wrapr_applicable
In wrapr: Wrap R Tools for Debugging and Parametric Programming

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

wrapr includes de-referencing, function evaluation, and a new concept called "wrapr_applicable". "wrapr_applicable" is dispatch by type of right hand side argument scheme.

Basic `wrapr`

The wrapr pipe operators (%.>% and %>.%) are roughly defined as: a %>.% b ~ { . <- a; b };. This works under the assumption that b is an expression with free-instances of ".". A typical use is:

library("wrapr")

5 %.>% sin(.)

The above is performed by standard S3 dispatch on the left argument of an exported generic functions called apply_left() and apply_right(). A formal description of wrapr piping can be found here.

Dereferencing and function evaluation

wrapr works primarily over expressions and ".". wrapr does tries to de-reference names found in the right-hand side of pipe stages, and also dispatches functions. One can also write the following.

5 %.>% sin
5 %.>% base::sin

`"wrapr_applicable"`

Arbitrary objects ask wrapr to treat them as special expressions by overriding one or more of apply_left() and apply_right() for the S3 class they wish managed.

For example:

function_reference <- list(f = sin)
class(function_reference) <- c("wrapr_applicable", "ourclass")

apply_right.ourclass <- function(pipe_left_arg,
                                 pipe_right_arg,
                                 pipe_environment,
                                 left_arg_name,
                                 pipe_string,
                                 right_arg_name) {
  pipe_right_arg$f(pipe_left_arg)
}

function_reference

5 %.>% function_reference

function_reference$f <- sqrt
5 %.>% function_reference

The signature arguments work as follows:

pipe_left_arg: The value moving down the pipeline.
pipe_right_arg: The right pipeline operator (essentially "self" or "this" in object oriented terms, used for S3 dispatch).
pipe_environment: The environment the pipeline is working in (not usually needed).
left_arg_name: If the left arguement was passed in by name, what that name was.
pipe_string: The name of the pipe operator (not usually needed).
right_arg_name: If the right arguement was passed in by name, what that name was.

This functionality allows arbitrary objects to directly specify their intended pipeline behavior.

Let's use a debugging function to see the values of all of the arguments.

apply_right.ourclass <- function(pipe_left_arg,
                                 pipe_right_arg,
                                 pipe_environment,
                                 left_arg_name,
                                 pipe_string,
                                 right_arg_name) {
  print("pipe_left_arg")
  print(pipe_left_arg)
  print("pipe_right_arg")
  print(pipe_right_arg)
  print("pipe_environment")
  print(pipe_environment)
  print("left_arg_name")
  print(left_arg_name)
  print("pipe_string")
  print(pipe_string)
  print("right_arg_name")
  print(right_arg_name)
  pipe_right_arg$f(pipe_left_arg)
}

5 %.>% function_reference

a <- 5

a %.>% function_reference

Conclusion

wrapr values (left-hand sides of pipe expressions) are completely general. wrapr operators (right-hand sides of pipe expressions) are primarily intended to be expressions that have "." as a free-reference. wrapr can also be used with right-hand sides that are function references or with arbitrary annotated objects.