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src/Research.hs

@@ -227,10 +227,18 @@ toNumber (Fork (Stem Leaf) rest) = case toNumber rest of
   Left err -> Left err
 toNumber _ = Left "Invalid Tree Calculus number"
 
+toChar :: Integer -> Either String Char
+toChar n
+  | n < 0                    = Left "Negative character code"
+  | n > 0x10FFFF             = Left "Character code out of Unicode range"
+  | n >= 0xD800 && n <= 0xDFFF = Left "Surrogate character code not allowed"
+  | otherwise                = Right (toEnum (fromInteger n))
+
 toString :: T -> Either String String
-toString tc = case toList tc of
-  Right list -> traverse (fmap (toEnum . fromInteger) . toNumber) list
-  Left _ -> Left "Invalid Tree Calculus string"
+toString tc = do
+  list <- toList tc
+  nums <- mapM toNumber list
+  mapM toChar nums
 
 toList :: T -> Either String [T]
 toList Leaf = Right []