# eval_bare() works just like base::eval() but you have to create
# the evaluation environment yourself:
eval_bare(quote(foo), env(foo = "bar"))
# eval() has different evaluation semantics than eval_bare(). It
# can return from the supplied environment even if its an
# environment that is not on the call stack (i.e. because you've
# created it yourself). The following would trigger an error with
# eval_bare():
ret <- quote(return("foo"))
eval(ret, env())
# eval_bare(ret, env()) # "no function to return from" error
# Another feature of eval() is that you can control surround loops:
bail <- quote(break)
while (TRUE) {
eval(bail)
# eval_bare(bail) # "no loop for break/next" error
}
# To explore the consequences of stack inconsistent semantics, let's
# create a function that evaluates `parent.frame()` deep in the call
# stack, in an environment corresponding to a frame in the middle of
# the stack. For consistency with R's lazy evaluation semantics, we'd
# expect to get the caller of that frame as result:
fn <- function(eval_fn) {
list(
returned_env = middle(eval_fn),
actual_env = current_env()
)
}
middle <- function(eval_fn) {
deep(eval_fn, current_env())
}
deep <- function(eval_fn, eval_env) {
expr <- quote(parent.frame())
eval_fn(expr, eval_env)
}
# With eval_bare(), we do get the expected environment:
fn(rlang::eval_bare)
# But that's not the case with base::eval():
fn(base::eval)
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