Flatten lacks some functionality

[wolfv]

I just coded some list functionality for Mathics, and noticed that unfortunately Flatten seems to lack some functionality:

>> Flatten[{{1,2,3}, {4,5,6}, {7,8,9}}] 
 = {1, 2, 3, 4, 5, 6, 7, 8, 9} // Correct!
>> Flatten[{{1,2,3}, {4,5,6}, {7,8,9}}, {2}]
: Level to be flattened together in {2} should be a non-negative integer.
Out[2]= Flatten[{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}, {2}, List]

Expected Result:
{{1,4,7},{2,5,8},{3,6,9}}

I tried to fix it but couldn't wrap my head completely around where the issue exactly is. Flatten seems to be a pretty generic function in Mathics.

Otherwise, cool stuff!

Wolf

[sn6uv]

The way Mathics evaluation works is rules are applied first. The Flatten[expr_, n_] -> Flatten[expr, n, List] rule is being applied but {2} is not an integer so Mathics doesn't know what to do and reports an error.

[wolfv]

Is there another Wolfram Language Spec this was based on? Because with the latest, "my" syntax should work.

Also, could you show me how to flatten the second dimension? I wasn't able to find another way to do it (ie. I tried: Flatten[{{1,2,3}, {4,5,6}}, 1, {2}] )

[sn6uv]

I'm reading through the online docs trying to get my head around what Flatten is spposed to do when the second argument is a list.

[wolfv]

Let me know if I can be of any help.

As I understand, the list contains dimensions along which to flatten.

cf: