qenv

In teal.code: Code Storage and Execution Class for 'teal' Applications

Introduction to qenv

A qenv inherits from the environment class, behaves like an environment, and has the following characteristics:

• It inherits from the environment and methods such as $, get, ls, as.list() work out of the box.
• qenv is a locked environment, and data modification is only possible through the eval_code and within functions.
• It stores metadata about the code used to create the data (see get_code).
• It supports slicing by [.
• It is immutable which means that each code evaluation does not modify the original qenv environment directly.

Initialization

The qenv() function serves as the gateway to create an initial qenv object:

library(teal.code)

# create a new qenv object
empty_qenv <- qenv()
print(empty_qenv)

qenv basic usage

To modify the data use eval_code to execute R code within the environment, yielding a new qenv object as the output.

# evaluate code in qenv
my_qenv <- eval_code(empty_qenv, "x <- 2")
print(my_qenv)

q1 <- eval_code(my_qenv, "y <- x * 2")
q1 <- eval_code(q1, "z <- y * 2")

# my_qenv still contains only x
print(my_qenv)
names(my_qenv)

# q1 contains x, y and z
print(q1)
names(q1)

The same result can be achieved with the within method.

q2 <- within(my_qenv, y <- x * 2)
q2 <- within(q2, z <- y * 2)
print(q2)

To extract objects from a qenv, use [[; this is particularly useful for displaying them in a shiny app. You can retrieve the code used to generate the qenv using the get_code() function.

print(q2[["y"]])

cat(get_code(q2))

Substitutions

In some cases, one may want to substitute some elements of the code before evaluation. Consider a case when a subset of iris is defined by an input value.

q <- qenv()
q <- eval_code(q, quote(i <- subset(iris, Species == "setosa")))
q <- eval_code(q, substitute(
  ii <- subset(iris, Species == species),
  env = list(species = "versicolor")
))
input_value <- "virginica"
q <- eval_code(q, substitute(
  iii <- subset(iris, Species == species),
  env = list(species = input_value)
))

summary(q[["i"]]$Species)
summary(q[["ii"]]$Species)
summary(q[["iii"]]$Species)

A more convenient way to pass code with substitution is to use the within method.

qq <- qenv()
qq <- within(qq, i <- subset(iris, Species == "setosa"))
qq <- within(qq, ii <- subset(iris, Species == species), species = "versicolor")
input_value <- "virginica"
qq <- within(qq, iii <- subset(iris, Species == species), species = input_value)

summary(qq[["i"]]$Species)
summary(qq[["ii"]]$Species)
summary(qq[["iii"]]$Species)

See ?qenv for more details.

Combining qenv objects

Given a pair of qenv objects, you may be able to "join" them, creating a new qenv object encompassing the union of both environments, along with the requisite code for reproduction:

common_q <- eval_code(qenv(), quote(x <- 1))

x_q <- eval_code(common_q, quote(y <- 5))
y_q <- eval_code(common_q, quote(z <- 5))

join_q <- c(x_q, y_q)

print(join_q)
names(join_q)

The feasibility of joining qenv objects hinges on the contents of the environments and the code's order. Refer to the function documentation for further details.

Warnings and messages in qenv objects

In cases where warnings or messages arise while evaluating code within a qenv environment, these are captured and stored within the qenv object. Access these messages and warnings using below

q_message <- eval_code(qenv(), quote(message("this is a message")))
get_messages(q_message)

q_warning <- eval_code(qenv(), quote(warning("and this is a warning")))
get_warnings(q_warning)

If any of above returns NULLm then no warnings nor messages were present.

Utilizing qenv inside shiny applications

These functions can be seamlessly integrated into shiny applications to produce reproducible outputs. In the example below, the rcode section showcases the code employed for generating the output.

When employing a qenv to evaluate code, should an error occur, an object of type qenv.error is generated. This object can be utilized wherever a qenv object is used, alleviating the need for code alterations to handle these errors. Select the error_option in the example below to witness qenv error handling in action.

library(shiny)
# create an initial qenv with the data in
data_q <- qenv()
data_q <- eval_code(data_q, "iris_data <- iris")

ui <- fluidPage(
  radioButtons(
    "option", "Choose a column to plot:",
    c("Sepal.Length", "Sepal.Width", "Petal.Length", "Petal.Width", "error_option")
  ),
  verbatimTextOutput("rcode"),
  plotOutput("plot")
)

server <- function(input, output, session) {
  # create a qenv containing the reproducible output
  output_q <- reactive({
    req(input$option)
    eval_code(
      data_q,
      bquote(p <- hist(iris_data[, .(input$option)]))
    )
  })

  # display output
  output$plot <- renderPlot(output_q()[["p"]])
  # display code
  output$rcode <- renderText(get_code(output_q()))
}

if (interactive()) {
  shinyApp(ui, server)
}

qenv and teal applications

The versatile qenv object can seamlessly integrate into teal modules. Explore the teal vignette Creating Custom Modules for detailed guidance.

teal.code documentation built on April 4, 2025, 5:16 a.m.