Update

Author: ZeppLu

long_calc.hs

@@ -0,0 +1,85 @@
+import Data.Ratio
+import Data.List
+
+data Digit = Neg
+           | Zero
+           | One
+           | Two
+           | Three
+           | Four
+           | Five
+           | Six
+           | Seven
+           | Eight
+           | Nine
+             deriving (Show, Eq, Ord, Enum)
+
+instance Num Digit where
+    fromInteger x
+        | x == 0           = Zero
+        | x >= 1 && x <= 9 = succ $ fromInteger (x-1)
+    (+) x y = fromInteger (s `mod` 10)
+        where s = toInteger x + toInteger y
+    (-) x y = fromInteger (r `mod` 10)
+        where r = toInteger x - toInteger y
+    (*) x y = fromInteger (p `mod` 10)
+        where p = toInteger x * toInteger y
+    abs = id
+    signum Zero = Zero
+    signum _    = One
+
+instance Real Digit where
+    toRational Zero = 0 % 1
+    toRational x    = 1 + toRational (pred x)
+
+instance Integral Digit where
+    toInteger Zero = 0
+    toInteger x    = 1 + toInteger (pred x)
+    quotRem x y = (fromInteger a, fromInteger b)
+        where (a, b) = quotRem (toInteger x) (toInteger y)
+
+carryOrLend :: (Integer -> Integer -> Integer) -> Digit -> Digit -> Integer
+carryOrLend f x y
+    | result < 0   = -1
+    | result >= 10 = quot result 10
+    | otherwise    = 0
+          where result = f (toInteger x) (toInteger y)
+
+
+join :: [a] -> [[a]] -> [a]
+join _  []  = []
+join xs xss = foldl1 joinNew xss
+    where joinNew acc ys = acc ++ xs ++ ys
+
+data Expr = Cst Int
+          | Var
+          | Add [Expr]
+          | Mul [Expr]
+instance Show Expr where
+    show (Cst c) = show c
+    show Var = "x"
+    show (Mul xs) = "(" ++ join "*" (map show xs) ++ ")"
+    show (Add xs) = "(" ++ join "+" (map show xs) ++ ")"
+data Equal = Equal Expr Expr
+
+--normalize :: Expr -> Expr
+--normalize (Cst x) = Cst x
+--normalize Var = Var
+--normalize (Mul (Add x y) z) = Add (normalize (Mul x z)) (normalize (Mul y z))
+--normalize (Mul z (Add x y)) = normalize (Mul (Add x y) z)
+--normalize (Mul (Cst x) (Cst y)) = Cst (x * y)
+--normalize (Mul x y) = Mul (normalize x) (normalize y)
+--normalize (Add x y) = Add (normalize x) (normalize y)
+
+{-
+normalize :: Expr -> Expr
+normalize (Add xs) = merge $
+
+testExpr = Mul [Add [Var, Cst 2], Add [Mul [Cst 2, Var], Cst 3]]
+
+solve :: Equal -> Maybe Int
+solve (Equal    (Cst _) (Cst _)) = Nothing
+solve (Equal    (Cst c) all@_  ) = solve (Equal all (Cst c))
+solve (Equal    Var     (Cst c)) = Just c
+-}
+

rwh04.hs

@@ -32,6 +32,8 @@ concatBySpace = unwords ["Can", "you", "hear", "me,", "Alice?"]
 -- `splitAt' "tuples up" the results of `take' and `drop'
 -- `span' "tuples up" those of `takeWhile'
 -- `break' "tuples up" those of `dropWhile'
+-- Further further more:
+-- from Data.List: `nub', `group', `sort'
 
 -- ex02, both work
 splitWith :: (a -> Bool) -> [a] -> [[a]]