[Gretl-devel] interaction effects with only series (no lists)

Hi everybody, in chapter 15 of the user guide under "Generating lists of transformed variables" there is an explanation of how to use the "^" operator to create a list of interaction effects. This is nice, but to the average user it might be surprising to read in the footnote "this construct does not work if neither D nor Z are of the the list type." The reason for this, AFAIK, is that if both D and Z are series objects, then D^Z is just the exponentiation of the corresponding values of D by Z, and so we get a single series in return with a totally different meaning. (And since this series is then anonymous, trying to assign it to a list type yields an error.) I believe there are some little problems with this situation. 1) This is not my main point, but there might be a danger of inadvertently doing the exponentiation although it wasn't intended. Example: <hansl> open mroz87.gdt # KL6 is a discrete number of children, WA is (pseudo-) continuous inter = KL6 ^ WA    # note no explicit "list" type is indicated </hansl> This will create a series "inter" which has nothing to do with interaction effects. Possibly this could imply subtle bugs, if "inter" is mistakenly treated as a regressor list down the road. Possible solutions for this problem: Perhaps mention this danger explicitly in the documentation. Also, why actually is the "^" operator overloaded with these very different things? Couldn't the interaction thing be indicated through, say, "^^" instead? 2) It's a bit of a pity that interactions through this mechanism aren't possible for single series, i.e. that 'list inter = KL6 ^ WA' currently must fail. It would be nice if this could be generalized, no? Possible solutions for this second point: Agani, if we had a different and exclusive operator like "^^", then it would seem to be fairly easy to implement, right? Apart from that, I guess that a pretty straightforward workaround could be mentioned in the guide, namely something like: list H = deflist(D) ^ Z (which in the specific context above becomes: list inter = deflist(KL6) ^ WA, tested as working). OK, so bottom line, I think we should add some remarks to the documentation, and then maybe discuss the pros and cons of having a separate operator. thanks sven