added error message

Main.hs

@@ -4,10 +4,12 @@ import CSVParser
 import Apriori
 import qualified Data.Set as Set
 import System.Environment (getArgs)
+import Control.Monad
 
 main :: IO()
 main = do
 	args <- getArgs
+	when (2 /= length args) (error "Usage: Main <file.csv> <threshold>")
 	let filename = head args
 	let threshold = read $ last args
 	file <- readFile filename