You could implement monad as a class that List, Either, Writer and all the rest extended. There are practical issues: you need higher-kinded types to be able to write the monad interface, which not many OO languages have (Scala is the only one I can think of), and writing the functions as instance methods would require "F-bounded polymorphism" which is tricky ( http://logji.blogspot.co.uk/2012/11/f-bounded-type-polymorph... - that link even has an example of how you'd write Monadic as an interface you extend rather than a typeclass (don't be intimidated by the type lambda, it's a scala wart), although it doesn't compile in scala as implemented today), and you also need a way to associate the "zero" with the type in general rather than a particular instance of it (i.e. you need a static virtual method - I think maybe C# supports them?) but in principle there's no reason you couldn't do it.

(There are arguments that type classes are a more expressive way to solve these kind of N*M problems than inheritance - see https://en.wikipedia.org/wiki/Expression_problem - but that's orthogonal really)