaign.com/
Basically it's advocating removing if/switch statements and replacing them with polymorphic method calls. I understand that polymorphism has its value, but think that it's only valuable when, for lack of a better phrase, the thing that's being polymorphosed is "big and varied" enough that it makes sense to impose this extra level of indirection. I think that it being hard to explain when something is worth it, is enough justification that principles shouldn't be arbitrarily decided based on that.