scoping/non-standard evaluation issue

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?

class(mod.1$call[[2]])

class(mod.2$call[[2]])

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