Evaluate an expression within a given environment
These functions evaluate expr within a given environment (env
for with_env(), or the child of the current environment for
locally). They rely on eval_bare() which features a lighter
evaluation mechanism than base R base::eval(), and which also has
some subtle implications when evaluting stack sensitive functions
(see help for eval_bare()).
with_env(env, expr) locally(expr)
env |
An environment within which to evaluate |
expr |
An expression to evaluate. |
locally() is equivalent to the base function
base::local() but it produces a much cleaner
evaluation stack, and has stack-consistent semantics. It is thus
more suited for experimenting with the R language.
These functions are experimental. Expect API changes.
# with_env() is handy to create formulas with a given environment:
env <- child_env("rlang")
f <- with_env(env, ~new_formula())
identical(f_env(f), env)
# Or functions with a given enclosure:
fn <- with_env(env, function() NULL)
identical(get_env(fn), env)
# Unlike eval() it doesn't create duplicates on the evaluation
# stack. You can thus use it e.g. to create non-local returns:
fn <- function() {
g(current_env())
"normal return"
}
g <- function(env) {
with_env(env, return("early return"))
}
fn()
# Since env is passed to as_environment(), it can be any object with an
# as_environment() method. For strings, the pkg_env() is returned:
with_env("base", ~mtcars)
# This can be handy to put dictionaries in scope:
with_env(mtcars, cyl)Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.