Extensible query operations/commands

Hi David,
Let's continue the discussion based on that post of Julia Discourse.

You did misunderstand what I mean, but after a sufficient sleep I found my previous comment could be a little vague because you call something like @take and @drop a command . And what I actually mean is about

• Is there a way to define our custom command(what you refer to as the standalone query command)

• If not such a chance, could/should we bring this into Query.jl?

e.g., we can implement an extension command to provide such a

@extension_command function apply_col(df,  mapper :: Pair{Symbol, F}) where F <: Function
       update_col, func = mapper
       col_names = names(df)
       map(col_names) do col_name
         if col_name !== update_col
             col_name => df[col_name]    
         else
             col_name => map(func, df[col_name])
         end
       end |> x -> DataFrame(x...)
end

df |> 
   @apply_col(:x => x -> x  > 20 ? 0.5x : -2x) |>
    ...

I know that it could be implemented via @select but, extensibility could be attractive.

Start with the linked Julia Discourse discussion and inspect Query.jl's existing standalone query commands, including @take, @drop, and @select. Determine whether users can define custom commands such as the proposed @apply_col, and clarify whether supporting that extension belongs in Query.jl. Done should include a decided scope and an agreed user-facing behavior.