Writers.Shared: metaToJSON, generalized type so it can take a Text.

Previously a String was needed as argument; now any ToJSON
instance will do.

API change.

src/Text/Pandoc/Writers/Shared.hs

@@ -62,10 +62,10 @@ import Text.Pandoc.XML (escapeStringForXML)
 -- Variables overwrite metadata fields with the same names.
 -- If multiple variables are set with the same name, a list is
 -- assigned.  Does nothing if 'writerTemplate' is Nothing.
-metaToJSON :: (Functor m, Monad m)
+metaToJSON :: (Functor m, Monad m, ToJSON a)
            => WriterOptions
-           -> ([Block] -> m String)
-           -> ([Inline] -> m String)
+           -> ([Block] -> m a)
+           -> ([Inline] -> m a)
            -> Meta
            -> m Value
 metaToJSON opts blockWriter inlineWriter meta
@@ -75,9 +75,9 @@ metaToJSON opts blockWriter inlineWriter meta
 
 -- | Like 'metaToJSON', but does not include variables and is
 -- not sensitive to 'writerTemplate'.
-metaToJSON' :: Monad m
-           => ([Block] -> m String)
-           -> ([Inline] -> m String)
+metaToJSON' :: (Monad m, ToJSON a)
+           => ([Block] -> m a)
+           -> ([Inline] -> m a)
            -> Meta
            -> m Value
 metaToJSON' blockWriter inlineWriter (Meta metamap) = do
@@ -98,9 +98,9 @@ addVariablesToJSON opts metadata =
   where combineMetadata (Object o1) (Object o2) = Object $ H.union o1 o2
         combineMetadata x _                     = x
 
-metaValueToJSON :: Monad m
-                => ([Block] -> m String)
-                -> ([Inline] -> m String)
+metaValueToJSON :: (Monad m, ToJSON a)
+                => ([Block] -> m a)
+                -> ([Inline] -> m a)
                 -> MetaValue
                 -> m Value
 metaValueToJSON blockWriter inlineWriter (MetaMap metamap) = liftM toJSON $