Pretty print settings parsing errors

src/Main.hs

@@ -6,7 +6,7 @@ module Main
     ( main
     ) where
 
-import Control.Exception (Exception(..), handle)
+import Control.Exception (Exception(..), handle, SomeException(..))
 import Data.Char (isNumber)
 import Control.Monad.Catch (MonadThrow(..))
 import Control.Monad.IO.Class (MonadIO(..))
@@ -20,6 +20,7 @@ import SlackBuilder.Update
 import qualified Toml
 import Data.Text (Text)
 import qualified Data.Text as Text
+import qualified Data.Text.IO as Text
 import Control.Monad.Trans.Reader (ReaderT(..))
 import SlackBuilder.Package (PackageDescription(..))
 import qualified SlackBuilder.Package as Package
@@ -258,15 +259,22 @@ check = for_ autoUpdatable go
         >> liftIO (putStrLn "")
 
 main :: IO ()
-main = do
-    programCommand <- execParser slackBuilderParser
-    settings <- Toml.decodeFile settingsCodec "config/config.toml"
-    handle handleException
-        $ flip runReaderT settings
+main = execParser slackBuilderParser
+    >>= handle handleException . withCommandLine
+  where
+    withCommandLine programCommand = do
+        settingsResult <- Toml.decodeFileEither settingsCodec configurationFile
+        case settingsResult of
+            Right settings -> flip runReaderT settings
                 $ runSlackBuilderT
                 $ executeCommand programCommand
-  where
-    handleException :: SlackBuilderException -> IO ()
+            Left settingsErrors
+                -> setSGR [SetColor Foreground Dull Red]
+                >> putStrLn (configurationFile <> " parsing failed.")
+                >> setSGR [Reset]
+                >> Text.putStr (Toml.prettyTomlDecodeErrors settingsErrors)
+    configurationFile = "config/config.toml"
+    handleException :: SomeException -> IO ()
     handleException slackBuilderException
         = setSGR [SetColor Foreground Dull Red]
         >> putStrLn (displayException slackBuilderException)