21 Translating R code

21.1 Introduction

The combination of first-class environments, lexical scoping, and metaprogramming gives us a powerful toolkit for translating R code into other languages. One fully-fledged example of this idea is dbplyr, which powers the database backends for dplyr, allowing you to express data manipulation in R and automatically translate it into SQL. You can see the key idea in translate_sql() which takes R code and returns the equivalent SQL:

library(dbplyr)
translate_sql(x ^ 2)
#> <SQL> POWER("x", 2.0)
translate_sql(x < 5 & !is.na(x))
#> <SQL> "x" < 5.0 AND NOT((("x") IS NULL))
translate_sql(!first %in% c("John", "Roger", "Robert"))
#> <SQL> NOT("first" IN ('John', 'Roger', 'Robert'))
translate_sql(select == 7)
#> <SQL> "select" = 7.0

Translating R to SQL is complex because of the many idiosyncrasies of SQL dialects, so here I’ll develop two simple, but useful, domain specific languages (DSL): one to generate HTML, and the other to generate mathematical equations in LaTeX.

If you’re interested in learning more about domain specific languages in general, I highly recommend Domain Specific Languages (Fowler 2010). It discusses many options for creating a DSL and provides many examples of different languages.

Outline

  • Section 21.2 creates a DSL for generating HTML, using quasiquotation and purrr to generate a function for each HTML tag, then tidy evaluation to easily access them.

  • Section 21.3 transforms mathematically R code into its LaTeX equivalent using a combination of tidy evaluation and expression walking.

Prerequisites

This chapter pulls together many techniques discussed elsewhere in the book. In particular, you’ll need to understand environments, expressions, tidy evaluation, and a little functional programming and S3. We’ll use rlang for metaprogramming tools, and purrr for functional programming.

library(rlang)
library(purrr)

21.2 HTML

HTML (HyperText Markup Language) underlies the majority of the web. It’s a special case of SGML (Standard Generalised Markup Language), and it’s similar but not identical to XML (eXtensible Markup Language). HTML looks like this:

<body>
  <h1 id='first'>A heading</h1>
  <p>Some text &amp; <b>some bold text.</b></p>
  <img src='myimg.png' width='100' height='100' />
</body>

Even if you’ve never looked at HTML before, you can still see that the key component of its coding structure is tags, which look like <tag></tag> or <tag />. Tags can be nested within other tags and intermingled with text. There are over 100 HTML tags, but in this chapter we’ll focus on just a handful:

  • <body> is the top-level tag that contains all content.
  • <h1> defines a top level heading.
  • <p> defines a paragraph.
  • <b> emboldens text.
  • <img> embeds an image.

Tags can have named attributes which look like <tag name1='value1' name2='value2'></tag>. Two of the most important attributes are id and class, which are used in conjunction with CSS (Cascading Style Sheets) to control the visual appearance of the page.

Void tags, like <img>, don’t have any children, and are written <img />, not <img></img>. Since they have no content, attributes are more important, and img has three that are used with almost every image: src (where the image lives), width, and height.

Because < and > have special meanings in HTML, you can’t write them directly. Instead you have to use the HTML escapes: &gt; and &lt;. And since those escapes use &, if you want a literal ampersand you have to escape it as &amp;.

21.2.1 Goal

Our goal is to make it easy to generate HTML from R. To give a concrete example, we want to generate the following HTML:

<body>
  <h1 id='first'>A heading</h1>
  <p>Some text &amp; <b>some bold text.</b></p>
  <img src='myimg.png' width='100' height='100' />
</body>

Using the following code that matches the structure of the HTML as closely as possible:

with_html(
  body(
    h1("A heading", id = "first"),
    p("Some text &", b("some bold text.")),
    img(src = "myimg.png", width = 100, height = 100)
  )
)

This DSL has the following three properties:

  • The nesting of function calls matches the nesting of tags.

  • Unnamed arguments become the content of the tag, and named arguments become their attributes.

  • & and other special characters are automatically escaped.

21.2.2 Escaping

Escaping is so fundamental to translation that it’ll be our first topic. There are two related challenges:

  • In user input, we need to automatically escape &, < and >.

  • At the same time we need to make sure that the &, < and > we generate are not double-escaped (i.e. that we don’t accidentally generate &amp;amp;, &amp;lt; and &amp;gt;).

