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.
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())expr |
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envir |
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priority |
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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.
# 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()))
})
# defer and trigger events on the global environment
defer(print("one"))
defer(print("two"))
deferred_run()
defer(print("three"))
deferred_clear()
deferred_run()Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.