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https://github.com/jonthegeek/factory
factory: Build Function Factories
https://github.com/jonthegeek/factory
function-factory r
Last synced: 15 days ago
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factory: Build Function Factories
- Host: GitHub
- URL: https://github.com/jonthegeek/factory
- Owner: jonthegeek
- License: other
- Created: 2019-04-22T03:19:20.000Z (over 5 years ago)
- Default Branch: master
- Last Pushed: 2020-06-04T22:42:24.000Z (over 4 years ago)
- Last Synced: 2024-10-26T08:38:23.374Z (17 days ago)
- Topics: function-factory, r
- Language: R
- Homepage:
- Size: 202 KB
- Stars: 49
- Watchers: 4
- Forks: 6
- Open Issues: 14
-
Metadata Files:
- Readme: README.Rmd
- Contributing: .github/CONTRIBUTING.md
- License: LICENSE
- Code of conduct: CODE_OF_CONDUCT.md
Awesome Lists containing this project
- jimsghstars - jonthegeek/factory - factory: Build Function Factories (R)
README
---
output: github_document
---
```{r, include = FALSE}
knitr::opts_chunk$set(
collapse = TRUE,
comment = "#>",
fig.path = "man/figures/README-",
out.width = "100%"
)
```
# factory 
[](https://www.tidyverse.org/lifecycle/#maturing)
[](https://travis-ci.org/jonthegeek/factory)
[](https://ci.appveyor.com/project/jonthegeek/factory)
[](https://codecov.io/gh/jonthegeek/factory?branch=master)
[](https://CRAN.R-project.org/package=factory)
The goal of factory is to make construction of function factories more straightforward, without requiring the user to learn the `rlang` package.
## Installation
Install the released version of factory from CRAN:
```{r cran, eval = FALSE}
install.packages("factory")
```
Or install the development version from [GitHub](https://github.com/jonthegeek/factory) with:
```{r dev, eval = FALSE}
# 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, as we'll see below, they can produce functions that are fragile, or that are confusing to work with as a user.
WARNING: All code shown below is "wrong" in some way until we get to the example at the end! These examples show the dangers of working with function factories, and why this package exists.
(examples adapted from [Advanced R by Hadley Wickham (2nd Edition), 10.2.3: Forcing Evaluation](https://adv-r.hadley.nz/function-factories.html#forcing-evaluation))
### The Simplest Factories are Fragile
`power1` is a function factory.
It returns a function based on the `exponent` argument.
```{r power1}
power1 <- function(exponent) {
function(x) {
x ^ exponent
}
}
```
For many use cases, `power1` works fine.
For example, we can define a square function by calling `power1` with `exponent = 2`.
```{r power1-simple-usage}
square1 <- power1(2)
square1(2)
# 2 ^ 2 = 4
square1(3)
# 3 ^ 2 = 9
```
However, `power1` is fragile.
Let's think about what the definition of power1 *means.*
The function returned by `power1` raises its argument to whatever the `exponent` variable is defined as.
Let's see what happens if we use a variable in the global environment to define our `square` function.
```{r power1-fragile}
my_exponent <- 2
square1a <- power1(my_exponent)
```
Due to R's lazy evaluation, when we call `power1`, the `exponent` variable gets a promise to take on the value of the `my_exponent` variable.
But `my_exponent` doesn't actually have the value of `2` yet.
Until we *use* `my_exponent`, it has a *promise* to get the value of `2`.
If we call `square1a` right away, it works as expected.
```{r power1-fragile-seems-ok}
square1a(2)
# 2 ^ 2 = 4
my_exponent <- 3
square1a(3)
# 3 ^ 2 = 9
```
The `my_exponent` promise (which was passed in during the definition of `square1a`) resolves to `2` the first time it is needed (when `square1a` is first called).
After that initial call, that is the value used in `square1a` forever.
But if `my_exponent` changes between definition of our function and first call of that function, we get a different result.
```{r power1-fragile-breaks}
my_exponent <- 2
square1b <- power1(my_exponent)
my_exponent <- 3
square1b(2)
# 2 ^ 3 = 8
square1b(3)
# 3 ^ 3 = 27
```
What happened?
When `square1b` was defined, `my_exponent` was passed in as a *promise.*
However, before `my_exponent` was ever actually *used*, its value changed.
The promise isn't evaluated *until it is used,* which, in this case, is the first time `square1b` is called.
Once the promise is evaluated, its value is "fixed," and the function works as expected.
### Forcing Arguments Trades Fragility for Complexity
We can make factories that are less fragile, if we remember to `force` the variables.
```{r power2}
power2 <- function(exponent) {
force(exponent) # Gah, easy to forget!
function(x) {
x ^ exponent
}
}
my_exponent <- 2
square2 <- power2(my_exponent)
my_exponent <- 3
square2(2)
# 2 ^ 2 = 4
square2(3)
# 3 ^ 2 = 9
```
Why does this work?
The `force` function forces the evaluation of its argument.
We don't really need to use `force`, per se.
Any function that forces evaluation would work, but `force` makes it obvious why we're doing it.
For example, we could produce the same result by `message`ing within the factory.
```{r power2-message}
power2b <- function(exponent) {
message("The exponent's value is ", exponent)
function(x) {
x ^ exponent
}
}
my_exponent <- 2
square2b <- power2b(my_exponent)
my_exponent <- 3
square2b(2)
# 2 ^ 2 = 4
square2b(3)
# 3 ^ 2 = 9
```
Since the value of `exponent` is needed for the message, the promise is evaluated when the factory is invoked, and the resulting function is stable.
While such factories are more stable, it's easy to miss a `force`.
And, in both of these cases, the resulting functions are difficult to understand as a user.
```{r resulting-functions}
square1
square2
cube <- power2(3)
cube
```
It isn't clear what these functions will do, since the definitions of `exponent` are hidden inside the function environments.
### Using rlang
We can use {rlang} to make functions that are easier to understand, but building the function factory is much more difficult (from [Advanced R by Hadley Wickham (2nd Edition), 19.7.4: Creating functions](https://adv-r.hadley.nz/quasiquotation.html#new-function)):
```{r power3}
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.
```{r square3}
square3 <- power3(2)
square3
```
The {rlang} calls are very difficult to understand, though.
It would be nice to get the stability and interpretability of the rlang-produced functions, with the ease-of-programming of the simplest function factories.
## Enter {factory}
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`.
Right now, the calls are still a *little* more complicated than I would like, but they're definitely easier to understand than the {rlang} calls.
```{r power4}
library(factory)
power4 <- build_factory(
fun = function(x) {
x ^ exponent
},
exponent
)
my_exponent <- 2
square4 <- power4(my_exponent)
my_exponent <- 3
square4(2)
# 2 ^ 2 = 4
```
The resulting function makes sense, as with `power3`.
```{r square4}
square4
```