In reflex-dom, how to pass a value from a dynamic function to external javascript (FFI)

Question

I am new to Haskell and reflex-dom, but really love the language. I used https://github.com/hansroland/reflex-dom-inbits/blob/master/tutorial.md and it was very useful.

I'm currently trying to create a function that takes a dynamic and creates an element, and calls the FFI function every time the value changes in the dynamics. Here is a simplified version of what I'm trying to do.

{-# LANGUAGE OverloadedStrings #-}
import Data.Text as T
import qualified GHCJS.DOM.Types as GDT
import GHCJS.Types
import Reflex.Dom

foreign import javascript safe
  "$1.value = $2"
  testSet :: JSVal -> JSVal -> IO()

testTB :: DomBuilder t m => Dynamic t T.Text -> m ()
testTB dt = do
  (e, _) <- elAttr' "input" ("type" =: "text") blank
  bob <- (testSet (GDT.pToJSVal e) . GDT.pToJSVal) <$> dt
  return ()

main = mainWidget $ testTB $ constDyn "Hello World!"

As a result, a compile-time error occurs:

reflex-canvas.hs:14:10: error:
    • Couldn't match type ‘m’ with ‘Dynamic t’
      ‘m’ is a rigid type variable bound by
        the type signature for:
          testTB :: forall t (m :: * -> *).
                    DomBuilder t m =>
                    Dynamic t Text -> m ()
        at reflex-canvas.hs:11:11
      Expected type: m (IO ())
        Actual type: Dynamic t (IO ())
    • In a stmt of a 'do' block:
        bob <- (testSet (GDT.pToJSVal e) . GDT.pToJSVal) <$> dt
      In the expression:
        do { (e, _) <- elAttr' "input" ("type" =: "text") blank;
             bob <- (testSet (GDT.pToJSVal e) . GDT.pToJSVal) <$> dt;
             return () }
      In an equation for ‘testTB’:
          testTB dt
            = do { (e, _) <- elAttr' "input" ("type" =: "text") blank;
                   bob <- (testSet (GDT.pToJSVal e) . GDT.pToJSVal) <$> dt;
                   return () }
    • Relevant bindings include
        e :: Element EventResult (DomBuilderSpace m) t
          (bound at reflex-canvas.hs:13:4)
        dt :: Dynamic t Text (bound at reflex-canvas.hs:12:8)
        testTB :: Dynamic t Text -> m () (bound at reflex-canvas.hs:12:1)

I tried various ways to convert Dynamic to m (), but can't figure it out. What is the best practice for this?

Answer 1

performEvent_ javascript, performEvent_ Event t (WidgetHost m ()), , , Dynamic t (IO ()).

updated Dynamic t (IO ()) Event t (IO ()), fmap liftIO, IO () Event WidgetHost m (), Event t (WidgetHost m ()), performEvent_

. testSet testTB, /. . , .

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}
import Data.Text as T (Text)
import qualified GHCJS.DOM.Types as GDT (pToJSVal)
import GHCJS.Types (JSVal)
import Reflex.Dom
import Control.Monad.Trans (liftIO)

foreign import javascript safe
  "console.log $1"
  testSet :: JSVal -> IO()

testTB :: forall t m.  MonadWidget t m => Dynamic t T.Text -> m ()
testTB dt = do 
    let bob :: Dynamic t (IO ())
        bob = (testSet.(GDT.pToJSVal)) <$> dt  

        bobIOEvent :: Event t (IO ())
        bobIOEvent = updated bob

        bobWidgetHostEvent :: Event t (WidgetHost m ())
        bobWidgetHostEvent = fmap liftIO bobIOEvent

    performEvent_ bobWidgetHostEvent

main = mainWidget $ do
    ti <- textInput def 
    let dt = value ti
    testTB dt