switching mode phantom

Foo.hs

{-# LANGUAGE ScopedTypeVariables #-}

class HasMode a where
  mode :: Proxy a -> Mode

instance HasMode 'BrowseMail where                                              
  mode _ = BrowseMail

-- | instantiate this class to specify a valid mode transition
class ModeTransition a b where

-- | transition from 'a' to 'a' is no transition at all
-- thus it is vacuously valid!
instance ModeTransition a a where

-- use infix
chain :: Action ctx AppState -> Action ctx a -> Action ctx a
chain (Action d1 f1) (Action d2 f2) =
  Action (d1 <> d2) (f1 >=> f2) 

-- | Chain two actions, allowing a mode tranisition (if such a transition
-- is defined). 
--
chain'
  :: forall ctx ctx' a. (HasMode ctx', ModeTranisiton ctx ctx')
  => Action ctx AppState
  -> Action ctx' a
  -> Action ctx a  -- ^ the composed action is in the ORIGINAL context, because that is the starting point!
chain' (Action d1 f1) (Action d2 f2) =
  Action (d1 <> d2) (f1 >=> switchMode >=> f2) 
  where
    switchMode = pure . set asAppMode (mode (Proxy :: Proxy ctx'))


-- this action switches to 'SearchMail, then focuses the search widget
--
switchThenFocus :: Action 'BrowseMail (Brick.Next AppState)
switchThenFocus = noop `chain'` (focus :: Action 'SearchMail AppState) `chain` continue

-- an noop action; this just returns the current 'AppState'.
-- This action can be used at the start of action chain where
-- an immediate mode switch is required.
noop :: Action ctx AppState
noop = Action "no-op" pure

@romanofski yo