locate: Find the environment which defines a name.

In crowding/fexpr: Tools for Lazy and Non-Standard Evaluation

Find the environment which defines a name.

Description

locate starts at a given environment, and searches enclosing environments for a name. It returns the first enclosing environment which defines sym.

locate_ is the normally evaluating method; locate(x) is equivalent to locate_(quo(x)) or locate_(quote(x), environment()).

When sym is a quotation or dots, any env argument is ignored.

Usage

locate(sym, env = arg_env_(quote(sym), environment()), mode = "any", ...)

locate_(sym, env = arg_env_(quote(sym), environment()), mode = "any", ...)

locate_.list(sym, env = arg_env_(quote(sym), environment()), mode = "any", ...)

locate_.quotation(sym, env = "ignored", mode = "any", ...)

locate_.character(
  sym,
  env = arg_env_(quote(sym), environment()),
  mode = "any",
  ...
)

`locate_.(`(sym, env = arg_env_(quote(sym), environment()), mode = "any", ...)

locate_.dots(sym, env = "ignored", mode = "any", ...)

locate_.name(
  sym,
  env = arg_env_(quote(sym), environment()),
  mode = "any",
  ifnotfound = stop("Binding ", deparse(sym), " not found")
)

Arguments

sym	A name. For locate the argument is used literally. For locate_ it should be a name or list of names.
env	Which environment to begin searching from.
mode	Either "any" or "function". "any" finds the lowest enclosing environment which gives any definiton for sym. "function" searches for an environment which defines sym as a function. This may force lazy arguments in the process, in the same way as get.
...	Further arguments passed to methods.
ifnotfound	What is returned if the symbol is not found. By default an exception is raised.

Value

An environment object which defines sym, if one is found.

If sym is a list (of names) or a dots object, locate_(sym) returns a list.

Note

To locate where ... is bound, you can wrap it in parens, as locate( (...) ).

If you use a literal character argument, as in locate("x", environment()), you must also provide the environment argument explicitly; locate("x") won't work in compiled functions. However using a literal name like locate(x) will work OK. See note under arg.

Examples

# Here is how to implement R's `<<-` operator, using `locate_`:
`%<<-%` <- function(lval, rval) {
  lval_ <- arg(lval)
  name <- expr(lval_)
  target.env <- locate_(name, parent.env(env(lval_)))
  assign(as.character(name), rval, envir=target.env)
}

x <- "not this one"
local({
  x <- "this one"
  local({
    x <- "not this one either"
    x %<<-% "this works like builtin <<-"
  })
  print(x)
})
print(x)

crowding/fexpr documentation built on July 3, 2024, 7:54 p.m.