Implement freeVars

Exercises/exercise-7/Lambda.hs

@@ -1,4 +1,4 @@
-import Data.Set (Set)
+import Data.Set (Set, empty, delete, union, singleton)
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 -- Slides
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@@ -21,6 +21,17 @@ pretty x = case x of
 -- >>> pretty (Abs "x" $ Abs "y" $ Inv (Inv Add (Var "x")) (Var "y"))
 -- "Lx.Ly.((Add x) y)"
 
+freeVars :: Term -> Set String
+freeVars x = case x of
+    (Abs param term) -> delete param (freeVars term)
+    (Inv func param) -> freeVars func `union` freeVars param
+    (Var name) -> singleton name
+    _ -> empty
+
+-- example
+-- >>> freeVars (Inv (Var "y") $ Abs "x" Add)
+-- fromList ["y"]
+
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 -- Exercise 1
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