The easiest way to do this is to create an S3 class (Section 13.3) that distinguishes between regular text (that needs escaping) and HTML (that doesn’t).

html <- function(x) structure(x, class = "advr_html")

print.advr_html <- function(x, ...) {
  out <- paste0("<HTML> ", x)
  cat(paste(strwrap(out), collapse = "\n"), "\n", sep = "")
}

We then write an escape generic. It has two important methods:

  • escape.character() takes a regular character vector and returns an HTML vector with special characters (&, <, >) escaped.

  • escape.advr_html() leaves already escaped HTML alone.

escape <- function(x) UseMethod("escape")

escape.character <- function(x) {
  x <- gsub("&", "&amp;", x)
  x <- gsub("<", "&lt;", x)
  x <- gsub(">", "&gt;", x)

  html(x)
}

escape.advr_html <- function(x) x

Now we check that it works

escape("This is some text.")
#> <HTML> This is some text.
escape("x > 1 & y < 2")
#> <HTML> x &gt; 1 &amp; y &lt; 2

# Double escaping is not a problem
escape(escape("This is some text. 1 > 2"))
#> <HTML> This is some text. 1 &gt; 2

# And text we know is HTML doesn't get escaped.
escape(html("<hr />"))
#> <HTML> <hr />

Conveniently, this also allows a user to opt out of our escaping if they know the content is already escaped.

21.2.3 Basic tag functions

Next, we’ll write a one-tag function by hand, then figure out how to generalise it so we can generate a function for every tag with code.

Let’s start with <p>. HTML tags can have both attributes (e.g., id or class) and children (like <b> or <i>). We need some way of separating these in the function call. Given that attributes are named and children are not, it seems natural to use named and unnamed arguments for them respectively. For example, a call to p() might look like:

p("Some text. ", b(i("some bold italic text")), class = "mypara")

We could list all the possible attributes of the <p> tag in the function definition, but that’s hard because there are many attributes, and because it’s possible to use custom attributes. Instead, we’ll use ... and separate the components based on whether or not they are named. With this in mind, we create a helper function that wraps around rlang::list2() (Section 19.6) and returns named and unnamed components separately:

dots_partition <- function(...) {
  dots <- list2(...)
  
 if (is.null(names(dots))) {
  is_named <- rep(FALSE, length(dots))
} else {
  is_named <- names(dots) != ""
}
  
  list(
    named = dots[is_named],
    unnamed = dots[!is_named]
  )
}

str(dots_partition(a = 1, 2, b = 3, 4))
#> List of 2
#>  $ named  :List of 2
#>   ..$ a: num 1
#>   ..$ b: num 3
#>  $ unnamed:List of 2
#>   ..$ : num 2
#>   ..$ : num 4

We can now create our p() function. Notice that there’s one new function here: html_attributes(). It takes a named list and returns the HTML attribute specification as a string. It’s a little complicated (in part, because it deals with some idiosyncrasies of HTML that I haven’t mentioned here), but it’s not that important and doesn’t introduce any new programming ideas, so I won’t discuss it in detail. You can find the source online if you want to work through it yourself.

source("dsl-html-attributes.r")
p <- function(...) {
  dots <- dots_partition(...)
  attribs <- html_attributes(dots$named)
  children <- map_chr(dots$unnamed, escape)

  html(paste0(
    "<p", attribs, ">",
    paste(children, collapse = ""),
    "</p>"
  ))
}

p("Some text")
#> <HTML> <p>Some text</p>
p("Some text", id = "myid")
#> <HTML> <p id='myid'>Some text</p>
p("Some text", class = "important", `data-value` = 10)
#> <HTML> <p class='important' data-value='10'>Some text</p>

21.2.4 Tag functions

It’s straightforward to adapt p() to other tags: we just need to replace "p" with the name of the tag. One elegant way to do that is to create a function with rlang::new_function() (Section 19.7.4), using unquoting and paste0() to generate the starting and ending tags.

tag <- function(tag) {
  new_function(
    exprs(... = ),
    expr({
      dots <- dots_partition(...)
      attribs <- html_attributes(dots$named)
      children <- map_chr(dots$unnamed, escape)

      html(paste0(
        !!paste0("<", tag), attribs, ">",
        paste(children, collapse = ""),
        !!paste0("</", tag, ">")
      ))
    }),
    caller_env()
  )
}
tag("b")
#> function (...) 
#> {
#>     dots <- dots_partition(...)
#>     attribs <- html_attributes(dots$named)
#>     children <- map_chr(dots$unnamed, escape)
#>     html(paste0("<b", attribs, ">", paste(children, collapse = ""), 
#>         "</b>"))
#> }

We need the weird exprs(... = ) syntax to generate the empty ... argument in the tag function. See Section 18.6.2 for more details.

