2016 Update

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

@@ -2,6 +2,7 @@ import Text.Parsec
 import Text.Parsec.Expr
 import Text.Parsec.Combinator
 import Data.Functor
+import Data.Function (on)
 
 data Exp = Num Int
          | Add Exp Exp
@@ -15,15 +16,13 @@ expr = buildExpressionParser table factor
           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)
 
 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
+eval (Num x)   = fromIntegral x
+eval (Add a b) = eval a   +   eval b
+eval (Sub a b) = eval a   -   eval b
+eval (Mul a b) = eval a   *   eval b
+eval (Div a b) = eval a `div` eval b
 
 solution :: Num a => String -> a
 solution = either (const (error "Did not parse")) eval . parse expr ""