Author: Yihui Xie
Date: 2020-07-23

# An Introduction to xfun

### A Collection of Miscellaneous Functions

#### Yihui Xie

#### 2020-07-23

After writing about 20 R packages, I found I had accumulated several utility functions that I used across different packages, so I decided to extract them into a separate package. Previously I had been using the evil triple-colon `:::` to access these internal utility functions. Now with xfun, these functions have been exported, and more importantly, documented. It should be better to use them under the sun instead of in the dark.

This page shows examples of a subset of functions in this package. For a full list of functions, see the help page `help(package = 'xfun')`. The source package is available on Github: https://github.com/yihui/xfun.

## No more partial matching for lists!

I have been bitten many times by partial matching in lists, e.g., when I want `x$a` but the element `a` does not exist in the list `x`, it returns the value `x$abc` if `abc` exists in `x`. This is very annoying to me which is why I created strict lists. A strict list is a list for which the partial matching of the `$` operator is disabled. The functions `xfun::strict_list()` and `xfun::as_strict_list()` are the equivalents to `base::list()` and `base::as.list()` respectively which always return as strict list, e.g.,

    library(xfun)
    (z = strict_list(aaa = "I am aaa", b = 1:5))

    ## $aaa
    ## [1] "I am aaa"
    ##
    ## $b
    ## [1] 1 2 3 4 5

    z$a  # NULL (strict matching)

    ## NULL

    z$aaa  # I am aaa

    ## [1] "I am aaa"

    z$b

    ## [1] 1 2 3 4 5

    z$c = "you can create a new element"

    z2 = unclass(z)  # a normal list
    z2$a  # partial matching

    ## [1] "I am aaa"

    z3 = as_strict_list(z2)  # a strict list again
    z3$a  # NULL (strict matching) again!

    ## NULL

Similarly, the default partial matching in `attr()` can be annoying, too. The function `xfun::attr()` is simply a shorthand of `attr(..., exact = TRUE)`.

I want it, or I do not want. There is no “I probably want”.

## Output character vectors for human eyes

When R prints a character vector, your eyes may be distracted by the indices like `[1]`, double quotes, and escape sequences. To see a character vector in its “raw” form, you can use `cat(..., sep = '\n')`. The function `raw_string()` marks a character vector as “raw”, and the corresponding printing function will call `cat(sep = '\n')` to print the character vector to the console.

    library(xfun)
    raw_string(head(LETTERS))

    A
    B
    C
    D
    E
    F

    (x = c("a \"b\"", "hello\tworld!"))

    [1] "a \"b\""       "hello\tworld!"

    raw_string(x)  # this is more likely to be what you want to see

    a "b"
    hello   world!

## Print the content of a text file

I have used `paste(readLines('foo'), collapse = '\n')` many times before I decided to write a simple wrapper function `xfun::file_string()`. This function also makes use of `raw_string()`, so you can see the content of a file in the console as a side-effect, e.g.,

    f = system.file("LICENSE", package = "xfun")
    xfun::file_string(f)

    YEAR: 2018
    COPYRIGHT HOLDER: Yihui Xie

    as.character(xfun::file_string(f))  # essentially a character string

    [1] "YEAR: 2018\nCOPYRIGHT HOLDER: Yihui Xie"

## Search and replace strings in files

I can never remember how to properly use `grep` or `sed` to search and replace strings in multiple files. My favorite IDE, RStudio, has not provided this feature yet (you can only search and replace in the currently opened file). Therefore I did a quick and dirty implementation in R, including functions `gsub_files()`, `gsub_dir()`, and `gsub_ext()`, to search and replace strings in multiple files under a directory. Note that the files are assumed to be encoded in UTF-8. If you do not use UTF-8, we cannot be friends. Seriously.

All functions are based on `gsub_file()`, which performs searching and replacing in a single file, e.g.,

    library(xfun)
    f = tempfile()
    writeLines(c("hello", "world"), f)
    gsub_file(f, "world", "woRld", fixed = TRUE)
    file_string(f)

    hello
    woRld

The function `gsub_dir()` is very flexible: you can limit the list of files by MIME types, or extensions. For example, if you want to do substitution in text files, you may use `gsub_dir(..., mimetype = '^text/')`.

