Great package!

I would recommend that you use a custom class internally to handle the measure column. It doesn't have to do much, just enough that you can do inherits(x, "measure_obj"). This will save you some headache in the long run.

So steps like step_measure_input_*() produces this class type. This automatically gives the benefit that you don't have to worry about name clashes as it is dependent on the class, not name of column.

All the remaining steps can then use the class.

A function like check_has_measure() can do any(inherits(x, "measure_obj")). You also get the flexibility to easily have multiple measure columns, as talked about in #13.