Consider the following Haskell datatypes representing sugared and unsugared calculus terms. data PureLC = Lambda String PureLC | App PureLC PureLC | Var String deriving Show data SugaredLC = LambdaS [String] SugaredLC | AppS [SugaredLC] | VarS String deriving Show Write fv::PureLC[String]desugar::SugaredLCPureLC which finds a list of the free variables in a pure calculus term, and de-sugars a sugared calculus term, respectively. So for example Prelude> desugar (LambdaS ["f", "x"] (AppS [(VarS "f"), AppS [Vars "t", Vars "g", Vars "g"]])) Lambda "f" (Lambda "x" (App (Var "f") (App (App (Var "t") (Var "g")) (Var "g"))))) Prelude>fv (Lambda "f" (Lambda "x" (App (Var "f") (App (App (Var "t") (Var "g")) (Var "g"))))) [ "t","g"]