Add SyntheticEvent utilities

[maddie927]

• targetValue
• bubbles
• cancelable
• currentTarget
• defaultPrevented
• eventPhase
• isTrusted
• nativeEvent
• preventDefault
• isDefaultPrevented
• stopPropagation
• isPropagationStopped
• target
• timeStamp
• type

[paf31]

I'm in favor generally, but we need to be careful about things like targetValue as we've noticed already. Just putting it in Eff seems okay, but not ideal, and I'm wondering if there's something better we can do. Ideally, we want a way to say "this event doesn't escape this scope" which seems like a job for ST/linear types, and I'm wondering if there's a lightweight way to get that without making the API unusable.

[paf31]

API sketch:

data EventFn a b = EventFn (a -> b)

instance Category EventFn

preventDefault :: EventFn SyntheticEvent SyntheticEvent
stopPropagation :: EventFn SyntheticEvent SyntheticEvent  

targetValue :: EventFn SyntheticEvent String
targetChecked :: EventFn SyntheticEvent String

Crucially, we don't export the EventFn data constructor, or any instances which let real functions creep in.

The functions for a symmetric monoidal category could be defined if we want to support extracting multiple things, or we could do something fancy with RowToList like

-- Insert RowToList magic here
smoosh 
  :: { foo :: EventFn a foo, bar :: EventFn a bar, ... }
  -> EventFn a { foo :: foo, bar :: bar, ... }

Then you could do stuff like

preventDefault >>> smoosh { targetValue, timeStamp }

and extract the value and timestamp at the same time.

[maddie927]

Awesome, thanks