Финальная функция для обработки строк

coursework/part1/src/Lib.hs

@@ -61,10 +61,10 @@ data Operation = Add | Sub | Mul | Div deriving Show
 
 operationToString :: Operation -> String
 operationToString op = case op of
-    Add -> " + "
+    Add -> "+"
-    Sub -> " - "
+    Sub -> "-"
-    Mul -> " * "
+    Mul -> "*"
-    Div -> " / " 
+    Div -> "/" 
 
 operationToOperator :: Operation -> (Int -> Int -> Int)
 operationToOperator op = case op of
@@ -84,3 +84,19 @@ operation = skipSpaces *> (
 
 expression :: Parser Char (Int, Operation, Int)
 expression = (,,) <$> number <*> operation <*> number <* skipSpaces
+
+calculateExpression :: (Int, Operation, Int) -> Int
+calculateExpression (a, op, b) = (operationToOperator op) a b
+
+processExpression :: String -> String
+processExpression s = case runParser expression s of
+    Nothing -> err
+    Just (cs, (a, op, b)) -> case cs of
+        [] -> 
+            show a ++ " " ++ 
+            operationToString op ++ " " ++ 
+            show b ++ " = " ++ 
+            show (calculateExpression (a, op, b))
+        _ -> err
+    where
+        err = error $ "Unable to parse the following expression: \"" ++ s ++ "\""