Preface

"Everything I Know Is From Jenny Bryan"

- Sharla Gelfand

- Tan Ho

Preface

There are some excellent talks on debugging (some are better on debugging than Adv R itself!)

• Jenny Bryan's "Object of type closure is not subsettable" talk from rstudio::conf 2020
• Jenny Bryan and Jim Hester's book: "What They Forgot to Teach You About R" Ch11
• Hadley's video on a minimal reprex for a shiny app <- insanely useful!

and I suggest that you visit these resources later if you're looking for more of a talk or text-based content on debugging.

Today's Plan

We've all "read" the chapter, so I'll (obviously) take the liberty to live-code some debugging. Along the way, we'll try to hit:

• traceback
• print debugging
• browser()
• debug/debugonce/undebug
• options(error = recover)

by looking through some examples, a few contrived, a few less-contrived, and lastly (possibly?) a Shiny app?

Examples, easy mode

Let's use one of the canned examples from rstats.wtf to get us started.

usethis::use_course("rstd.io/wtf-debugging")
# Example 1: Spartan

Examples, AdvR mode

Maya: "How do I debug this"? (SURPRISE, METAPROGRAMMING REVIEW!)

Does anyone remember what this problem was about?

subset2 <- function(data, rows) {
  rows <- rlang::enquo(rows)
  # change val from 2 to 3, breaking things
  rlang::env_bind(rlang::caller_env(), val = 3)
  rows_val <- rlang::eval_tidy(rows, data)
  stopifnot(is.logical(rows_val))
  data[rows_val, , drop = FALSE]
}
resample <- function(df, n) {
  idx <- sample(nrow(df), n, replace = TRUE)
  df[idx, , drop = FALSE]
}
subsample <- function(df, cond, n = nrow(df)) {
  df <- subset2(df, cond)
  resample(df, n)
}
df <- data.frame(x = c(1,1,1,2,2), y = 1:5)
subsample(df, x == 1)
# Error in rlang::eval_tidy(rows, data) : object 'x' not found

Debug a Shiny app?

I can do this too if people are keen!