Type safe RPC call example from GHCJS client to Yesod/Snap/Servant

Question

I have two, related questions about RPC call to Yesod/Snap/Servant:

1. Say I have a function on the server, written in Haskell, compiled by GHC:

   add x y = x+y+42

How can:

1. I call this function from the client (the client is also written in Haskell, compiled with GHCJS),
2. execute the function on the server,
3. return the result to the client?

Do all this in a type-safe way?

So for example, I would like to write something like this on the client:

main= putStrLn $ show $ add 2 3

Such that the add function gets executed on the server.

What is the simplest way of doing this using Yesod/Snap/Servant on the server and GHCJS on the client?

2) Similarly, I would like to write a "function" : getPhaseOfMoon :: IO Phase which runs on the server and I would like to call getPhaseOfMoon from the client in a type-safe way.

So that I can write this on the client:

main= do 
   p<-getPhaseOfMoon
   putStrLn $ show p
   return ()

Which

1. goes to the server
2. the server consults the phase of the moon
3. the server returns Phase to the client
4. the client displays the phase of the moon in the JavaScript Console.

Question

How can I do this with Yesod/Snap/Servant (server) + GHCJS (client)?