made pretty printer use -> if possible

app/Expr.hs

@@ -14,6 +14,14 @@ data Expr where
     Pi :: Expr -> Expr -> Expr
     deriving (Show, Eq)
 
+occursFree :: Integer -> Expr -> Bool
+occursFree n (Var k) = n == k
+occursFree _ Star = False
+occursFree _ Square = False
+occursFree n (App a b) = occursFree n a || occursFree n b
+occursFree n (Abs a b) = occursFree n a || occursFree (n + 1) b
+occursFree n (Pi a b) = occursFree n a || occursFree (n + 1) b
+
 {- --------------------- PRETTY PRINTING ----------------------------- -}
 
 -- TODO : store parsed identifiers for better printing
@@ -33,7 +41,10 @@ pretty = helper 0
     helper k (Abs ty b) =
         "(λ" ++ genName k ++ " : " ++ helper k ty ++ " . " ++ helper (k + 1) b ++ ")"
     helper k (Pi ty b) =
-        "(∏" ++ genName k ++ " : " ++ helper k ty ++ " . " ++ helper (k + 1) b ++ ")"
+        if occursFree 0 b
+            then
+                "(∏" ++ genName k ++ " : " ++ helper k ty ++ " . " ++ helper (k + 1) b ++ ")"
+            else "(" ++ helper k ty ++ " -> " ++ helper (k + 1) b ++ ")"
 
 {- --------------- ACTUAL MATH STUFF ---------------- -}