A function, formula, or atomic vector.
If a function, it is used as is.
If a formula, e.g. ~ .x + 2
, it is converted to a function. There
are three ways to refer to the arguments:
For a single argument function, use .
For a two argument function, use .x
and .y
For more arguments, use ..1
, ..2
, ..3
etc
This syntax allows you to create very compact anonymous functions.
If character vector, numeric vector, or list, it
is converted to an extractor function. Character vectors index by name
and numeric vectors index by position; use a list to index by position
and name at different levels. Within a list, wrap strings in get-attr()
to extract named attributes. If a component is not present, the value of
.default
will be returned.