Stop dying in Path validation and return an Either instead so we can handle expected errors cleanly

src/Blog.hs

@@ -41,6 +41,7 @@ import Page (Page)
 import qualified Page (at)
 import Prelude hiding (lookup)
 import System.Directory (doesFileExist, withCurrentDirectory)
+import System.Exit (die)
 import System.FilePath ((</>), dropTrailingPathSeparator, takeExtension, takeFileName)
 import Text.Parsec (ParseError)
 
@@ -115,7 +116,7 @@ build arguments = do
   templates <- Template.build wording
   root <- Files.absolute . Dir $ Arguments.sourceDir arguments
   withCurrentDirectory root $ do
-    path <- Path.build root arguments
+    path <- either die return =<< Path.build root arguments
     let name = maybe (takeFileName $ dropTrailingPathSeparator root) id
               $ Arguments.name arguments
     skin <- Skin.build name arguments

src/Blog/Path.hs

@@ -12,14 +12,13 @@ import Data.Aeson (ToJSON(..), (.=), pairs)
 import Data.Monoid ((<>))
 import Files (File(..), filePath, filePathMaybe)
 import GHC.Generics (Generic)
-import System.Exit (die)
 
 data Path = Path {
       articlesPath :: Maybe FilePath
     , pagesPath :: Maybe FilePath
     , remarkableConfig :: Maybe FilePath
     , root :: FilePath
-  } deriving Generic
+  } deriving (Eq, Generic, Show)
 
 instance ToJSON Path where
   toEncoding (Path {articlesPath, pagesPath}) = pairs (
@@ -31,14 +30,14 @@ getMarkdownPath :: FilePath -> Maybe FilePath -> IO (Maybe FilePath)
 getMarkdownPath defaultPath Nothing = filePathMaybe (Dir defaultPath)
 getMarkdownPath _ (Just path) = Just <$> filePath (Dir path)
 
-build :: FilePath -> Arguments -> IO Path
+build :: FilePath -> Arguments -> IO (Either String Path)
 build root arguments = do
   articlesPath <- getMarkdownPath "articles" $ Arguments.articlesPath arguments
   pagesPath <- getMarkdownPath "pages" $ Arguments.pagesPath arguments
-  checkForContent articlesPath pagesPath
   remarkableConfig <- mapM (filePath . File) $ Arguments.remarkableConfig arguments
-  return $ Path {articlesPath, pagesPath, remarkableConfig, root}
+  checkForContent articlesPath pagesPath remarkableConfig
   where
-    checkForContent Nothing Nothing =
-      die "No articles ? No pages ? Why did you wake me up ? I'm going back to sleep"
-    checkForContent _ _ = return ()
+    checkForContent Nothing Nothing _ = return $
+      Left "No articles ? No pages ? Why did you wake me up ? I'm going back to sleep"
+    checkForContent articlesPath pagesPath remarkableConfig =
+      return . Right $ Path {articlesPath, pagesPath, remarkableConfig, root}