I like that the article starts with describing Functor as "something almost every developer knows about". That might be a bit of a stretch. I would say almost all developers has interacted with what could be called a functor, but knowing the concept is something different. In certain areas it is starting to get well known though.
I agree, though I think developers should be aware of type-parameters, and should be aware that if A is a subtype of B then List[A] should be considered a subtype of List[B] (which turns List into a functor on types by inclusion). In fact I reckon almost every developer is aware of these things but perhaps doesn't realise the underlying pattern.
> if A is a subtype of B then List[A] should be considered a subtype of List[B]
Well, careful -- I'm sure you know this by your name, but the naive treatment of Java's native array type as A[] <: B[] when A <: B leads to well-known soundness issues.
Fundamentally that's because you can both read and write to Java's arrays; if you assume arrays are read-only then A[] <: B[] is indeed sound.
