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Create a new "with" or "local" function

Source: R/local_.R, R/with_.R
with_.Rd

These are constructors for with_... or local_... functions. They are only needed if you want to alter some global state which is not covered by the existing with_... functions, see withr for an overview.

Usage

local_(
  set,
  reset = set,
  get = NULL,
  ...,
  envir = parent.frame(),
  new = TRUE,
  dots = FALSE
)

with_(set, reset = set, get = NULL, ..., envir = parent.frame(), new = TRUE)

Arguments

set

[function(...)]
Function used to set the state. The return value from this function should be the old state, which will then be passed back into the reset() function to clean up the state. The function can have arbitrarily many arguments, they will be replicated in the formals of the returned function.

reset

[function(x)]
Function used to reset the state. The first argument can be named arbitrarily, further arguments with default values, or a "dots" argument, are supported but not used: The function will be called as reset(old).

get

[function(...)]
Optionally, a getter function. If supplied, the on.exit() restoration is set up before calling set. This is more robust in edge cases.

For technical reasons, this getter function must have the same interface as set, which means it is passed the new values as well. These can be safely ignored.

...

These dots are for future extensions and must be empty.

envir

[environment]
Environment of the returned function.

new

[logical(1)]
Replace the first argument of the set function by new? Set to FALSE if the set function only has optional arguments.

Value

[function(new, code, ...)] A function with at least two arguments,

  • new: New state to use

  • code: Code to run in that state.

If there are more arguments to the function passed in set they are added to the returned function. If set does not have arguments, or new is FALSE, the returned function does not have a code argument.

Details

The with_... functions reset the state immediately after the code argument has been evaluated. The local_... functions reset their arguments after they go out of scope, usually at the end of the function body.

Examples

with_(setwd)
#> function (new, code) 
#> {
#>     old <- setwd(dir = new)
#>     on.exit(setwd(old))
#>     force(code)
#> }
#> <environment: 0x5590ff747460>

global_stack <- list()
set_global_state <- function(state, msg = "Changing global state.") {
  global_stack <- c(list(state), global_stack)
  message(msg)
  state
}
reset_global_state <- function(state) {
  old_state <- global_stack[[1]]
  global_stack <- global_stack[-1]
  stopifnot(identical(state, old_state))
}
with_(set_global_state, reset_global_state)
#> function (new, code, msg = "Changing global state.") 
#> {
#>     old <- set_global_state(state = new, msg = msg)
#>     on.exit(reset_global_state(old))
#>     force(code)
#> }
#> <environment: 0x5590ff747460>