Now we can run our earlier example:

p <- tag("p")
b <- tag("b")
i <- tag("i")
p("Some text. ", b(i("some bold italic text")), class = "mypara")
#> <HTML> <p class='mypara'>Some text. <b><i>some bold italic
#> text</i></b></p>

Before we generate functions for every possible HTML tag, we need to create a variant that handles void tags. void_tag() is quite similar to tag(), but it throws an error if there are any unnamed tags, and the tag itself looks a little different.

void_tag <- function(tag) {
  new_function(
    exprs(... = ),
    expr({
      dots <- dots_partition(...)
      if (length(dots$unnamed) > 0) {
        abort(!!paste0("<", tag, "> must not have unnamed arguments"))
      }
      attribs <- html_attributes(dots$named)

      html(paste0(!!paste0("<", tag), attribs, " />"))
    }),
    caller_env()
  )
}

img <- void_tag("img")
img
#> function (...) 
#> {
#>     dots <- dots_partition(...)
#>     if (length(dots$unnamed) > 0) {
#>         abort("<img> must not have unnamed arguments")
#>     }
#>     attribs <- html_attributes(dots$named)
#>     html(paste0("<img", attribs, " />"))
#> }
img(src = "myimage.png", width = 100, height = 100)
#> <HTML> <img src='myimage.png' width='100' height='100' />

21.2.5 Processing all tags

Next we need to generate these functions for every tag. We’ll start with a list of all HTML tags:

tags <- c("a", "abbr", "address", "article", "aside", "audio",
  "b","bdi", "bdo", "blockquote", "body", "button", "canvas",
  "caption","cite", "code", "colgroup", "data", "datalist",
  "dd", "del","details", "dfn", "div", "dl", "dt", "em",
  "eventsource","fieldset", "figcaption", "figure", "footer",
  "form", "h1", "h2", "h3", "h4", "h5", "h6", "head", "header",
  "hgroup", "html", "i","iframe", "ins", "kbd", "label",
  "legend", "li", "mark", "map","menu", "meter", "nav",
  "noscript", "object", "ol", "optgroup", "option", "output",
  "p", "pre", "progress", "q", "ruby", "rp","rt", "s", "samp",
  "script", "section", "select", "small", "span", "strong",
  "style", "sub", "summary", "sup", "table", "tbody", "td",
  "textarea", "tfoot", "th", "thead", "time", "title", "tr",
  "u", "ul", "var", "video"
)

void_tags <- c("area", "base", "br", "col", "command", "embed",
  "hr", "img", "input", "keygen", "link", "meta", "param",
  "source", "track", "wbr"
)

If you look at this list carefully, you’ll see there are quite a few tags that have the same name as base R functions (body, col, q, source, sub, summary, table). This means we don’t want to make all the functions available by default, either in the global environment or in a package. Instead, we’ll put them in a list (like in Section 10.5) and then provide a helper to make it easy to use them when desired. First, we make a named list containing all the tag functions:

html_tags <- c(
  tags %>% set_names() %>% map(tag),
  void_tags %>% set_names() %>% map(void_tag)
)

This gives us an explicit (but verbose) way to create HTML:

html_tags$p(
  "Some text. ",
  html_tags$b(html_tags$i("some bold italic text")),
  class = "mypara"
)
#> <HTML> <p class='mypara'>Some text. <b><i>some bold italic
#> text</i></b></p>

We can then finish off our HTML DSL with a function that allows us to evaluate code in the context of that list. Here we slightly abuse the data mask, passing it a list of functions rather than a data frame. This is quick hack to mingle the execution environment of code with the functions in html_tags.

with_html <- function(code) {
  code <- enquo(code)
  eval_tidy(code, html_tags)
}

This gives us a succinct API which allows us to write HTML when we need it but doesn’t clutter up the namespace when we don’t.

with_html(
  body(
    h1("A heading", id = "first"),
    p("Some text &", b("some bold text.")),
    img(src = "myimg.png", width = 100, height = 100)
  )
)
#> <HTML> <body><h1 id='first'>A heading</h1><p>Some text
#> &amp;<b>some bold text.</b></p><img src='myimg.png' width='100'
#> height='100' /></body>

If you want to access the R function overridden by an HTML tag with the same name inside with_html(), you can use the full package::function specification.

