Okay, sounds like a misunderstanding about what "web app" means. Sorry about that!
I think of a web app as the client side alone, which can be quite a substantial amount of code with something like GMail or Google Maps. Of course on the server side, you can make a mostly-independent choice of language, RPC mechanisms aside. But for the client side, I'd choose a language based on how well it compiles to JavaScript and can use browser API's and JavaScript libraries.
I don't know whether OCaml is ahead of Haskell in this space. I've tinkered with Elm a little bit.
I think of a web app as the client side alone, which can be quite a substantial amount of code with something like GMail or Google Maps. Of course on the server side, you can make a mostly-independent choice of language, RPC mechanisms aside. But for the client side, I'd choose a language based on how well it compiles to JavaScript and can use browser API's and JavaScript libraries.
I don't know whether OCaml is ahead of Haskell in this space. I've tinkered with Elm a little bit.