# Custom knitr language engines

Glue provides a few custom language engines for knitr, which allows you to use glue directly in knitr chunks.

## `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_.
    ```

#### mtcars has 32 rows and 11 columns.

If you want to pass additional arguments into the glue call, simply include them as chunk options.

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

The median waiting time between eruptions is 76.

## `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

Displaying records 1 - 10
model | hp | mpg
--- | --- | ---
Hornet Sportabout | 175 | 18.7
Duster 360 | 245 | 14.3
Merc 450SE | 180 | 16.4
Merc 450SL | 180 | 17.3
Merc 450SLC | 180 | 15.2
Cadillac Fleetwood | 205 | 10.4
Lincoln Continental | 215 | 10.4
Chrysler Imperial | 230 | 14.7
Camaro Z28 | 245 | 13.3
Pontiac Firebird | 175 | 19.2