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

[Jan T Kim]

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
> 
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