gistfile1.txt

not_found_cookbooks(AllVersions, Cookbooks) ->
    case lists:filter(fun(CB) -> cookbook_exists(CB, AllVersions) end, Cookbooks) of
        [] -> ok;
        NotFound -> {not_found, [ cookbook_name(NFCookbook) || NFCookbook <- NotFound] }
    end.

Can become :

not_found_cookbooks(AllVersions, Cookbooks) ->
    NotFound = [ cookbook_name(Cookbook) || Cookbook <- Cookbooks, cookbook_exists(Cookbook, AllVersions)],
    case NotFound
        [] -> ok;
        NotFound -> {not_found, NotFound}
    end.