feature(cli): default arguments

Also add src/Utils & src/System to move some helpers there, and changed
(again) the way the reading, parsing and writing is composed

app/Main.hs

@@ -2,21 +2,21 @@ module Main
   ( main
   ) where
 
-import qualified Data.Aeson         as Aeson
-import           Parsing            (parseEntries)
-import qualified System.Environment as Env
-import qualified System.IO          as Sys
+import qualified Data.Aeson as Aeson
+import           Parsing    (parseEntries)
+import           System     (argOr, putStderr)
 
-putStderr :: String -> IO ()
-putStderr = Sys.hPutStrLn Sys.stderr
+parse :: String -> Aeson.Value
+parse = Aeson.toJSON . parseEntries
 
-doStuff :: FilePath -> FilePath -> IO ()
-doStuff input output =
-  Aeson.encodeFile output =<< Aeson.toJSON . parseEntries <$> readFile input
+parseToFile :: FilePath -> FilePath -> IO ()
+parseToFile input output =
+  pure parse <*> readFile input >>= Aeson.encodeFile output
 
 main :: IO ()
 main = do
-  (input:output:_) <- Env.getArgs -- TODO handle missing files
+  input <- argOr 0 "statements.txt"
+  output <- argOr 1 "data.json"
   putStderr $ "Processing statements from: \"" ++ input ++ "\""
-  doStuff input output
-  putStderr $ "Data written to: \"" ++ input ++ "\""
+  parseToFile input output
+  putStderr $ "Data written to: \"" ++ output ++ "\""

src/Date.hs

@@ -5,8 +5,8 @@ module Date
   , readFormatedDate
   ) where
 
-import qualified Data.Aeson   as Aeson
-import qualified Text.Regex   as R
+import qualified Data.Aeson as Aeson
+import qualified Text.Regex as R
 
 newtype Date =
   Date (String, String, String)

src/System.hs

@@ -0,0 +1,20 @@
+module System
+  ( nthArg
+  , argOr
+  , putStderr
+  ) where
+
+import qualified Control.Applicative as Ap
+import qualified Data.Maybe          as M
+import qualified System.Environment  as Env
+import qualified System.IO           as Sys
+import           Utils               (nth)
+
+nthArg :: Int -> IO (Maybe String)
+nthArg = Ap.liftA2 nth Env.getArgs . pure
+
+argOr :: Int -> String -> IO String
+argOr n def = M.fromMaybe def <$> nthArg n
+
+putStderr :: String -> IO ()
+putStderr = Sys.hPutStrLn Sys.stderr

src/Utils.hs

@@ -0,0 +1,8 @@
+module Utils
+  (nth) where
+
+nth :: [a] -> Int -> Maybe a
+nth xs n
+  | n >= length xs = Nothing
+  | n < 0 = Nothing
+  | otherwise = Just (xs !! n)

vigilant-engine.cabal

@@ -25,6 +25,8 @@ library
                      , Matchers 
                      , Date
                      , Types
+                     , Utils
+                     , System
 
   build-depends:       base ^>= 4.12.0.0
                      , regex-compat ^>= 0.95.1