T8044.hs:16:13: error: [GHC-83865]
    • Couldn't match type ‘Frob a’ with ‘Char’
      Expected: X (Frob a)
        Actual: X Char
    • In the expression: XChar
      In an equation for ‘frob’: frob _ = XChar
    • Relevant bindings include
        frob :: X a -> X (Frob a) (bound at T8044.hs:15:1)