Introduce `within.qenv`

[gogonzo]

As an alternative to eval_code, within could be a nice way of specifying the code. Most important thing about this function should be substitution of dynamic symbols. We need to be able to have a full control over a call.

qenv <- new_qenv() |> within(qenv, {
  iris2 <- dplyr::filter(iris, {varname} = {as.character(var_value)})
})

eval_code shouldn't be discontinued, as it is still important to have a way of evaluating character or language.

Update:

• within should have arguments:
  • expr literal R expression
  • code R object containing valid R expression (class - language, expression or character)
  • ... replacement arguments similar to env in substitute

[chlebowa]

I have been working on a prototype but my assumptions so far were slightly different:

1. qenv is an environment, not an S4 object holding an environment in a slot
2. code and condition messages are stored in attributes, not slots
3. all code is evaluated in the qenv, modifying it by reference, not returning a (deep) copy
4. syntax is based on the S3 generic with and code is passed as expressions:

q <- qenv()
with(q, i <- iris)

I added a within method to copy, modify and return a qenv.

qq <- within(q, m <- mtcars)

I tried to keep everything in base R, no rlang.

My suggestions and more details can be found in the R/with-qenv.R file here.