tower: Create a tower of functions.

In syberia/director: Load, track, and manage dependencies in a file structure.

Description

A tower is equivalent to the Ruby gem Rack's notion of middleware.

Usage

tower(functions = list())

Arguments

functions

list. A list of functions in the tower. The first argument of each function must be named "object" and each function must take a ... parameter. By default list(), which creates an identity tower that performs no operation.

Details

Imagine a function f1 that, in the middle of its processing, calls another function, f2, which in the middle of its processing, calls another function, f3, and so on.

To construct such a structure from the list of functions list(f1, f2, ...) we need to be able to call the next "inner" function from the previous outer function.

This is accomplished by providing a yield keyword which simply calls the next function in the tower.

The purpose of a tower is to modify some primary object throughout its operation. To this end, an object keyword will be provided to each tower function. If object is modified prior to a yield call, the next function in the tower will receive the modified object.

For composability, every function in a tower should have a yield keyword. The last function in the tower will yield to an identity function that simply returns the object.

Value

An S3 "tower" object, which is a callable function and must be passed the object as the first argument. Additional arguments will be passed to the first function in the tower.

Examples

functions <- list(
  function(object, ...) {
    object <- object + 1
    object <- yield()
    object + 1
  },

  function(object, ...) {
    object <- object * 2
    yield()
  }
)

t <- tower(functions)
v <- t(1) # This is now 5, since in the tower, we increment object,
          # Multiply it by 2 in the next function, then increment it
          # again after receiving the previous function.
stopifnot(v == 5)

syberia/director documentation built on May 30, 2019, 10:40 p.m.