21.2.6 Exercises

  1. The escaping rules for <script> tags are different because they contain JavaScript, not HTML. Instead of escaping angle brackets or ampersands, you need to escape </script> so that the tag isn’t closed too early. For example, script("'</script>'"), shouldn’t generate this:

    <script>'</script>'</script>

    But

    <script>'<\/script>'</script>

    Adapt the escape() to follow these rules when a new argument script is set to TRUE.

  2. The use of ... for all functions has some big downsides. There’s no input validation and there will be little information in the documentation or autocomplete about how they are used in the function. Create a new function that, when given a named list of tags and their attribute names (like below), creates tag functions with named arguments.

    list(
      a = c("href"),
      img = c("src", "width", "height")
    )

    All tags should get class and id attributes.

  3. Reason about the following code that calls with_html() referencing objects from the environment. Will it work or fail? Why? Run the code to verify your predictions.

    greeting <- "Hello!"
    with_html(p(greeting))
    
    p <- function() "p"
    address <- "123 anywhere street"
    with_html(p(address))
  4. Currently the HTML doesn’t look terribly pretty, and it’s hard to see the structure. How could you adapt tag() to do indenting and formatting? (You may need to do some research into block and inline tags.)

21.3 LaTeX

The next DSL will convert R expressions into their LaTeX math equivalents. (This is a bit like ?plotmath, but for text instead of plots.) LaTeX is the lingua franca of mathematicians and statisticians: it’s common to use LaTeX notation whenever you want to express an equation in text, like in email. Since many reports are produced using both R and LaTeX, it might be useful to be able to automatically convert mathematical expressions from one language to the other.

Because we need to convert both functions and names, this mathematical DSL will be more complicated than the HTML DSL. We’ll also need to create a default conversion, so that symbols that we don’t know about get a standard conversion. This means that we can no longer use just evaluation: we also need to walk the abstract syntax tree (AST).

21.3.1 LaTeX mathematics

Before we begin, let’s quickly cover how formulas are expressed in LaTeX. The full standard is very complex, but fortunately is well documented, and the most common commands have a fairly simple structure:

  • Most simple mathematical equations are written in the same way you’d type them in R: x * y, z ^ 5. Subscripts are written using _ (e.g., x_1).

  • Special characters start with a \: \pi = \(\pi\), \pm = \(\pm\), and so on. There are a huge number of symbols available in LaTeX: searching online for latex math symbols returns many lists. There’s even a service that will look up the symbol you sketch in the browser.

  • More complicated functions look like \name{arg1}{arg2}. For example, to write a fraction you’d use \frac{a}{b}. To write a square root, you’d use \sqrt{a}.

  • To group elements together use {}: i.e., x ^ a + b versus x ^ {a + b}.

  • In good math typesetting, a distinction is made between variables and functions. But without extra information, LaTeX doesn’t know whether f(a * b) represents calling the function f with input a * b, or is shorthand for f * (a * b). If f is a function, you can tell LaTeX to typeset it using an upright font with \textrm{f}(a * b). (The rm stands for “Roman”, the opposite of italics.)

21.3.2 Goal

Our goal is to use these rules to automatically convert an R expression to its appropriate LaTeX representation. We’ll tackle this in four stages:

  • Convert known symbols: pi\pi

  • Leave other symbols unchanged: xx, yy

  • Convert known functions to their special forms: sqrt(frac(a, b))\sqrt{\frac{a, b}}

  • Wrap unknown functions with \textrm: f(a)\textrm{f}(a)

We’ll code this translation in the opposite direction of what we did with the HTML DSL. We’ll start with infrastructure, because that makes it easy to experiment with our DSL, and then work our way back down to generate the desired output.

