Dixon graceful error handling

app/FactorizationUI.hs

@@ -42,7 +42,11 @@ dixonUI = do
     n <- askNumber "Enter an integer n > 1:"
     b <- askNumber "Enter the bound B:"
     x <- askNumber "Enter the max random integer x:"
-    let (p, q) = dixon n b x
-    putStrLn ("n = " ++ show n)
-    putStrLn ("p = " ++ show p)
-    putStrLn ("q = " ++ show q)
+
+    case dixon n b x of
+        Left err -> do
+            putStrLn ("Error: " ++ err)
+        Right (p, q) -> do
+            putStrLn ("n = " ++ show n)
+            putStrLn ("p = " ++ show p)
+            putStrLn ("q = " ++ show q)

src/Factorization/Dixon.hs

@@ -14,10 +14,10 @@ factorOverBase primes a = go primes a []
                         in go ps num' (e : acc)
 
 
-dixon :: Integer -> Integer -> Integer -> (Integer, Integer)
+dixon :: Integer -> Integer -> Integer -> Either String (Integer, Integer)
 dixon n b maxX
-  | n <= 3    = error "n must be greater than 3"
-  | even n    = (2, n `div` 2)
+  | n <= 3    = Left "n must be greater than 3"
+  | even n    = Right (2, n `div` 2)
   | otherwise =
       let base = erathosteneSieve b
           numCols = length base
@@ -33,21 +33,21 @@ dixon n b maxX
                                Just exps <- [factorOverBase base a]]
           numCollected = length pairs
       in if numCollected <= numCols
-         then error $ "Not enough smooth squares found up to " ++ show maxX
+         then Left $ "Not enough smooth squares found up to " ++ show maxX
          else let xs = map fst pairs
                   allExps = map snd pairs
                   bitvecs = map (fromBools . map (\e -> e `mod` 2 == 1)) allExps
               in case gaussianEliminationMask numCols bitvecs of
-                   Nothing -> error "No linear dependency found"
+                   Nothing -> Left "No linear dependency found"
                    Just mask ->
                      let indices = maskToIndicesInt mask
                      in if null indices
-                        then error "Empty dependency"
+                        then Left "Empty dependency"
                         else let productX = foldl (\acc idx -> modMul acc (xs !! idx) n) 1 indices
                                  fs = [ sum [ (allExps !! idx !! j) | idx <- indices ] `div` 2 | j <- [0 .. numCols - 1] ]
                                  z = foldl (\acc (p, f) -> if f == 0 then acc else modMul (modExp p f n) acc n) 1 (zip base fs)
                                  diff = productX - z
                                  g = gcd (abs diff) n
                              in if g == 1 || g == n
-                                then error "non trivial factors not found, try larger b or maximum x"
-                                else (g, n `div` g)
+                                then Left "non trivial factors not found, try larger b or maximum x"
+                                else Right (g, n `div` g)