[Haskell-cafe] an advanced foldification problem

I have a data type "Group", representing a group of geographic information that is all referring to the same location, and a function "mergeGroups" that tries to merge two groups: mergeGroups :: Group -> Group -> Maybe Group Then I have a function "mergeGroupToList" that tries to merge its first argument with every element of the second argument: mergeGroupToList :: Group -> [Group] -> [Group] mergeGroupToList g [] = [g] mergeGroupToList g1 (g2:gs) = case (mergeGroups g1 g2) of Nothing -> g2 : (mergeGroupToList g1 gs) Just g3 -> mergeGroupToList g3 gs How can I rewrite mergeGroupToList in terms of foldr?