Haskell - Expressions

Este código es más feo que pegarle a un padre con un calcetín sudado lleno de clavos.

LP/Víctor/Haskell/3-5_P70540.hs

@@ -0,0 +1,52 @@
+data Expr = Val Int | Add Expr Expr | Sub Expr Expr | Mul Expr Expr | Div Expr Expr
+
+eval1 :: Expr -> Int
+eval1 (Val n) = n
+eval1 (Add a b) = eval1 a   +   eval1 b
+eval1 (Sub a b) = eval1 a   -   eval1 b
+eval1 (Mul a b) = eval1 a   *   eval1 b
+eval1 (Div a b) = eval1 a `div` eval1 b
+
+eval2 :: Expr -> Maybe Int
+eval2 (Val n)   = Just n
+eval2 (Add a b) = case (eval2 a, eval2 b) of
+    (Just a, Just b) -> Just (a+b)
+    (Nothing, _)     -> Nothing
+    (_, Nothing)     -> Nothing
+eval2 (Sub a b) = case (eval2 a, eval2 b) of
+    (Just a, Just b) -> Just (a-b)
+    (Nothing, _)     -> Nothing
+    (_, Nothing)     -> Nothing
+eval2 (Mul a b) = case (eval2 a, eval2 b) of
+    (Just a, Just b) -> Just (a*b)
+    (Nothing, _)     -> Nothing
+    (_, Nothing)     -> Nothing
+eval2 (Div a b) = case (eval2 a, eval2 b) of
+    (Just a, Just 0) -> Nothing
+    (Just a, Just b) -> Just (div a b)
+    (Nothing, _)     -> Nothing
+    (_, Nothing)     -> Nothing
+
+eval3 :: Expr -> Either String Int
+eval3 (Val n)   = Right n
+eval3 (Add a b) = case (eval3 a, eval3 b) of
+    (Right a, Right b) -> Right (a+b)
+    (Left k, _)        -> Left k
+    (_, Left k)        -> Left k
+eval3 (Sub a b) = case (eval3 a, eval3 b) of
+    (Right a, Right b) -> Right (a-b)
+    (Left k, _)        -> Left k
+    (_, Left k)        -> Left k
+eval3 (Mul a b) = case (eval3 a, eval3 b) of
+    (Right a, Right b) -> Right (a*b)
+    (Left k, _)        -> Left k
+    (_, Left k)        -> Left k
+eval3 (Div a b) = case (eval3 a, eval3 b) of
+    (Right a, Right 0) -> Left "div0"
+    (Right a, Right b) -> Right (div a b)
+    (Left k, _)        -> Left k
+    (_, Left k)        -> Left k
+
+
+
+