optimization of list comprehensions

[Ulf Wiger (AL/EAB)]

 Richard A. O'Keefe wrote:

> In fact, I spent about 20 minutes looking at foldl calls in 
> the OTP sources, and *most* of them had complex functions and 
> list arguments that were just simple variable names.

Let me then provide a more interesting example.
Based on (and supposedly equivalent to) real-life code, 
but never compiled nor tested:

mk_tops(TIds1, TIds2, Dir, SId)
 when is_list(TIds1), is_list(TIds2) ->
  [#ctxtTop{frTId = TId1, toTId = TId2,
            dir = Dir, strId = SId} ||
    TId1 <- TIds1,
    TId2 <- TIds2,
    TId1 =/= TId2].

expand(Tops, TIds) ->
  lists:foldr(
    fun(#ctxtTop{frTId = '*', toTId = '*', dir = Dir, strId = SId}, Acc)
->
          Acc ++ mk_tops(TIds, TIds, Dir, Sid);
       (#ctxtTop{frTId = T1, toTId = '*', dir = Dir, strId = SId}, Acc)
->
          Acc ++ mk_tops([T1], TIds, Dir, SId);
       (#ctxtTop{frTId = '*', toTId = T1, dir = Dir, strId = SId}, Acc)
->
          Acc ++ mk_tops(TIds, [T1], Dir, SId)
    end, [], Tops).


or, rewritten without ++:


expand(Tops, TIds) ->
  lists:foldl(
    fun(#ctxtTop{frTId = '*', toTId = '*', dir = Dir, strId = SId}, Acc)
->
          mk_tops(TIds, TIds, Dir, Sid, Acc);
       (#ctxtTop{frTId = T1, toTId = '*', dir = Dir, strId = SId}, Acc)
->
          mk_tops([T1], TIds, Dir, SId, Acc);
       (#ctxtTop{frTId = '*', toTId = T1, dir = Dir, strId = SId}, Acc)
->
          mk_tops(TIds, [T1], Dir, SId, Acc)
    end, [], Tops).

mk_tops(TIds1, TIds2, Dir, SId, Acc) ->
  lists:foldl(
    fun(TId1, Acc1) ->
      lists:foldl(
        fun(TId2, Acc11) when TId1 =/= TId2 ->
            [#ctxtTop{frTId = TId1, toTId = TId2,
                      dir = Dir, strId = SId} | Acc11]
        end, Acc1, TIds2)
    end, Acc, TIds1).

I have no own idea on how to improve it, with or without
folding lcs.

Regards,
Ulf W