Version of “Nested function structure” from https://coolbutuseless.github.io/2022/06/15/nesting-functions/ without NSE

doc-api.r

doc = function (...) {
    doc = new.env(parent = emptyenv())
    doc$words = lapply(list(...), add_node, doc = doc)
    doc
}

par = function (...) {
    structure(list(...), class = c("par", "docnode"))
}

sentence = function (words) {
    structure(words, class = c("sentence", "docnode"))
}

# Note: for this simple example the `doc` argument to `add_node` is unused but
# for a real document API that allows nessted structures it becomes
# indispensable.
add_node = function (node, doc) {
    UseMethod('add_node')
}

add_node.par = function (node, doc) {
    lapply(node, add_node, doc = doc)
}

add_node.sentence = function (node, doc) {
    unclass(node)
}

doc-groups.r

doc = function (expr) {
    parent = parent.frame()
    parent$`__doc__` = new.env(parent = emptyenv())
    on.exit(rm(`__doc__`, envir = parent))
    parent$`__doc__`$words = list()
    expr
    parent$`__doc__`
}

par = function (expr) {
    parent = parent.frame()
    parent$`__par__` = c()
    on.exit(rm(`__par__`, envir = parent))
    expr
    parent$`__doc__`$words = c(parent$`__doc__`$words, list(parent$`__par__`))
}

sentence = function (words) {
    parent = parent.frame()
    parent$`__par__` = c(parent.frame()$`__par__`, words)
}

my_doc = doc({
    par({
        sentence("Hello there.")
        sentence("My name is Greg.")
    })
    par({
        sentence("Start of second par.")
    })
})

print_doc(my_doc)