function_new: Create a function by "hand"
In lazyeval: Lazy (Non-Standard) Evaluation

This constructs a new function given it's three components: list of arguments, body code and parent environment.

1	function_new(args, body, env = parent.frame())

`args`	A named list of default arguments. Note that if you want arguments that don't have defaults, you'll need to use the special function `alist`, e.g. `alist(a = , b = 1)`
`body`	A language object representing the code inside the function. Usually this will be most easily generated with `quote`
`env`	The parent environment of the function, defaults to the calling environment of `make_function`

f <- function(x) x + 3
g <- function_new(alist(x = ), quote(x + 3))

# The components of the functions are identical
identical(formals(f), formals(g))
identical(body(f), body(g))
identical(environment(f), environment(g))

# But the functions are not identical because f has src code reference
identical(f, g)

attr(f, "srcref") <- NULL
# Now they are:
stopifnot(identical(f, g))