windmill/system_prompts/languages/rlang.md

R

Structure

Define a main function using <- or = assignment. Parameters become the script inputs:

library(dplyr)
library(jsonlite)

main <- function(x, name = "default", flag = TRUE) {
    df <- tibble(x = x, name = name)
    result <- df %>% mutate(greeting = paste("Hello", name))
    return(toJSON(result, auto_unbox = TRUE))
}

Important:

• The main function is required
• Use library() to load packages — they are resolved and installed automatically
• jsonlite is always available (used internally for argument parsing)
• Return values must be JSON-serializable

Parameters

R types map to Windmill types:

• numeric → float/int
• character → string
• logical → bool (use TRUE/FALSE)
• list → object/dict
• NULL → null

Default values are inferred from the function signature:

main <- function(
    name,              # required string
    count = 10,        # optional int, default 10
    verbose = FALSE    # optional bool, default FALSE
) {
    # ...
}

Resources and Variables

Use the built-in Windmill helpers (no import needed):

main <- function() {
    # Get a variable
    api_key <- get_variable("f/my_folder/api_key")

    # Get a resource (returns a list)
    db <- get_resource("f/my_folder/postgres_config")
    host <- db$host
    port <- db$port

    return(list(host = host, port = port))
}

Output

Return any JSON-serializable value from main. The return value becomes the step result:

main <- function(x) {
    # Return a scalar
    return(x + 1)

    # Or a list (becomes JSON object)
    return(list(result = x + 1, status = "ok"))
}

Annotations

Control execution behavior with comment annotations:

#renv_verbose = true        # Show verbose renv output during resolution
#renv_install_verbose = true # Show verbose output during package installation
#sandbox = true              # Run in nsjail sandbox (requires nsjail)