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FAQ.md

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Frequently Asked Questions

How do I configure DOM events with flags like preventDefault and stopPropagation?

Using element, a low-level DOM builder function. Its second argument, ElementConfig, has a _elementConfig_eventSpec field that can be modified using addEventSpecFlags. It takes a Proxy that specifies the DomSpace, an event name, and a callback which takes the event result and returns an event flag, which can enable behavior like preventDefault and stopPropagation.

The type signature on def is needed because the fields of the ElementConfig can change the type of the ElementConfig. GHC can probably figure out the type of the resulting ElementConfig, but it doesn't know what the type of def was before the modifications, since it never gets used with that configuration.

For example:

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}

import Control.Lens ((&), (%~), (.~))
import Data.Proxy
import Reflex.Dom

main :: IO ()
main = mainWidget $ do
  (click, ()) <- linkPreventDefault $ text "reflex-frp"
  display =<< count click

linkPreventDefault :: forall t m a. DomBuilder t m => m a -> m (Event t (), a)
linkPreventDefault c = do
  let cfg = (def :: ElementConfig EventResult t (DomBuilderSpace m))
        & elementConfig_initialAttributes .~ ("href" =: "https://reflex-frp.org")
        & elementConfig_eventSpec %~ addEventSpecFlags (Proxy :: Proxy (DomBuilderSpace m)) Click (const preventDefault)
  (link, a) <- element "a" cfg c
  return (domEvent Click link, a)