Avoid eager evaluation of arg in vec_as_location*() and vec_recycle()

[krlmlr]

for pretty error messages in tibble.

Do we need to replace .Call() by something different?

It would be great to move the check_dots_empty() check to C code too.

system.time(vctrs::vec_as_location(1, 1L, arg = Sys.sleep(1)))
#>    user  system elapsed 
#>   0.023   0.000   1.025

^{Created on 2020-06-15 by the reprex package (v0.3.0)}

[lionel-]

This would require a new kind of vctrs_arg object that takes an environment and a symbol name.

[krlmlr]

The way I see it we could use a technique similar to 9ef15b6 (#998) at the front-end. Are you suggesting we need to introduce a new internal structure to pass along the environment and the name where to look up arg?

[krlmlr]

Would you review a PR?

[krlmlr]

Simpler reprex:

library(vctrs)

vec_as_subscript(1, arg = { print("oof"); deparse(quote(i)) })
#> [1] "oof"
#> [1] 1

^{Created on 2021-07-20 by the reprex package (v2.0.0)}

The deparse() call is slow, it should be run only if there is an actual error.

[krlmlr]

@lionel-: I now understand your remark regarding the new vctrs_arg. What should the constructor look like: new_deferred_arg(struct vctrs_arg* parent, SEXP environment, const char* arg_name) ?

I think we can do this in baby steps. I have found 8 instances of new_wrapper_arg(), all have parent = NULL . Do we even need the parent in new_deferred_arg()?

[lionel-]

I think you can omit parent, I can't think of a case where this wouldn't be the root arg, since it's for function arguments.

Maybe call it lazy_arg rather than deferred_arg? Shorter and refers to lazy evaluation.

[krlmlr]

I need to pass two items to the new lazy_arg_fill(): environment and symbol name. I'm thinking about passing a formula from the R side instead, this would also minimize the code changes necessary. How do you feel about it? How do I evaluate a formula in rlang?

[krlmlr]

Example: in vec_as_location(), I would do

  .Call(
    vctrs_as_location,
    ...,
    arg = ~arg
  )

instead of

  .Call(
    vctrs_as_location,
    ...,
    arg = arg
  )

.

[krlmlr]

I'll go with using the C-callable equivalents new_quosure() and eval_tidy() for a first draft.

[lionel-]

I'd rather not bring quosures into this. Let's use a list that contains an argument name and an environment. These are internal data structures.

[krlmlr]

Lists and quosures suffer from the same problem, we'd need to protect these objects globally. Going with an internal caller-allocated struct.