Shared: export extractIds.

This will be used to help with #1745.

src/Text/Pandoc/Shared.hs

@@ -64,6 +64,7 @@ module Text.Pandoc.Shared (
                      Element (..),
                      hierarchicalize,
                      uniqueIdent,
+                     extractIds,
                      inlineListToIdentifier,
                      isHeaderBlock,
                      headerShift,
@@ -529,6 +530,25 @@ headerLtEq level (Header l _ _) = l <= level
 headerLtEq level (Div ("",["references"],[]) (Header l _ _ : _))  = l <= level
 headerLtEq _ _ = False
 
+-- | Extract the identifiers from a block element.
+extractBlockIds :: Block -> Set.Set String
+extractBlockIds (Header _ (ident,_,_) _) | not (null ident) =
+  Set.singleton ident
+extractBlockIds (Div (ident,_,_) _) | not (null ident) =
+  Set.singleton ident
+extractBlockIds _ = Set.empty
+
+-- | Extract the identifiers from an inline element.
+extractInlineIds :: Inline -> Set.Set String
+extractInlineIds (Span (ident,_,_) _) | not (null ident) =
+  Set.singleton ident
+extractInlineIds _ = Set.empty
+
+-- | Extract the identifiers from a pandoc document.
+extractIds :: Pandoc -> Set.Set String
+extractIds doc =
+  query extractBlockIds doc `Set.union` query extractInlineIds doc
+
 -- | Generate a unique identifier from a list of inlines.
 -- Second argument is a list of already used identifiers.
 uniqueIdent :: [Inline] -> Set.Set String -> String