Docx Reader: Add a compatibility layer for Except.

mtl switched from ErrorT to ExceptT, but we're not sure which mtl we'll
be dealing with. This should make errors work with both.

The main difference (beside the name of the module and the monad
transformer) is that Except doesn't require an instance of an Error
Typeclass. So we define that for compatability. When we switch to a
later mtl, using Control.Monad.Exception, we can just erase the instance
declaration, and all should work fine.

1 files changed, 27 insertions, 0 deletions

diff --git a/src/Text/Pandoc/Compat/Except.hs b/src/Text/Pandoc/Compat/Except.hs
new file mode 100644
index 000000000..7f5648e7a
--- /dev/null
+++ b/src/Text/Pandoc/Compat/Except.hs
@@ -0,0 +1,27 @@
+{-# LANGUAGE CPP #-}
+module Text.Pandoc.Compat.Except ( ExceptT
+                                 , Error(..)
+                                 , runExceptT
+                                 , throwError
+                                 , catchError )
+       where
+
+#if MIN_VERSION_mtl(2,2,1)
+import Control.Monad.Except
+
+class Error a where
+  noMsg  :: a
+  strMsg :: String -> a
+
+  noMsg    = strMsg ""
+  strMsg _ = noMsg
+
+#else
+import Control.Monad.Error
+type ExceptT = ErrorT
+
+runExceptT :: ExceptT e m a -> m (Either e a)
+runExceptT = runErrorT
+#endif
+
+