[Rd] glm Argument-Evaluation Does Not Match Documentation.

[Jorgen Harmse]

Details in documentation: "All of ‘weights’, ‘subset’, ‘offset’, ‘etastart’ and ‘mustart’ are evaluated in the same way as variables in ‘formula’, that is first in ‘data’ and then in the environment of ‘formula’."
In fact, `data` is usually not an environment, and I have not seen arguments evaluated in `environment(formula)` when `data` is provided. (Information in `environment(formula)` is used, so presumably they are evaluated in an environment whose parent is `environment(formula)`.)


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> environment(glm)
<environment: namespace:stats>
> maintainer('stats')
[1] "R Core Team <R-core using r-project.org>"
> df <- data.frame(x=1:3, y=c(pi,7,sqrt(2)), z=c(1,2.3,3))
> a <- 4:6; b <- 7:9
> lm <- glm(z ~ x+y, data=df, weights = a*b -> v)
> v # Check whether weights was evaluated in .GlobalEnv, the environment of the formula.
Error: object 'v' not found
> env.df <- as.environment(df); lm <- glm(z ~ x+y, data=env.df, weights = a*b -> v)
Error in list(z, x, y) : could not find function "list"
> parent.env(env.df) <- .GlobalEnv; lm <- glm(z ~ x+y, data=env.df, weights = a*b -> v)
> v
Error: object 'v' not found
> ls(env.df) # show that weights was evaluated in env.df
[1] "v" "x" "y" "z"


Suggested rewrite: "All of ‘weights’, ‘subset’, ‘offset’, ‘etastart’ and ‘mustart’ are evaluated in the same way as variables in ‘formula’. Values needed are found first in `data` and then usually in the environment of ‘formula’ & its ancestors. (If `data` is provided but is not an environment then expressions are evaluated in an environment with content taken from `data` whose parent is `environment(formula)`.) Side effects of evaluating the arguments will usually NOT occur in the caller's frame."
I think this is correct, but what really happens is inside other functions with unusual argument evaluation, so I'm not certain.


Regards,
Jorgen Harmse
 
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