WARNING: Before using these functions, make sure that you have backed up your files, or version control your files. The files will be modified in-place. If you do not back up or use version control, there is no chance to regret.

## Manipulate filename extensions

Functions `file_ext()` and `sans_ext()` are based on functions in tools. The function `with_ext()` adds or replaces extensions of filenames, and it is vectorized.

    library(xfun)
    p = c("abc.doc", "def123.tex", "path/to/foo.Rmd")
    file_ext(p)

    ## [1] "doc" "tex" "Rmd"

    sans_ext(p)

    ## [1] "abc"         "def123"      "path/to/foo"

    with_ext(p, ".txt")

    ## [1] "abc.txt"         "def123.txt"      "path/to/foo.txt"

    with_ext(p, c(".ppt", ".sty", ".Rnw"))

    ## [1] "abc.ppt"         "def123.sty"      "path/to/foo.Rnw"

    with_ext(p, "html")

    ## [1] "abc.html"         "def123.html"      "path/to/foo.html"

## Types of operating systems

The series of functions `is_linux()`, `is_macos()`, `is_unix()`, and `is_windows()` test the types of the OS, using the information from `.Platform` and `Sys.info()`, e.g.,

    xfun::is_macos()

    ## [1] TRUE

    xfun::is_unix()

    ## [1] TRUE

    xfun::is_linux()

    ## [1] FALSE

    xfun::is_windows()

    ## [1] FALSE

## Loading and attaching packages

Oftentimes I see users attach a series of packages in the beginning of their scripts by repeating `library()` multiple times. This could be easily vectorized, and the function `xfun::pkg_attach()` does this job. For example,

    library(testit)
    library(parallel)
    library(tinytex)
    library(mime)

is equivalent to

    xfun::pkg_attach(c('testit', 'parallel', 'tinytex', 'mime'))

I also see scripts that contain code to install a package if it is not available, e.g.,

    if (!requireNamespace('tinytex')) install.packages('tinytex')
    library(tinytex)

This could be done via

    xfun::pkg_attach2('tinytex')

The function `pkg_attach2()` is a shorthand of `pkg_attach(..., install = TRUE)`, which means if a package is not available, install it. This function can also deal with multiple packages.

The function `loadable()` tests if a package is loadable.

## Read/write files in UTF-8

Functions `read_utf8()` and `write_utf8()` can be used to read/write files in UTF-8. They are simple wrappers of `readLines()` and `writeLines()`.

## Convert numbers to English words

The function `numbers_to_words()` (or `n2w()` for short) converts numbers to English words.

    n2w(0, cap = TRUE)

    ## [1] "Zero"

    n2w(seq(0, 121, 11), and = TRUE)

    ##  [1] "zero"                       "eleven"
    ##  [3] "twenty-two"                 "thirty-three"
    ##  [5] "forty-four"                 "fifty-five"
    ##  [7] "sixty-six"                  "seventy-seven"
    ##  [9] "eighty-eight"               "ninety-nine"
    ## [11] "one hundred and ten"        "one hundred and twenty-one"

    n2w(1e+06)

    ## [1] "one million"

    n2w(1e+11 + 12345678)

    ## [1] "one hundred billion, twelve million, three hundred forty-five thousand, six hundred seventy-eight"

    n2w(-987654321)

    ## [1] "minus nine hundred eighty-seven million, six hundred fifty-four thousand, three hundred twenty-one"

    n2w(1e+15 - 1)

    ## [1] "nine hundred ninety-nine trillion, nine hundred ninety-nine billion, nine hundred ninety-nine million, nine hundred ninety-nine thousand, nine hundred ninety-nine"

## Cache an R expression to an RDS file

The function `cache_rds()` provides a simple caching mechanism: the first time an expression is passed to it, it saves the result to an RDS file; the next time it will read the RDS file and return the value instead of evaluating the expression again. If you want to invalidate the cache, you can use the argument `rerun = TRUE`.

    res = xfun::cache_rds({
      # pretend the computing here is a time-consuming
      Sys.sleep(2)
      1:10
    })

When the function is used in a code chunk in a knitr document, the RDS cache file is saved to a path determined by the chunk label (the base filename) and the chunk option `cache.path` (the cache directory), so you do not have to provide the `file` and `dir` arguments of `cache_rds()`.