21.3.3 to_math()

To begin, we need a wrapper function that will convert R expressions into LaTeX math expressions. This will work like to_html() by capturing the unevaluated expression and evaluating it in a special environment. There are two main differences:

  • The evaluation environment is no longer constant, as it has to vary depending on the input. This is necessary to handle unknown symbols and functions.

  • We never evaluate in the argument environment because we’re translating every function to a LaTeX expression. The user will need to use explicitly !! in order to evaluate normally.

This gives us:

to_math <- function(x) {
  expr <- enexpr(x)
  out <- eval_bare(expr, latex_env(expr))

  latex(out)
}

latex <- function(x) structure(x, class = "advr_latex")
print.advr_latex <- function(x) {
  cat("<LATEX> ", x, "\n", sep = "")
}

Next we’ll build up latex_env(), starting simply and getting progressively more complex.

21.3.4 Known symbols

Our first step is to create an environment that will convert the special LaTeX symbols used for Greek characters, e.g., pi to \pi. We’ll use the trick from Section 20.4.3 to bind the symbol pi to the value "\pi".

greek <- c(
  "alpha", "theta", "tau", "beta", "vartheta", "pi", "upsilon",
  "gamma", "varpi", "phi", "delta", "kappa", "rho",
  "varphi", "epsilon", "lambda", "varrho", "chi", "varepsilon",
  "mu", "sigma", "psi", "zeta", "nu", "varsigma", "omega", "eta",
  "xi", "Gamma", "Lambda", "Sigma", "Psi", "Delta", "Xi",
  "Upsilon", "Omega", "Theta", "Pi", "Phi"
)
greek_list <- set_names(paste0("\\", greek), greek)
greek_env <- as_environment(greek_list)

We can then check it:

latex_env <- function(expr) {
  greek_env
}

to_math(pi)
#> <LATEX> \pi
to_math(beta)
#> <LATEX> \beta

Looks good so far!

21.3.5 Unknown symbols

If a symbol isn’t Greek, we want to leave it as is. This is tricky because we don’t know in advance what symbols will be used, and we can’t possibly generate them all. Instead, we’ll use the approach described in Section 18.5: walking the AST to find all symbols. This gives us all_names_rec() and helper all_names():

all_names_rec <- function(x) {
  switch_expr(x,
    constant = character(),
    symbol =   as.character(x),
    call =     flat_map_chr(as.list(x[-1]), all_names)
  )
}

all_names <- function(x) {
  unique(all_names_rec(x))
}

all_names(expr(x + y + f(a, b, c, 10)))
#> [1] "x" "y" "a" "b" "c"

We now want to take that list of symbols and convert it to an environment so that each symbol is mapped to its corresponding string representation (e.g., so eval(quote(x), env) yields "x"). We again use the pattern of converting a named character vector to a list, then converting the list to an environment.

latex_env <- function(expr) {
  names <- all_names(expr)
  symbol_env <- as_environment(set_names(names))

  symbol_env
}

to_math(x)
#> <LATEX> x
to_math(longvariablename)
#> <LATEX> longvariablename
to_math(pi)
#> <LATEX> pi

This works, but we need to combine it with the Greek symbols environment. Since we want to give preference to Greek over defaults (e.g., to_math(pi) should give "\\pi", not "pi"), symbol_env needs to be the parent of greek_env. To do that, we need to make a copy of greek_env with a new parent. This gives us a function that can convert both known (Greek) and unknown symbols.

latex_env <- function(expr) {
  # Unknown symbols
  names <- all_names(expr)
  symbol_env <- as_environment(set_names(names))

  # Known symbols
  env_clone(greek_env, parent = symbol_env)
}

to_math(x)
#> <LATEX> x
to_math(longvariablename)
#> <LATEX> longvariablename
to_math(pi)
#> <LATEX> \pi

21.3.6 Known functions

Next we’ll add functions to our DSL. We’ll start with a couple of helpers that make it easy to add new unary and binary operators. These functions are very simple: they only assemble strings.

unary_op <- function(left, right) {
  new_function(
    exprs(e1 = ),
    expr(
      paste0(!!left, e1, !!right)
    ),
    caller_env()
  )
}

binary_op <- function(sep) {
  new_function(
    exprs(e1 = , e2 = ),
    expr(
      paste0(e1, !!sep, e2)
    ),
    caller_env()
  )
}

unary_op("\\sqrt{", "}")
#> function (e1) 
#> paste0("\\sqrt{", e1, "}")
binary_op("+")
#> function (e1, e2) 
#> paste0(e1, "+", e2)

Using these helpers, we can map a few illustrative examples of converting R to LaTeX. Note that with R’s lexical scoping rules helping us, we can easily provide new meanings for standard functions like +, -, and *, and even ( and {.

