Function in default parameter value closing over variables defined later in the enclosing function

Hi Ivan & All,

R's scoping system basically goes to all environments along the call
stack when trying to resolve an unbound variable, see the language
definition [1], section 4.3.4, and perhaps also 2.1.5.

Generally, unbound variables should be used with care. It's a bit
difficult to decide whether and how the code should be rewritten,
I'd say that depends on the underlying intentions / purposes. As it
is, the code could be simplified to just

     print("secret");

but that's probably missing the point.

Best regards, Jan


[1] https://cran.r-project.org/doc/manuals/r-release/R-lang.html

On Wed, Jan 23, 2019 at 12:53:01PM +0300, Ivan Krylov wrote:

Hi!

I needed to generalize a loss function being optimized inside another
function, so I made it a function argument with a default value. It
worked without problems, but later I noticed that the inner function,
despite being defined in the function arguments, somehow closes over a
variable belonging to the outer function, which is defined later.

Example:

outside <- function(inside = function() print(secret)) {
	secret <- 'secret'
	inside()
}
outside()

I'm used to languages that have both lambdas and variable declaration
(like perl5 -Mstrict or C++11), so I was a bit surprised.

Does this work because R looks up the variable by name late enough at
runtime for the `secret` variable to exist in the parent environment of
the `inside` function? Can I rely on it? Is this considered bad style?
Should I rewrite it (and how)?

-- 
Best regards,
Ivan