If providing multiple column names, the result will be an expansion of
validation steps to that number of column names (e.g., vars(col_a, col_b)
will result in the entry of two validation steps). Aside from column names
in quotes and in vars(), tidyselect helper functions are available for
specifying columns. They are: starts_with(), ends_with(), contains(),
matches(), and everything().
Often, we will want to specify actions for the validation. This argument,
present in every validation step function, takes a specially-crafted list
object that is best produced by the action_levels() function. Read that
function's documentation for the lowdown on how to create reactions to
above-threshold failure levels in validation. The basic gist is that you'll
want at least a single threshold level (specified as either the fraction test
units failed, or, an absolute value), often using the warn_at argument.
This is especially true when x is a table object because, otherwise,
nothing happens. For the col_is_*()-type functions, using
action_levels(warn_at = 1) or action_levels(stop_at = 1) are good choices
depending on the situation (the first produces a warning, the other
stop()s).
Want to describe this validation step in some detail? Keep in mind that this
is only useful if x is an agent. If that's the case, brief the agent
with some text that fits. Don't worry if you don't want to do it. The
autobrief protocol is kicked in when brief = NULL and a simple brief will
then be automatically generated.