First of all, a Ruby developer would need to be convinced of the value of static types, in his/her domain. For the maintenence of large complex systems, they are invaluable, for web development perhaps less so. Advanced type systems simply allow one to statically verify more invariants, or in the case of higher-kinded types, allow one to make more sophisticated abstractions and reduce boilerplate. It's important to remember that every statically typed language is capable of being dynamically typed also. I can use string-keyed dictionaries and runtime tagged types in Haskell if I ever need to. What you loose is ease-of-use, which I guess is why Ruby is more popular than Haskell.
After a while using Haskell, I'm not sure about that anymore. I keep thinking that an "easier than rails" web framework is just one breakthrough by some random developer somewhere on the Internet away.
Even if I can't imagine the form such library would take, the features I see every day are so powerful that it just look possible.
