:5:1: error: • Illegal generalised algebraic data declaration for ‘T’ (Enable the GADTs extension to allow this) • In the data declaration for ‘T’ ghci057.hs:4:3: error: • Data constructor ‘C’ has existential type variables, a context, or a specialised result type C :: T Int (Enable ExistentialQuantification or GADTs to allow this) • In the definition of data constructor ‘C’ In the data type declaration for ‘T’ ghci057.hs:4:3: error: • Data constructor ‘C’ has existential type variables, a context, or a specialised result type C :: T Int (Enable ExistentialQuantification or GADTs to allow this) • In the definition of data constructor ‘C’ In the data type declaration for ‘T’