source_df should inherit from data.frame

[hadley]

So you can use it as, without having to manually coerce to a data frame. And possibly should do the same thing for data.table

[dmenne]

Yep, and I know of a good test case, lme/nlme (and friends). That's the one which fails (or failed? did not check today) with the implicit casting in data.table.

[hadley]

But in general, you can't assume that a data source is a data frame, and it will be safer to force the user to acknowledge this by using as.data.frame(). I think one of the things that makes data.table hard to use is that it tries to hard to behave like data frame, even though it works in a profoundly different way.

[dmenne]

Point taken, I had this trouble a few times already. A good error message would be nice, because I remember using quite some time to locate the problem I thought was in lme.