[Rd] scoping/non-standard evaluation issue

[Gabor Grothendieck]

On Tue, Jan 4, 2011 at 4:35 PM, John Fox <jfox at mcmaster.ca> wrote:
> Dear r-devel list members,
>
> On a couple of occasions I've encountered the issue illustrated by the
> following examples:
>
> --------- snip -----------
>
>> mod.1 <- lm(Employed ~ GNP.deflator + GNP + Unemployed +
> +         Armed.Forces + Population + Year, data=longley)
>
>> mod.2 <- update(mod.1, . ~ . - Year + Year)
>
>> all.equal(mod.1, mod.2)
> [1] TRUE
>>
>> f <- function(mod){
> +     subs <- 1:10
> +     update(mod, subset=subs)
> +     }
>
>> f(mod.1)
>
> Call:
> lm(formula = Employed ~ GNP.deflator + GNP + Unemployed + Armed.Forces +
>    Population + Year, data = longley, subset = subs)
>
> Coefficients:
>  (Intercept)  GNP.deflator           GNP    Unemployed  Armed.Forces
>   3.641e+03     8.394e-03     6.909e-02    -3.971e-03    -8.595e-03
>  Population          Year
>   1.164e+00    -1.911e+00
>
>> f(mod.2)
> Error in eval(expr, envir, enclos) : object 'subs' not found
>
> --------- snip -----------
>
> I *almost* understand what's going -- that is, clearly mod.1 and mod.2, or
> the formulas therein, are associated with different environments, but I
> don't quite see why.
>
> Anyway, here are two "solutions" that work, but neither is in my view
> desirable:
>
> --------- snip -----------
>
>> f1 <- function(mod){
> +     assign(".subs", 1:10, envir=.GlobalEnv)
> +     on.exit(remove(".subs", envir=.GlobalEnv))
> +     update(mod, subset=.subs)
> +     }
>
>> f1(mod.1)
>
> Call:
> lm(formula = Employed ~ GNP.deflator + GNP + Unemployed + Armed.Forces +
>    Population + Year, data = longley, subset = .subs)
>
> Coefficients:
>  (Intercept)  GNP.deflator           GNP    Unemployed  Armed.Forces
>   3.641e+03     8.394e-03     6.909e-02    -3.971e-03    -8.595e-03
>  Population          Year
>   1.164e+00    -1.911e+00
>
>> f1(mod.2)
>
> Call:
> lm(formula = Employed ~ GNP.deflator + GNP + Unemployed + Armed.Forces +
>    Population + Year, data = longley, subset = .subs)
>
> Coefficients:
>  (Intercept)  GNP.deflator           GNP    Unemployed  Armed.Forces
>   3.641e+03     8.394e-03     6.909e-02    -3.971e-03    -8.595e-03
>  Population          Year
>   1.164e+00    -1.911e+00
>
>> f2 <- function(mod){
> +     env <- new.env(parent=.GlobalEnv)
> +     attach(NULL)
> +     on.exit(detach())
> +     assign(".subs", 1:10, pos=2)
> +     update(mod, subset=.subs)
> +     }
>
>> f2(mod.1)
>
> Call:
> lm(formula = Employed ~ GNP.deflator + GNP + Unemployed + Armed.Forces +
>    Population + Year, data = longley, subset = .subs)
>
> Coefficients:
>  (Intercept)  GNP.deflator           GNP    Unemployed  Armed.Forces
>   3.641e+03     8.394e-03     6.909e-02    -3.971e-03    -8.595e-03
>  Population          Year
>   1.164e+00    -1.911e+00
>
>> f2(mod.2)
>
> Call:
> lm(formula = Employed ~ GNP.deflator + GNP + Unemployed + Armed.Forces +
>    Population + Year, data = longley, subset = .subs)
>
> Coefficients:
>  (Intercept)  GNP.deflator           GNP    Unemployed  Armed.Forces
>   3.641e+03     8.394e-03     6.909e-02    -3.971e-03    -8.595e-03
>  Population          Year
>   1.164e+00    -1.911e+00
>
> --------- snip -----------
>
> The problem with f1() is that it will clobber a variable named .subs in the
> global environment; the problem with f2() is that .subs can be masked by a
> variable in the global environment.
>
> Is there a better approach?
>

I think there is something wrong with R here since the formula in the
call component of mod.1 has a "call" class whereas the corresponding
call component of mod.2 has "formula" class:

> class(mod.1$call[[2]])
[1] "call"
> class(mod.2$call[[2]])
[1] "formula"

If we reset call[[2]] to have "call" class then it works:

> mod.2a <- mod.2
> mod.2a$call[[2]] <- as.call(as.list(mod.2a$call[[2]]))
> f(mod.2a)

Call:
lm(formula = Employed ~ GNP.deflator + GNP + Unemployed + Armed.Forces +
    Population + Year, data = longley, subset = subs)

Coefficients:
 (Intercept)  GNP.deflator           GNP    Unemployed  Armed.Forces
 Population          Year
   3.641e+03     8.394e-03     6.909e-02    -3.971e-03    -8.595e-03
  1.164e+00    -1.911e+00


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