Adverb that starts browser() on failure

[hadley]

Like this but in correct environment

hopefully <- function(f) {
  function(...) {
    tryCatch(
      f(...),
      error = function(e) browser()
    )
  }
  
}

[robertness]

You're welcome!

[Deleetdk]

I've been using this function:

try_browse = function(expr) {
  #try
  .trial = try({
    y = eval(substitute(expr), parent.frame())
  })
  
  #catch
  if (inherits(.trial, "try-error")) eval(quote(browser()), parent.frame(n = 1))
  
  y
}

So basically encapsulate the error prone call in that and when it fails, you step right into where you want to be, including when inside implicit loops like ddply or map_df.

I've also suggested trying to make a pipe version of this, but I couldn't figure out how to do that.

[hadley]

This works:

hopefully <- function(f) {
  function(...) {
    withCallingHandlers(
      f(...),
      error = function(e) {
        # 1: h(simpleError(msg, call))
        # 2: .handleSimpleError(function (e)  <...>
        # 3: stop(...)
        ef <- eval_frame(4)
        eval(quote(browser()), env = ef$env)
      }
    )
  }
}

f <- function(x) {
  y <- 20
  if (x > 5) {
    stop("!") 
  } else {
    x
  }
}

map(1:6, hopefully(f))

But is fragile:

options(warn = 2)
f <- function(x) {
  y <- 20
  if (x > 5) {
    warning("!") 
  } else {
    x
  }
}

map(1:6, hopefully(f))

@lionel- is there a better way here?

[lionel-]

This doesn't work in ESS because of some limitations of how it handles step-debugging.

I don't think it's that fragile, this just needs another handler for warnings that checks for options("warn"). The R code establishing restarts and handlers hasn't changed for 15 years, so in practice it's not as bad as it feels to just count the frames. Here is a version that works with warnings and in ESS:

hopefully <- function(f) {
  function(...) {
    withCallingHandlers(
      f(...),
      error = function(e) {
        # 1: h(simpleError(msg, call))
        # 2: .handleSimpleError(function (e)  <...>
        # 3: stop(...)
        with_env(frame$env, on.exit({
          browser()
          NULL # Workaround ESS issue
        }))
        return_from(frame)
      },
      warning = function(e) {
        if (getOption("warn") == 2) {
          frame <- eval_frame(7)
          with_env(frame$env, on.exit({ browser(); NULL }))
          return_from(frame)
        }
      }
    )
  }
}

[hadley]

Unfortunately your work around doesn't work in RStudio. I think we'll have to switch based on.Platform$GUI. What does ESS use?

For reasons I don't understand it no longer works when I extract the browser() logic into a separate function:

browse_in_frame <- function(frame) {
  if (.Platform$GUI == "ESS") {
    # Workaround ESS issue
    with_env(frame$env, on.exit({
      browser()
      NULL
    }))
    return_from(frame)
  } else {
    eval(quote(browser()), envir = frame$env)
  }
}

[hadley]

Complete code in #304