Custom knitr language engines

glue engine

The first engine is the glue engine, which evaluates the chunk contents as a glue template.

1 + 1 = {1 + 1}

## 1 + 1 = 2

Maybe the most useful use of the glue engine is to set the knitr option results = 'asis' and output markdown or HTML directly into the document.

```{glue, results = 'asis', echo = FALSE}
#### mtcars has **{nrow(mtcars)} rows** and _{ncol(mtcars)} columns_.
```

```{glue, .open = "<<", .close = ">>", results = 'asis', echo = FALSE}
The **median waiting time** between eruptions is <<median(faithful$waiting)>>.
```

glue_sql engine

The second engine is glue_sql, which will use glue::glue_sql() to generate a SQL query and then run the query using the sql engine.

First we create a new connection to an in-memory SQLite database, and write a new table to it.

con <- DBI::dbConnect(RSQLite::SQLite(), ":memory:")
mtcars$model <- rownames(mtcars)
DBI::dbWriteTable(con, "mtcars", mtcars)

Next define some variables we that we can use with glue to interpolate.

var <- "mpg"
tbl <- "mtcars"
num <- 150

Then we can use glue_sql to construct and run a query using those variables into that database. Note you need to provide the connection object as a connection chunk option.

In this example there are two type of quotes. The first is a bare backtick, these are passed directly to the SQL engine unchanged. The second is backticks inside of braces, which are specially interpreted to do the proper quoting for the given SQL engine by glue. In this example we use the sqlite engine, which uses backticks for quoting, but you would use the same backticks inside brace syntax for postgreSQL, and glue_sql() would automatically use double quotes for quoting instead.

```{glue_sql, connection = con}
SELECT `model`, `hp`, {`var`}
FROM {`tbl`}
WHERE {`tbl`}.`hp` > {num}
```

SELECT `model`, `hp`, `mpg`
FROM `mtcars`
WHERE `mtcars`.`hp` > 150