function(){
     var handler = function( val ){
         return cb( val )()
     }
     target.on( eventName, handler )
     return function(){ 
         console.log( "Detached" ) 
     }
}

class Foo a b where
    test :: a -> b


type Test a b =
    Foo a b => { x :: List a }


type TestWrap a b =
    { wrap :: Test a b
    }


func :: forall a b. Foo a b => TestWrap a b -> List a
func { wrap } =
    wrap.x

  Could not match constrained type

    Foo a0 b3 => { x :: List a0
                 }

  with type

    { x :: List a0
    | t1
    }

func :: forall a b. Foo a b => Test a b -> List a
func { x } = x

class Foo a where
    test :: a

type Test a =
    { x :: Foo a => List a }

type TestWrap a =
    { wrap :: Test a
    }

func :: forall a. Foo a => TestWrap a -> List a
func { wrap } =
    wrap.x