[pdeffebach]

In Julia you could have an `AbstractVector` type also be callable, or more likely a vector of callable objects (and the operation is performed row-wise).

I agree it's unlikely that a user will name their column `.data`. But it certainly saves developer effort from thinking about these issues.

The larger concern, really, is that Julia needs to know which things are columns and which things are variables in an expression at parse time in order to generate fast code for a DataFrame. It needs to do this without inspecting the data frame, since the data frame's contents aren't known at parse time.

One option would be to make all literals columns. But then you run into issues with things like `missing`, which would have to be escaped or not recognized as a column. Its hard to predict all the problems there, and any escaping rules would definitely have to be more complicated than R's. So we require `:` and take the easy way out, which has the added benefit for new users who might get confused about the variable-column distinction.