Implement the pretty function

Exercises/exercise-7/Lambda.hs

@@ -4,3 +4,13 @@ data Term =
     | Inv Term Term     -- EApp Invocation (a.k.a. Application) of a function
     | Var String        -- EVar a variable
     | Add
+
+pretty x = case x of
+    (Abs name term) -> "L" ++ name ++ "." ++ pretty term
+    (Inv func param) -> "(" ++ pretty func ++ " " ++ pretty param ++ ")"
+    (Var name) -> name
+    Add -> "Add"
+
+-- example
+-- >>> pretty (Abs "x" $ Abs "y" $ Inv (Inv Add (Var "x")) (Var "y"))
+-- "Lx.Ly.((Add x) y)"