phantom1.hs

data PrimExpr =
  BinExpr BinOp PrimExpr PrimExpr
  | UnExpr UnOp PrimExpr
  | ConstExpr String
  | IntExpr Int
  | BoolExpr Bool

data BinOp =
  OpEq
  | OpAnd
  | OpPlus

data UnOp

-- phantom types for well-formedness
data Expr a = Expr PrimExpr

(.==.) :: Eq a => Expr a -> Expr a -> Expr Bool
(.==.) x y = Expr (BoolExpr True) -- at least at the type-level.

createExprInt :: Int -> Expr Int
createExprInt x = Expr (IntExpr x)

createExprString :: String -> Expr String
createExprString s = Expr (ConstExpr s)