glue .transformer support (particularly for length-0 and NULL values)

[mmuurr]

Since {glue} is vectorized, the default behavior when an interpolated value is length-0 (which includes NULL) is to return character(0):

str(glue::glue("foo is {NULL}")
# 'glue' chr(0)

This causes problems with the msg disappearing for LoggerGlue log events if the glue template includes a value that evaluates to length-0 or NULL.

With {glue}, one can use a transformer to convert these annoyances into something useful:

null0_transformer <- function(text, envir) {
  out <- glue::identity_transformer(text, envir)
  if (is.null(out)) {
    "NULL"
  } else if (length(out) == 0) {
    sprintf("%s(0)", mode(out))
  } else {
    out
  }
}

glue::glue("foo is {NULL}", .transformer = null0_transformer)
# foo is NULL

How about a .transformer optional arg (with glue::identity_transformer as the default) for the logging methods in a LoggerGlue, forwarding on those values to glue::glue()?

Perhaps the same for the .na optional arg in glue::glue()?

(I'm also happy making these changes in a PR if you think it's a good idea but are otherwise busy.)

[mmuurr]

Ah, I see now that the do.call() dispatching to glue::glue() already can pass along additional named args (e.g. .transformer), but perhaps it'd make sense to set options like these as part of the Logger initialization (i.e. get_logger_glue())?

[s-fleck]

Hmm handling NULL and zero length vectors is definitely something that should be addressed... This problem applies to normal loggers as well as logger glue... Whether or not I want logger glue to have a default transformer or not is something I still have to think about. Logger config is already quite complex and I don't want to add to many features; at some point "power users" are just better of creating their own subclasses.

Regardless of that, when displaying variable values I recommend doing this as custom fields, eg:

theuser <- NULL
lg$info("signing in user", user = theuser)

This will lead to logs that are more machine-readable/parsable.