## 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)

### References

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