This caching mechanism is much simpler than knitr’s caching. Cache invalidation is often tricky (see this post), so this function may be helpful if you want more transparency and control over when to invalidate the cache (for `cache_rds()`, the cache is invalidated when the cache file is deleted, which can be achieved via the argument `rerun = TRUE`).

As documented on the help page of `cache_rds()`, there are two common cases in which you may want to invalidate the cache:

1. The code in the expression has changed, e.g., if you changed the code from `cache_rds({x + 1})` to `cache_rds({x + 2})`, the cache will be automatically invalidated and the expression will be re-evaluated. However, please note that changes in white spaces or comments do not matter. Or generally speaking, as long as the change does not affect the parsed expression, the cache will not be invalidated, e.g., the two expressions below are essentially identical (hence if you have executed `cache_rds()` on the first expression, the second expression will be able to take advantage of the cache):

       res = xfun::cache_rds({
         Sys.sleep(3  );
         x=1:10;  # semi-colons won't matter
         x+1;
       })

       res = xfun::cache_rds({
         Sys.sleep(3)
         x = 1:10  # a comment
         x +
           1  # feel free to make any changes in white spaces
       })

2. The value of a global variable in the expression has changed, e.g., if `y` has changed, you are most likely to want to invalidate the cache and rerun the expression below:

       res = xfun::cache_rds({
         x = 1:10
         x + y
       })

   This is because `x` is a local variable in the expression, and `y` is an external global variable (not created locally like `x`). To invalidate the cache when `y` has changed, you may let `cache_rds()` know through the `hash` argument that `y` needs to be considered when deciding if the cache should be invalidated:

       res = xfun::cache_rds({
         x = 1:10
         x + y
       }, hash = list(y))

   If you do not want to provide this list of value(s) to the `hash` argument, you may try `hash = "auto"` instead, which asks `cache_rds()` to try to figure out all global variables automatically and use a list of their values as the value for the `hash` argument.

       res = xfun::cache_rds({
         x = 1:10
         x + y
       }, hash = "auto")

## Check reverse dependencies of a package

Running `R CMD check` on the reverse dependencies of knitr and rmarkdown is my least favorite thing in developing R packages, because the numbers of their reverse dependencies are huge. The function `rev_check()` reflects some of my past experience in this process. I think I have automated it as much as possible, and made it as easy as possible to discover possible new problems introduced by the current version of the package (compared to the CRAN version). Finally I can just sit back and let it run.

## Input a character vector into the RStudio source editor

The function `rstudio_type()` inputs characters in the RStudio source editor as if they were typed by a human. I came up with the idea when preparing my talk for rstudio::conf 2018 (see this post for more details).

## Print session information

Since I have never been fully satisfied by the output of `sessionInfo()`, I tweaked it to make it more useful in my use cases. For example, it is rarely useful to print out the names of base R packages, or information about the matrix products / BLAS / LAPACK. Oftentimes I want additional information in the session information, such as the Pandoc version when rmarkdown is used. The function `session_info()` tweaks the output of `sessionInfo()`, and makes it possible for other packages to append information in the output of `session_info()`.

You can choose to print out the versions of only the packages you specify, e.g.,

    xfun::session_info(c('xfun', 'rmarkdown', 'knitr', 'tinytex'), dependencies = FALSE)

    ## R version 4.0.2 (2020-06-22)
    ## Platform: x86_64-apple-darwin17.0 (64-bit)
    ## Running under: macOS Catalina 10.15.5
    ##
    ## Locale: C / en_US.UTF-8 / en_US.UTF-8 / C / en_US.UTF-8 / en_US.UTF-8
    ##
    ## Package version:
    ##   knitr_1.29.2    rmarkdown_2.3.2 tinytex_0.24.1  xfun_0.16
    ##
    ## Pandoc version: 2.9.2.1