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 is a way to make it work. Assume we have some useful function, like

revString :: String -> String
revString = reverse

somethingUseful :: JSString -> IO JSString
somethingUseful = return . toJSString . revString  . fromJSString

In order to export that, we need to make it a callback via one of the *Callback functions in GHCJS.Foreign. But these would discard the return value, so we need a wrapper that puts the result into a second argument:

returnViaArgument :: (JSRef a -> IO (JSRef b)) -> JSRef a -> JSRef c -> IO ()
returnViaArgument f arg retObj = do
    r <- f arg
    setProp "ret" r retObj

My main function creates the callback, and saves it as something that’s global to JavaScript:

foreign import javascript unsafe "somethingUseful_ = $1"
    js_set_somethingUseful :: JSFun a -> IO ()

main = do
    callback <- syncCallback2 NeverRetain False (returnViaArgument somethingUseful)
    js_set_somethingUseful callback

Finally, we need a little un-wrapper on the JS side:

function somethingUseful (arg) {x = {}; somethingUseful_(arg, x); return x.ret};

and now we can use our nice Haskell-implemented function:

somethingUseful("Hello World!")
"!dlroW olleH"

I am using this trick in a real-world application. In JsInterface.hs, which is defined as main-in of the executable in the Cabal file, the main function sets the global java script variable incredibleLogic_, while the JavaScript glue code takes care of packing and unpacking the parameters.