mt-haskell Monad Transformers - Step By Step Play stack setup stack ghci λ: example1 Plus (Lit (-10)) (App (Abs "x" (Plus (Var "x") (Var "x"))) (App (Abs "x" (Var "x")) (Plus (Lit 2) (Lit 2)))) λ: execute example1 -10 2 2 ((Right (IntVal (-2)),["x","x","x"]),13)