Another update from ingydotnet^djgoku

Task/Arithmetic-evaluation/Haskell/arithmetic-evaluation.hs

@@ -1,5 +1,7 @@
-import Text.ParserCombinators.Parsec
-import Text.ParserCombinators.Parsec.Expr
+import Text.Parsec
+import Text.Parsec.Expr
+import Text.Parsec.Combinator
+import Data.Functor
 
 data Exp = Num Int
          | Add Exp Exp
@@ -8,22 +10,20 @@ data Exp = Num Int
          | Div Exp Exp
 
 expr = buildExpressionParser table factor
+    where table = [[op "*" (Mul) AssocLeft, op "/" (Div) AssocLeft]
+                  ,[op "+" (Add) AssocLeft, op "-" (Sub) AssocLeft]]
+          op s f assoc = Infix (f <$ string s) assoc
+          factor =  (between `on` char) '(' ')' expr
+                <|> (Num . read <$> many1 digit)
+          on f g = \x y -> f (g x) (g y)
 
-table = [[op "*" (Mul) AssocLeft, op "/" (Div) AssocLeft]
-        ,[op "+" (Add) AssocLeft, op "-" (Sub) AssocLeft]]
-        where op s f assoc = Infix (do string s; return f) assoc
+eval :: Num a => Exp -> a
+eval e = case e of
+    Num x   -> fromIntegral x
+    Add a b -> eval a   +   eval b
+    Sub a b -> eval a   -   eval b
+    Mul a b -> eval a   *   eval b
+    Div a b -> eval a `div` eval b
 
-factor =  do char '(' ; x <- expr ; char ')'
-             return x
-      <|> do ds <- many1 digit
-             return $ Num (read ds)
-
-evaluate (Num x) = fromIntegral x
-evaluate (Add a b) = (evaluate a)   +   (evaluate b)
-evaluate (Sub a b) = (evaluate a)   -   (evaluate b)
-evaluate (Mul a b) = (evaluate a)   *   (evaluate b)
-evaluate (Div a b) = (evaluate a) `div` (evaluate b)
-
-solution exp = case parse expr [] exp of
-                 Right expr -> evaluate expr
-                 Left _ -> error "Did not parse"
+solution :: Num a => String -> a
+solution = either (const (error "Did not parse")) eval . parse expr ""