You can do this???

why-am-I-just-learning-about-this-now.R

library(ggplot2)

# This retrieves the function body
body(get("compute_group", StatCount))

# This splits the execution sequence as a list
as.list(body(get("compute_group", StatCount)))

# The trace call
trace(
  "compute_group",  # the method to trace
  where = StatCount,  # the ggproto object that the method belongs to
  at = 8, # the step to trace
  tracer = quote({            # the expression to evaluate at that step
    cat("> print(bars)\n")
    print(bars)
  }),
  exit = quote(untrace("compute_group", where = StatCount)) # called at the end to have a "traceonce" effect
)

ggplot(mtcars, aes(cyl)) + geom_bar()

# For even more power, `edit = TRUE`
# trace("compute_group", where = StatCount, edit = TRUE)

Multiple tracer exprs at multiple execution steps !!!

If this ends up being wrapped in a function, needs to rm tracer_idx on exit

library(ggplot)

tracer_idx <- 1

# The trace call
trace(
    "compute_group",  # the method to trace
    where = StatCount,  # the ggproto object that the method belongs to
    at = c(5, 8), # the step to trace
    tracer = function() {
        tracer_expr <- list(
            quote(print(x)),
            quote(print(bars))
        )[[tracer_idx]]
        eval(tracer_expr, envir = parent.frame())
        tracer_idx <<- tracer_idx + 1
    },
    exit = quote(untrace("compute_group", where = StatCount)) # called at the end to have a "traceonce" effect
)

ggplot(mtcars, aes(cyl)) + geom_bar()