Add all the A tasks

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

@@ -0,0 +1,29 @@
+import Text.ParserCombinators.Parsec
+import Text.ParserCombinators.Parsec.Expr
+
+data Exp = Num Int
+         | Add Exp Exp
+         | Sub Exp Exp
+         | Mul Exp Exp
+         | Div Exp Exp
+
+expr = buildExpressionParser table factor
+
+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
+
+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"