# with_env

From rlang v0.2.0
by Lionel Henry

##### 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()`

).

##### Usage

`with_env(env, expr)`locally(expr)

##### Arguments

- env
An environment within which to evaluate

`expr`

. Can be an object with an`get_env()`

method.- expr
An expression to evaluate.

##### Details

`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.

##### Life cycle

These functions are experimental. Expect API changes.

##### Examples

`library(rlang)`

```
# NOT RUN {
# 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(get_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)
# }
```

*Documentation reproduced from package rlang, version 0.2.0, License: GPL-3*

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