rlang (version 0.0.0.9000)

with_env: Evaluate an expression within a given environment.

Description

These functions evaluate expr within a given environment (env for with_env(), or the child of the current environment for locally). They rely on expr_eval() which features a lighter evaluation mechanism than base R eval(), and which also has some subtle implications when evaluting stack sensitive functions (see help for expr_eval()).

Usage

with_env(env, expr)
locally(expr)

Arguments

env

An environment within which to evaluate expr. Can be an object with an env() method.

expr

An expression to evaluate.

Details

locally() is equivalent to the base function 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.

Examples

# with_env() is handy to create formulas with a given environment:
env <- new_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(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(env())
  "normal return"
}
g <- function(env) {
  with_env(env, return("early return"))
}
fn()


# Since env is passed to env(), it can be any object with an env()
# method. For strings, the pkg_env() is returned:
with_env("base", ~mtcars)

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