Enable a isInstance method in JavaScript.Cast

[louispan]

Hi, I'm having trouble using Javascript.Cast for more complex types like React's SyntheticEvent

AFAIK there is no prototype exported by React for SyntheticEvent, so my logic for detecting if a javascript value is a SyntheticEvent is e && e.nativeEvent && e.nativeEvent instanceof Event

Unfortunately, this means I cannot use Javascript.Cast since the logic for detecting instance is hard-coded to use js_checkcast, which is:

foreign import javascript unsafe 
  "$1 instanceof $2" js_checkCast :: JSVal -> JSVal -> Bool

Would it be possible to invert the hard-coding and make the Cast class as follows?

class InstanceRef a where
  instanceRef :: a -> JSVal

class Cast a where
  unsafeWrap  :: JSVal -> a
  isInstance :: JSVal -> Maybe a
  default isInstance :: InstanceRef a => JSVal -> Maybe a
  isInstance x = js_checkCast x (instanceRef (undefined :: a))

cast :: forall a. Cast a => JSVal -> Maybe a
cast x | isInstance x = Just (unsafeWrap x)
           | otherwise     = Nothing
{-# INLINE cast #-}