withr (version 2.1.0)

defer: Defer Evaluation of an Expression

Description

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"))

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.

Examples

# NOT RUN {
# 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()))
})
# }

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