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{-# LANGUAGE RankNTypes, KindSignatures, MultiParamTypeClasses, FunctionalDependencies #-}
 
module Test where 
 
import Prelude hiding (div)
 
class MonadWidget t (m :: * -> *) | m -> t 
 
-- div :: forall t m a . MonadWidget t m => m a -> m a
div = (undefined :: forall t m a . MonadWidget t m => m a -> m a)
 
app :: forall t m . MonadWidget t m => m ()
app = (div (undefined :: forall t m . MonadWidget t m => m ())
        :: forall t m . MonadWidget t m => m () )

[1 of 1] Compiling Test             ( prog.hs, prog.o )

prog.hs:10:8:
    No instance for (MonadWidget t0 m0)
      arising from an expression type signature
    In the expression:
      (undefined :: forall t m a. MonadWidget t m => m a -> m a)
    In an equation for `div':
        div = (undefined :: forall t m a. MonadWidget t m => m a -> m a)

prog.hs:13:8:
    Couldn't match type `m0' with `m1'
      because type variable `m1' would escape its scope
    This (rigid, skolem) type variable is bound by
      an expression type signature: MonadWidget t2 m1 => m1 ()
      at prog.hs:(13,8)-(14,47)
    Expected type: m1 ()
      Actual type: m0 ()
    In the expression:
      (div (undefined :: forall t m. MonadWidget t m => m ()) ::
         forall t m. MonadWidget t m => m ())
    In an equation for `app':
        app
          = (div (undefined :: forall t m. MonadWidget t m => m ()) ::
               forall t m. MonadWidget t m => m ())

Standard output is empty