## Overview

loopurrr makes purrr’s iterator functions more understandable and easier to debug. In this initial version, loopurrr consists only of one main function: as_loop().

as_loop() takes a function call to one of purrr’s iterator functions, such as purrr::map(), and translates it into a regular for loop.

You might ask: “Why would anyone want to do this?!”

as_loop() has at least three use cases:

1. Learning and Teaching Functional Programming
2. Debugging
3. Accessing and Extending purrr Functions

The remainder of this readme will expand on the uses cases above, show how to get started, and give a brief outlook on the development roadmap.

## Motivation and Use Cases

#### 1. Learning and Teaching Functional Programming

Beginners, and especially users new to functional-style programming, often have a hard time getting their head around R’s rather opaque iterator functions, such as base R’s lapply() or purrr::map(). for loops, on the other hand, are fairly well understood, even by users new to R.

as_loop() translates a function call to one of purrr’s iterator functions into a regular for loop. Through this as_loop() shows how purrr’s iterator functions work under the hood. After reading about what iterator functions do (for example here), LearneRs can start playing around with calling as_loop() on the examples in the purrr documentation. TeacheRs, on the other hand, can use as_loop() interactively when introducing the different types of iterator functions in the purrr package.

Finally, this package is not only for beginners and users new to R. When writing this package I was fairly confident in my understanding of purrr’s iterator functions. Nevertheless, translating each of them into a for loop was quite revealing, especially with the more complex functions, such as purrr::reduce() (specifically when the direction is set to "backward").

#### 2. Debugging

Once learneRs know what an iterator function does and how to use it, the next hurdle to take is dealing with failure. Iterator functions introduce an additional layer of complexity, because special knowledge is required to debug non-running code (see also here). By translating an iterator function into a regular for loop, as_loop() can help with debugging. Usually a for loop will run over an index, for example i. When executed in the global environment, useRs can easily inspect the code in the console at index i once the code throws an error - without any special knowledge of how to use a debugger, browser() or purrr::safely().

Of course, useRs are still highly encouraged to learn how to use R’s and purrr’s debugging tools and functions. However, in data science teams with different levels of programming knowledge, the possibility to translate complex iterator functions to regular for loops can help mitigate temporary knowledge gaps.

#### 3. Accessing and Extending {purrr} Functions

After getting used to purrr’s functions, they easily come to mind, when dealing with iteration problems. However, sometimes the purrr package is not available, for example in a production environment where new packages cannot easily be installed, or when writing a package that doesn’t depend on purrr. Although base R equivalents exist for purrr’s major functions, there are functions like purrr::imap() or purrr::reduce2() which are not available in base R and need to be constructed. In those cases, as_loop() provides a ready-to-use alternative.

Further, by translating purrr’s iterator functions into for loops, the underlying building blocks can easily be rearranged to create functions that are not included in the purrr package. For example, by translating a call to purrr::imap() and a call to purrr::map2() we could easily build a for loop that loops over two vectors and an index, as if a function like imap2() existed.

## Installation

loopurrr is not on CRAN yet. You can install the latest version from GitHub with:

# install.packages("remotes")
remotes::install_github("TimTeaFan/loopurrr")

## Getting Started

First, lets use get_supported_fns("as_loop") to get a glimpse of which iterator functions from the purrr package are currently supported by as_loop():

library(loopurrr)
get_supported_fns("as_loop")
#> $map #> [1] "map" "map_at" "map_chr" "map_dbl" "map_df" "map_dfc" "map_dfr" #> [8] "map_if" "map_int" "map_lgl" "map_raw" #> #>$imap
#> [1] "imap"     "imap_chr" "imap_dbl" "imap_dfc" "imap_dfr" "imap_int" "imap_lgl"
#> [8] "imap_raw"
#>
#> $map #> [1] "map2" "map2_chr" "map2_dbl" "map2_df" "map2_dfc" "map2_dfr" "map2_int" #> [8] "map2_lgl" "map2_raw" #> #>$pmap
#> [1] "pmap"     "pmap_chr" "pmap_dbl" "pmap_df"  "pmap_dfc" "pmap_dfr" "pmap_int"
#> [8] "pmap_lgl" "pmap_raw"
#>
#> $lmap #> [1] "lmap" "lmap_at" #> #>$modify
#> [1] "modify"    "modify_at" "modify_if" "modify2"   "imodify"
#>
#> $walk #> [1] "iwalk" "pwalk" "walk" "walk2" #> #>$accumulate
#> [1] "accumulate"  "accumulate2"
#>
#> \$reduce
#> [1] "reduce"  "reduce2"

Now lets take the first example of loopurrr’s documentation and start with translating a call to purrr::map(). First, lets look at the result:

x <- list(1, c(1:2), c(1:3))
x %>% purrr::map(sum)
#> [[1]]
#> [1] 1
#>
#> [[2]]
#> [1] 3
#>
#> [[3]]
#> [1] 6

Next, lets pipe the function call into as_loop().

x %>%
purrr::map(sum) %>%
as_loop()

Depending on the automatically detected output settings, the result will either be:

1. directly inserted in the original R script or the console, given that the code is run in RStudio and the rstudioapi package is installed,
2. copied to the clipboard, given that the above conditions are not met and the clipr package is installed,
3. or if none of the conditions above are met, the result will just be printed to the console.
# --- convert: map(x, sum) as loop --- #
out <- vector("list", length = length(x))

for (i in seq_along(x)) {
out[[i]] <- sum(x[[i]])
}
# --- end loop --- #

To see the result we need to print out:

out
#> [[1]]
#> [1] 1
#>
#> [[2]]
#> [1] 3
#>
#> [[3]]
#> [1] 6

For future versions of loopurrr the following milestones are planned:

• release loopurrr on CRAN
• enable support for more iterator functions from purrr (e.g. cross() etc.)
• support base R’s apply family in as_loop()
• translate purrr’s iterators to base R equivalents with as_base() (yet to be created)

If anyone is interested in collaborating on one or more of those milestones, any help is appreciated! Feel free to reach out to me, for example on Twitter @TimTeaFan.

## History

The idea of this package is based on an experience I had at work. After diving deeper into purrr’s iterator functions I started refactoring some old code by replacing loops with map functions. To my surprise, although the code was much cleaner now, not everybody liked it. For some users it made things harder to understand. Learning once how map functions work, was not enough to solve this, since things got more complicated when the code was throwing errors. loopurrr allows us to write clean code and translate it to regular for loops when needed.

## Acknowledgements

Credit goes to the creators and maintainers of the amazing purrr package! loopurrr is just an add-on which would not exist without it.

Further, credit goes to the {gradethis} package from which I adapted this code to make as_loop() work with piped expressions (function calls). {gradethis} license and copyrights apply!

I was further inpsired by Miles McBain’s {datapasta}’s different output options. Looking at the code alone wasn’t enough, I also got help on StackOverflow from user @Waldi to make the {rstudioapi} package work.

Finally, I adapted this answer on StackOverflow to replace the function arguments of the functions in map(.f = ) with the actual objects that are being used.

## Disclaimer

This package does not promote for loops over iterator functions. Rather the opposite is true. I love the purrr package and would be happy if people would use it more.

Although this package contains tests with more than 1000 lines of code, there are definitely a number of edge cases which won’t work correctly. If you find one, I’d be happy if you file an issue here.