With Java, it's surprisingly easy to load new classes and even dynamically generated classes via a class loader. Dynamically linking with C is very easy in Windows too, which is more prevalent than Unix.

I should have qualified that my statement was referring to statically typed functional languages, though; the strategies required for runtime parametric polymorphism are quite different to those required for just compile-time parametric polymorphism, and statically typed functional languages generally encourage flow of types throughout the forest of functional expressions, by which I mean that compile-time parametric polymorphism is very much sufficient before dynamic linking and polymorphic - runtime polymorphic - APIs have been considered.

I have tried neither xmonad nor yi, though yi looks interesting. The choice of testimonials for xmonad on their site is amusing, though - "I have to say, the greatest thing about xmonad thus far is its insane stability... I have zero issues with xmonad". Stability is the absence of certain kinds of defects, not a positive feature - I for one have never suffered from e.g. lack of stability in the Windows window manager.