# Binary operators
f_env <- child_env(
  .parent = empty_env(),
  `+` = binary_op(" + "),
  `-` = binary_op(" - "),
  `*` = binary_op(" * "),
  `/` = binary_op(" / "),
  `^` = binary_op("^"),
  `[` = binary_op("_"),

  # Grouping
  `{` = unary_op("\\left{ ", " \\right}"),
  `(` = unary_op("\\left( ", " \\right)"),
  paste = paste,

  # Other math functions
  sqrt = unary_op("\\sqrt{", "}"),
  sin =  unary_op("\\sin(", ")"),
  log =  unary_op("\\log(", ")"),
  abs =  unary_op("\\left| ", "\\right| "),
  frac = function(a, b) {
    paste0("\\frac{", a, "}{", b, "}")
  },

  # Labelling
  hat =   unary_op("\\hat{", "}"),
  tilde = unary_op("\\tilde{", "}")
)

We again modify latex_env() to include this environment. It should be the last environment R looks for names in so that expressions like sin(sin) will work.

latex_env <- function(expr) {
  # Known functions
  f_env

  # Default symbols
  names <- all_names(expr)
  symbol_env <- as_environment(set_names(names), parent = f_env)

  # Known symbols
  greek_env <- env_clone(greek_env, parent = symbol_env)

  greek_env
}

to_math(sin(x + pi))
#> <LATEX> \sin(x + \pi)
to_math(log(x[i]^2))
#> <LATEX> \log(x_i^2)
to_math(sin(sin))
#> <LATEX> \sin(sin)

21.3.7 Unknown functions

Finally, we’ll add a default for functions that we don’t yet know about. We can’t know in advance what the unknown funtions will be so we again walk the AST to find them:

all_calls_rec <- function(x) {
  switch_expr(x,
    constant = ,
    symbol =   character(),
    call = {
      fname <- as.character(x[[1]])
      children <- flat_map_chr(as.list(x[-1]), all_calls)
      c(fname, children)
    }
  )
}
all_calls <- function(x) {
  unique(all_calls_rec(x))
}

all_calls(expr(f(g + b, c, d(a))))
#> [1] "f" "+" "d"

We need a closure that will generate the functions for each unknown call:

unknown_op <- function(op) {
  new_function(
    exprs(... = ),
    expr({
      contents <- paste(..., collapse = ", ")
      paste0(!!paste0("\\mathrm{", op, "}("), contents, ")")
    })
  )
}
unknown_op("foo")
#> function (...) 
#> {
#>     contents <- paste(..., collapse = ", ")
#>     paste0("\\mathrm{foo}(", contents, ")")
#> }
#> <environment: 0x64c1c80>

And again we update latex_env():

latex_env <- function(expr) {
  calls <- all_calls(expr)
  call_list <- map(set_names(calls), unknown_op)
  call_env <- as_environment(call_list)

  # Known functions
  f_env <- env_clone(f_env, call_env)

  # Default symbols
  names <- all_names(expr)
  symbol_env <- as_environment(set_names(names), parent = f_env)

  # Known symbols
  greek_env <- env_clone(greek_env, parent = symbol_env)
  greek_env
}

This completes our original requirements:

to_math(sin(pi) + f(a))
#> <LATEX> \sin(\pi) + \mathrm{f}(a)

You could certainly take this idea further and translate types of mathematical expression, but you should not need any additional metaprogramming tools.

21.3.8 Exercises

  1. Add escaping. The special symbols that should be escaped by adding a backslash in front of them are \, $, and %. Just as with HTML, you’ll need to make sure you don’t end up double-escaping. So you’ll need to create a small S3 class and then use that in function operators. That will also allow you to embed arbitrary LaTeX if needed.

  2. Complete the DSL to support all the functions that plotmath supports.

References

Fowler, Martin. 2010. Domain-Specific Languages. Pearson Education. http://amzn.com/0321712943.