Evaluate an expression within an environment inside a function

Question

Consider:

guy <- new.env(FALSE)
guy$stuff <- mean
guy$lib <- library
guy$stole_this_data <- mtcars
ls(guy)

How can I evaluate an expression within an environment inside a function?

For instance I can do with(guy, args(stuff)) to the below and return:

> with(guy, args(stuff))
function (x, ...) 
NULL

But within a functon:

foo <- function(env, fun) {
    with(env, args(fun))
}

foo(guy, stuff)

## > foo(guy, stuff)
## Error in args(fun) : could not find function "stuff"

Answer 1

Try this:

> foo <- function(env, fun) eval(substitute(args(fun)), env)
> foo(guy, stuff)
function (x, ...) 
NULL

ADDED. Regarding the comment below here is an example where zz is not in env or its ancestors (but is in foo2 and in f, the caller of foo2) and it does give a not found error as the comment wished:

> foo2 <- function(env, fun, zz = 1) eval(substitute(fun), env)
> f <- function() { zz <- 100; foo2(guy, zz+1) }
> f()
Error in eval(expr, envir, enclos) : object 'zz' not found

Answer 2

If you want to continue to use the with construct, this is an alternative:

foo <- function(env, fun) {
      fun <- substitute(fun)
      eval(bquote(with(env, {
        .(fun)
      })))
}