wrapr_applicable

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(.)
#> [1] -0.9589243

Dereferencing and function evaluation

wrapr works primarily over expressions and “.”. As of version 1.1.0 wrapr does try to de-reference names found in the right-hand side of pipe stages, and also dispatches functions. That is: one can now write the following.

5 %.>% sin
#> [1] -0.9589243

"wrapr_applicable"

Arbitrary objects ask wrapr to treat them as special expressions by:

1. Adding the string "wrapr_applicable" to their class declarations.
2. Populating an list entry named "wrapr_function" with a function of signature (pipe_left_arg, pipe_right_arg, pipe_environment).

If these two conditions are met then wrapr instead of evaluating the object calls the function wrapr_function.

For example:

function_reference <- list(f = sin)
class(function_reference) <- "wrapr_applicable"
function_reference$wrapr_function <- function(pipe_left_arg, 
                                              pipe_right_arg, 
                                              pipe_environment) {
  pipe_right_arg$f(pipe_left_arg)
}

function_reference
#> $f
#> function (x)  .Primitive("sin")
#> 
#> $wrapr_function
#> function (pipe_left_arg, pipe_right_arg, pipe_environment) 
#> {
#>     pipe_right_arg$f(pipe_left_arg)
#> }
#> 
#> attr(,"class")
#> [1] "wrapr_applicable"

5 %.>% function_reference
#> [1] -0.9589243

function_reference$f <- sqrt
5 %.>% function_reference
#> [1] 2.236068

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).
• pipe_environment: The environment the pipeline is working in (not usually needed).

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

Application

rquery is a first application of this new "wrapr_applicable" feature. This allows rquery pipelines to be applied to directly to data.frames (even though rquery is primarily intended for databases). A discussion of the methodology can be found here.

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.