Factor out a findM function (#6125)

This adds a new function to the API: Text.Pandoc.Shared.findM.

src/Text/Pandoc/App.hs

@@ -58,7 +58,7 @@ import Text.Pandoc.PDF (makePDF)
 import Text.Pandoc.SelfContained (makeDataURI, makeSelfContained)
 import Text.Pandoc.Shared (eastAsianLineBreakFilter, stripEmptyParagraphs,
          headerShift, isURI, tabFilter, uriPathToPath, filterIpynbOutput,
-         defaultUserDataDirs, tshow)
+         defaultUserDataDirs, tshow, findM)
 import Text.Pandoc.Writers.Shared (lookupMetaString)
 import Text.Pandoc.Readers.Markdown (yamlToMeta)
 import qualified Text.Pandoc.UTF8 as UTF8
@@ -94,13 +94,7 @@ convertWithOpts opts = do
   datadir <- case optDataDir opts of
                   Nothing   -> do
                     ds <- defaultUserDataDirs
-                    let selectUserDataDir [] = return Nothing
-                        selectUserDataDir (dir:dirs) = do
-                              exists <- doesDirectoryExist dir
-                              if exists
-                                 then return (Just dir)
-                                 else selectUserDataDir dirs
-                    selectUserDataDir ds
+                    findM doesDirectoryExist ds
                   Just _    -> return $ optDataDir opts
 
   let runIO' :: PandocIO a -> IO a

src/Text/Pandoc/App/CommandLineOptions.hs

@@ -50,7 +50,7 @@ import Text.Pandoc
 import Text.Pandoc.App.Opt (Opt (..), LineEnding (..), IpynbOutput (..), addMeta)
 import Text.Pandoc.Filter (Filter (..))
 import Text.Pandoc.Highlighting (highlightingStyles)
-import Text.Pandoc.Shared (ordNub, elemText, safeStrRead, defaultUserDataDirs)
+import Text.Pandoc.Shared (ordNub, elemText, safeStrRead, defaultUserDataDirs, findM)
 import Text.Printf
 
 #ifdef EMBED_DATA_FILES
@@ -118,14 +118,6 @@ engines = map ("html",) htmlEngines ++
 pdfEngines :: [String]
 pdfEngines = ordNub $ map snd engines
 
-findFile :: PandocMonad m => [FilePath] -> m (Maybe FilePath)
-findFile [] = return Nothing
-findFile (f:fs) = do
-  exists <- fileExists f
-  if exists
-     then return $ Just f
-     else findFile fs
-
 -- | A list of functions, each transforming the options data structure
 --   in response to a command-line option.
 options :: [OptDescr (Opt -> IO Opt)]
@@ -996,7 +988,7 @@ applyDefaults opt file = runIOorExplode $ do
               Nothing -> map (</> ("defaults" </> fp))
                                dataDirs
               Just dd -> [dd </> "defaults" </> fp]
-  fp' <- fromMaybe fp <$> findFile fps
+  fp' <- fromMaybe fp <$> findM fileExists fps
   inp <- readFileLazy fp'
   case Y.decode1 inp of
       Right (f :: Opt -> Opt) -> return $ f opt

src/Text/Pandoc/Shared.hs

@@ -27,6 +27,7 @@ module Text.Pandoc.Shared (
                      splitTextByIndices,
                      substitute,
                      ordNub,
+                     findM,
                      -- * Text processing
                      ToString (..),
                      ToText (..),
@@ -198,6 +199,14 @@ ordNub l = go Set.empty l
     go s (x:xs) = if x `Set.member` s then go s xs
                                       else x : go (Set.insert x s) xs
 
+findM :: forall m t a. (Monad m, Foldable t) => (a -> m Bool) -> t a -> m (Maybe a)
+findM p = foldr go (pure Nothing)
+  where
+    go :: a -> m (Maybe a) -> m (Maybe a)
+    go x acc = do
+      b <- p x
+      if b then pure (Just x) else acc
+
 --
 -- Text processing
 --