Compile library with GHCJS?

dnalor · July 1, 2022, 7:26am

I have a small cabal package whose functions I’d like to call from JavaScript code in the browser. All the GHCJS examples I’ve seen load a main.js in the browser. Is it instead possible to call individual Haskell functions from JavaScript? Would one use main.js to register the Haskell functions in the DOM and then call them from JavaScript code afterwards or is there a better way?

jodak · July 1, 2022, 10:03am

Perhaps you can wrap your Haskell functions in a CGI program and execute that via javascript? If your functions have output, you can write it as json. I did it like this with golang in the past.

dnalor · July 1, 2022, 12:03pm

Thanks, but running on the client (browser) is a specific requirement in this case.

nomeata · July 1, 2022, 9:20pm

Maybe have a look at how the UI of https://incredible.pm (written in plain JS) invokes the functions exposed by the HTML core, in particular this file:

github.com

nomeata/incredible/blob/d18ada4ae7ce1c7ca268c050ee688b633a307c2e/logic/js/JSInterface.hs#L33

    
      
          
          
foreign import javascript unsafe "incredibleNewRule_ = $1"
              js_set_new_rule :: Callback a -> IO ()
          
          
foreign import javascript unsafe "incredibleFormatTerm_ = $1"
              js_set_formatter :: Callback a -> IO ()
          
          
foreign import javascript unsafe "$.holdReady(false)"
              js_unblock_jquery :: IO ()
          
          
main = do
              putStr "Haskell logic core starting..."
              hFlush stdout
          
          
    callback <- syncCallback1 ThrowWouldBlock $ \jsval -> do
                  let o = Object jsval
                  rawContext <- getProp "context" o
                  rawProof <- getProp "proof" o
          
          
        context <- toContext rawContext
                  proof <-   toProof   rawProof

It’s main sets writes callbacks to the Haskell functions you want to expose to global JS variables, and then the JS code can just call them.

This is the JS code that calls these callbacks and uses a hack to return values:

github.com

nomeata/incredible/blob/master/logic/js/js-interface-wrapper.js

/*
 * GHCJS does not allow to export functions that return a value, so
 * this is a work-around-wrapper.
 *
 * TODO: Proper error reporting. How is it done usually?
 */

function incredibleLogic(context, proof) {
    var tmp = {"context": context, "proof": proof};
    incredibleLogic_(tmp);
    if (tmp.analysis) {
        return tmp.analysis;
    } else {
        return tmp.error;
    }
}

function incredibleNewRule(context, proof) {
    var tmp = {"context": context, "proof": proof};
    incredibleNewRule_(tmp);

This file has been truncated. show original

This is rather old code, so maybe there are better ways to do it now, but it worked well for me

dnalor · July 3, 2022, 3:11pm

This sort of thing is more or less what I was after. Thanks!