Defer Evaluation of an Expression

Similar to on.exit(), but allows one to attach an expression to be evaluated when exiting any frame currently on the stack. This provides a nice mechanism for scoping side effects for the duration of a function's execution.

Usage

defer(expr, envir = parent.frame(), priority = c("first", "last"))

defer_parent(expr, priority = c("first", "last"))

deferred_run(envir = parent.frame())

deferred_clear(envir = parent.frame())

Arguments

expr

[expression]
An expression to be evaluated.

envir

[environment]
Attach exit handlers to this environment. Typically, this should be either the current environment or a parent frame (accessed through parent.frame()).

priority

[character(1)]
Specify whether this handler should be executed "first" or "last", relative to any other registered handlers on this environment.

Details

defer() works by attaching handlers to the requested environment (as an attribute called "handlers"), and registering an exit handler that executes the registered handler when the function associated with the requested environment finishes execution.

Deferred events can be set on the global environment, primarily to facilitate the interactive development of code that is intended to be executed inside a function or test. A message alerts the user to the fact that an explicit deferred_run() is the only way to trigger these deferred events. Use deferred_clear() to clear them without evaluation. The global environment scenario is the main motivation for these functions.

Running handlers within source()

withr handlers run within source() are run when source() exits rather than line by line.

This is only the case when the script is sourced in globalenv(). For a local environment, the caller needs to set options(withr.hook_source = TRUE). This is to avoid paying the penalty of detecting source() in the normal usage of defer().

Examples

# define a 'local' function that creates a file, and
# removes it when the parent function has finished executing
local_file <- function(path) {
  file.create(path)
  defer_parent(unlink(path))
}

# create tempfile path
path <- tempfile()

# use 'local_file' in a function
local({
  local_file(path)
  stopifnot(file.exists(path))
})

# file is deleted as we leave 'local' local
stopifnot(!file.exists(path))

# investigate how 'defer' modifies the
# executing function's environment
local({
  local_file(path)
  print(attributes(environment()))
})
#> NULL

# Note that examples lack function scoping so deferred calls are
# generally executed immediately
defer(print("one"))
#> [1] "one"
defer(print("two"))
#> [1] "two"