Build Function Factories

Function factories are functions that make functions. They can be confusing to construct. Straightforward techniques can produce functions that are fragile or hard to understand. While more robust techniques exist to construct function factories, those techniques can be confusing. This package is designed to make it easier to construct function factories.

factory

The goal of factory is to make construction of function factories more straightforward, without requiring the user to learn the rlang package.

Installation

You can install factory from GitHub with:

# install.packages("remotes")
remotes::install_github("jonthegeek/factory")

Motivation

Function factories are functions that make functions. They can be confusing to work with. For example, they can produce functions that are fragile (examples from Advanced R by Hadley Wickham (2nd Edition), 10.2.3: Forcing Evaluation, “Gah” comments are me):

power1 <- function(exponent) {
  function(x) {
    x ^ exponent
  }
}

x <- 2
square1 <- power1(x)

x <- 3
square1(2) # Gah, fragile!
#> [1] 8

You can make factories that are less fragile, if you remember to force the variables.

power2 <- function(exponent) {
  force(exponent) # Gah, easy to forget!
  function(x) {
    x ^ exponent
  }
}

x <- 2
square2 <- power2(x)
x <- 3
square2(2)
#> [1] 4

However, the resulting function can be hard to understand:

square2
#> function(x) {
#>     x ^ exponent
#>   }
#> <environment: 0x00000000163e5650>

You can make functions that are easier to understand, but building the function factory is much more difficulty (from Advanced R by Hadley Wickham (2nd Edition), 19.7.4: Creating functions):

power3 <- function(exponent) {
  rlang::new_function(
    rlang::exprs(x = ), 
    rlang::expr({
      x ^ !!exponent
    }), 
    rlang::caller_env()
  )
}

The resulting functions look like a “normal” function, though, and are thus easier for users to understand:

square3 <- power3(2)
square3
#> function (x) 
#> {
#>     x^2
#> }

The goal of factory is to make function factories as straightforward to create as in power1, but to make the resulting functions make as much sense as in power3:

library(factory)
power4 <- build_factory(
  fun = function(x) {
    x ^ exponent
  },
  exponent
)

x <- 2
square4 <- power4(x)
x <- 3
square4(2)
#> [1] 4

The resulting function is clear, as with power3:

square4
#> function (x) 
#> {
#>     x^2
#> }