Shared: Change defaultUserDataDirs -> defaultUserDataDir.

Rationale: the manual says that the XDG data directory will
be used if it exists, otherwise the legacy data directory.
So we should just determine this and use this directory,
rather than having a search path which could cause some
things to be taken from one data directory and others from
others.

[API change]

MANUAL.txt

@@ -361,15 +361,15 @@ header when requesting a document from a URL:
     will be used.  On \*nix and macOS systems this will be the `pandoc`
     subdirectory of the XDG data directory (by default,
     `$HOME/.local/share`, overridable by setting the `XDG_DATA_HOME`
-    environment variable).  If that directory does not exist,
-    `$HOME/.pandoc` will be used (for backwards compatibility).
-    In Windows the default user data directory is
+    environment variable).  If that directory does not exist and
+    `$HOME/.pandoc` exists, it will be used (for backwards compatibility).
+    On Windows the default user data directory is
     `C:\Users\USERNAME\AppData\Roaming\pandoc`.
     You can find the default user data directory on your system by
     looking at the output of `pandoc --version`.
-    A `reference.odt`, `reference.docx`, `epub.css`, `templates`,
-    `slidy`, `slideous`, or `s5` directory
-    placed in this directory will override pandoc's normal defaults.
+    Data files placed in this directory (for example, `reference.odt`,
+    `reference.docx`, `epub.css`, `templates`) will override
+    pandoc's normal defaults.
 
 `-d` *FILE*, `--defaults=`*FILE*
 

src/Text/Pandoc/Shared.hs

@@ -95,7 +95,7 @@ module Text.Pandoc.Shared (
                      safeRead,
                      safeStrRead,
                      -- * User data directory
-                     defaultUserDataDirs,
+                     defaultUserDataDir,
                      -- * Version
                      pandocVersion
                     ) where
@@ -1012,12 +1012,16 @@ safeStrRead s = case reads s of
 --
 
 -- | Return appropriate user data directory for platform.  We use
--- XDG_DATA_HOME (or its default value), but fall back to the
--- legacy user data directory ($HOME/.pandoc on *nix) if this is
--- missing.
-defaultUserDataDirs :: IO [FilePath]
-defaultUserDataDirs = E.catch (do
+-- XDG_DATA_HOME (or its default value), but for backwards compatibility,
+-- we fall back to the legacy user data directory ($HOME/.pandoc on *nix)
+-- if the XDG_DATA_HOME is missing and this exists.  If neither directory
+-- is present, we return the XDG data directory.
+defaultUserDataDir :: IO FilePath
+defaultUserDataDir = do
   xdgDir <- getXdgDirectory XdgData "pandoc"
   legacyDir <- getAppUserDataDirectory "pandoc"
-  return $ ordNub [xdgDir, legacyDir])
- (\(_ :: E.SomeException) -> return [])
+  xdgExists <- doesDirectoryExist xdgDir
+  legacyDirExists <- doesDirectoryExist legacyDir
+  if not xdgExists && legacyDirExists
+     then return legacyDir
+     else return xdgDir