Efficient way to remove empty lists from lists without evaluating held expressions?

Question

In previous thread an efficient way to remove empty lists ({}) from lists was suggested:

Replace[expr, x_List :> DeleteCases[x, {}], {0, Infinity}]

Using the Trott-Strzebonski in-place evaluation technique this method can be generalized for working also with held expressions:

f1[expr_] := 
 Replace[expr, 
  x_List :> With[{eval = DeleteCases[x, {}]}, eval /; True], {0, Infinity}]

This solution is more efficient than the one based on ReplaceRepeated:

f2[expr_] := expr //. {left___, {}, right___} :> {left, right}

But it has one disadvantage: it evaluates held expressions if they are wrapped by List:

In[20]:= f1[Hold[{{}, 1 + 1}]]

Out[20]= Hold[{2}]

So my question is: what is the most efficient way to remove all empty lists ({}) from lists without evaluating held expressions? The empty List[] object should be removed only if it is an element of another List itself.

---

Here are some timings:

In[76]:= expr = Tuples[Tuples[{{}, {}}, 3], 4];
First@Timing[#[expr]] & /@ {f1, f2, f3}
pl = Plot3D[Sin[x y], {x, 0, Pi}, {y, 0, Pi}]; 
First@Timing[#[pl]] & /@ {f1, f2, f3}

Out[77]= {0.581, 0.901, 5.027}

Out[78]= {0.12, 0.21, 0.18}

Definitions:

Clear[f1, f2, f3];
f3[expr_] := 
  FixedPoint[
   Function[e, Replace[e, {a___, {}, b___} :> {a, b}, {0, Infinity}]], expr];
f1[expr_] := 
  Replace[expr, 
   x_List :> With[{eval = DeleteCases[x, {}]}, eval /; True], {0, Infinity}];
f2[expr_] := expr //. {left___, {}, right___} :> {left, right};

Answer 1

How about:

Clear[f3];
f3[expr_] := 
 FixedPoint[
  Function[e, 
   Replace[e, {a___, {}, b___} :> {a, b}, {0, Infinity}]],
   expr]

It seems to live up to the specs:

In[275]:= f3[{a, {}, {b, {}}, c[d, {}]}]

Out[275]= {a, {b}, c[d, {}]}

In[276]:= f3[Hold[{{}, 1 + 1, {}}]]

Out[276]= Hold[{1 + 1}]

Answer 2

You can combine the solutions you mentioned with a minimal performance hit and maintain the code unevaluated by using a technique from this post, with a modification that the custom holding wrapper will be made private by using Module:

ClearAll[removeEmptyListsHeld];
removeEmptyListsHeld[expr_Hold] :=
  Module[{myHold},
     SetAttributes[myHold, HoldAllComplete];
     Replace[MapAll[myHold, expr, Heads -> True],
        x : myHold[List][___] :> 
           With[{eval = DeleteCases[x, myHold[myHold[List][]]]}, 
             eval /; True], 
       {0, Infinity}]//. myHold[x_] :> x];

The above function assumes that the input expression is wrapped in Hold. Examples:

In[53]:= expr = Tuples[Tuples[{{}, {}}, 3], 4];
First@Timing[#[expr]] & /@ {f1, f2, f3, removeEmptyListsHeld[Hold[#]] &}

Out[54]= {0.235, 0.218, 1.75, 0.328}


In[56]:= removeEmptyListsHeld[Hold[{{},1+1,{}}]]
Out[56]= Hold[{1+1}]