Well, yes. You can write a web application in Haskell and enjoy the beautiful and flexible type system, first order functions, monads and all the other wonderful things. But you shouldn't. Because when you leave, the client will never be able to find someone to maintain the damn thing and chances are they will hire someone else to rewrite it yet again in PHP. And possibly hire someone to make a voodoo doll of you and stick pins up its ass.