Integer as destination in `src => fun => dest`

[pdeffebach]

Motivated by this discourse discussion

MWE:

julia> df = DataFrame(a = [1, 2]);

julia> transform(df, :a => (identity => 1))
ERROR: ArgumentError: Unrecognized column selector: :a => (identity => 1)

[bkamins]

@pdeffebach I am not sure what you would want to achieve here. How should 1 as target column name be interpreted (as it is an invalid column name)?

Do you mean transform(df, :a => (identity => "1")) or transform(df, :a => identity, renamecols=false) or transform(df, :a => (identity => identity))?

[pdeffebach]

1 should be interpreted as positional. renamecols solves the problem where src and dest are the same integer. But if they were different one might want to write

2 => f => 1

to take the 2nd column, apply f, then stick the result in the 1st position.

fwiw I don't think this is a good idea or a feature that should be added at the point. Closing, at least we can point to this issue in the future if needed.

[bkamins]

stick the result in the 1st position.

position is determined by the order of transformations passed. The problem is that if you pass 1 as output position there is no way to determine what should be the name of this column (note that the name is for NEW data frame, which is initially empty i.e. does not have any names)

2 as source specifier is fully un-ambiguous as OLD data frame has some columns and we can check what is the name and value of column #2.