Change formatForFilePaths to return a Maybe.

Internal change. This will make it easier to emit messages
when we're guessing at a format.

3 files changed, 10 insertions, 8 deletions

diff --git a/src/Text/Pandoc/App.hs b/src/Text/Pandoc/App.hs
index aabefc64a..d48ae1932 100644
--- a/src/Text/Pandoc/App.hs
+++ b/src/Text/Pandoc/App.hs
@@ -104,7 +104,8 @@ convertWithOpts opts = do
   -- assign reader and writer based on options and filenames
   let readerName = case optReader opts of
                      Just f  -> f
-                     Nothing -> formatFromFilePaths fallback sources
+                     Nothing -> fromMaybe fallback $
+                                  formatFromFilePaths sources
                        where fallback = if any isURI sources
                                            then "html"
                                            else "markdown"
diff --git a/src/Text/Pandoc/App/FormatHeuristics.hs b/src/Text/Pandoc/App/FormatHeuristics.hs
index 6198e83dc..a02d8d15e 100644
--- a/src/Text/Pandoc/App/FormatHeuristics.hs
+++ b/src/Text/Pandoc/App/FormatHeuristics.hs
@@ -18,13 +18,13 @@ import Prelude
 import Data.Char (toLower)
 import System.FilePath (takeExtension)
 
--- Determine default reader based on source file extensions.
-formatFromFilePaths :: String -> [FilePath] -> String
-formatFromFilePaths fallback [] = fallback
-formatFromFilePaths fallback (x:xs) =
+-- Determine default format based on file extensions.
+formatFromFilePaths :: [FilePath] -> Maybe String
+formatFromFilePaths [] = Nothing
+formatFromFilePaths (x:xs) =
   case formatFromFilePath x of
-    Just f     -> f
-    Nothing    -> formatFromFilePaths fallback xs
+    Just f     -> Just f
+    Nothing    -> formatFromFilePaths xs
 
 -- Determine format based on file extension
 formatFromFilePath :: FilePath -> Maybe String
diff --git a/src/Text/Pandoc/App/OutputSettings.hs b/src/Text/Pandoc/App/OutputSettings.hs
index 837636503..a9034d6f2 100644
--- a/src/Text/Pandoc/App/OutputSettings.hs
+++ b/src/Text/Pandoc/App/OutputSettings.hs
@@ -71,7 +71,8 @@ optToOutputSettings opts = do
        then liftIO $ pdfWriterAndProg (optWriter opts) (optPdfEngine opts)
        else case optWriter opts of
               Nothing  ->
-                return (formatFromFilePaths "html" [outputFile], Nothing)
+                return (fromMaybe "html" $ formatFromFilePaths [outputFile],
+                        Nothing)
               Just f   -> return (f, Nothing)
 
   let format = if ".lua" `isSuffixOf` writerName