How to call Haskell from Javascript with GHCJS

Question

I\'ve been playing about with GHCJS. The FFI can be used to call javascript from Haskell but I can\'t figure out how do go the other way round. Say I had a super useful utility

Answer 1

Here's an example that shows how to call a Haskell function from Javascript. This is similar to the example provided by Joachim but compiles and runs with the latest ghcjs.

import GHCJS.Marshal(fromJSVal)
import GHCJS.Foreign.Callback (Callback, syncCallback1, OnBlocked(ContinueAsync))
import Data.JSString (JSString, unpack, pack)
import GHCJS.Types (JSVal)

sayHello :: String -> IO ()
sayHello name = print $ "hello, " ++ name

sayHello' :: JSVal -> IO ()
sayHello' jsval = do
    Just str <- fromJSVal jsval
    sayHello $ unpack str

foreign import javascript unsafe "js_callback_ = $1"
    set_callback :: Callback a -> IO ()

foreign import javascript unsafe "js_callback_($1)" 
    test_callback :: JSString -> IO ()

main = do
    callback <- syncCallback1 ContinueAsync sayHello'
    set_callback callback
    test_callback $ pack "world"

The test works by calling from Haskell into Javascript code that then calls back into Haskell. The variable, "js_callback_", becomes available within Javascript for use as a function that takes one string argument.