Extending sum types with data constructors would spare runtime errors or exception control, when applying functions to inappropriate branches, as in the example ... data List a = Nil | Cons a (List a) -- List!Nil and List!Cons -- as extended types * Actual system, with runtime errors (as in GHC Data.List head) or exception throwing hd :: List a -> a hd (Cons x _) -> x hd Nil -> error "error: hd: empty list" -- error or exception throwing * Proposed system extending types with constructors as Type!Constructor: User must do pattern matching before applying the constructor-specific type function. In ''var @ (Constructor _ _)'' the compiler should append the constructor to the type as a pair (Type, Constructor) as an extended type for ''var'' No need for runtime errors or exception control hd :: List!Cons a -> a hd (Cons x _) = x using it: headOf :: List a -> Maybe a headOf list = case list of li @ (Cons _ _) -> Just hd li -- extTypeOf li == ( 'List', 'Cons') -- should pass typechecker for List!Cons li @ Nil -> Just hd li -- compiler error !! -- extTypeOf ('List','Nil') don't match _ -> Just hd list -- compiler error !! -- extTypeOf ('List',Nothing) don't match Maybe we could take out importance on the number of _ wildcards (constructor arity) with a syntax like. li @ (Cons ...) li @ (Nil ...) Cheers! Gabriel Riba Faura.