[WIP] Check length of input in <&str>::try_from()

[yutannihilation]

Fix #102

While other try_from() implementations use the following logic, &str is a bit tricky in that it fails with cannot return value referencing local variable error,

        if let Some(v) = robj.as_<type>_slice() {
            match v.len() {
                0 => Err(Error::ExpectedNonZeroLength(robj)),
                1 => Ok(v[0]),
                _ => Err(Error::ExpectedScalar(robj)),
            }
        } else {
            Err(Error::ExpectedLogical(robj))
        }

so I chose to check the length before trying to convert the input to the actual variable.

[clauswilke]

Looks good to me.

However, shouldn't this require changes in the integration tests, here:

extendr/tests/extendrtests/tests/testthat/test-type-conversion.R

Line 29 in 2103979

expect_error(char_scalar(c("hello", "world")), "not a string object") # why this error message and not "Input must be of length 1"?

[yutannihilation]

In my understanding, try_from() isn't used yet by #[extendr] macro. It still uses from_robj() (c.f. #219 (comment)). I'm wondering if I should modify from_robj() as well at the moment, considering it is still used but will be replaced...

[clauswilke]

Ah, Ok. We probably need a complete implementation of try_from() for all types before we can switch over. It's not quite clear to me whether we're there yet or not.

[yutannihilation]

In my understanding, try_from() isn't used yet by #[extendr] macro.

Sorry, I was a bit confused. We can confirm the behavior with #[extendr(use_try_from = true)].

gen_fun <- function(fun_name, use_try_from) {
  use_try_from <- if(isTRUE(use_try_from)) "true" else "false"
  code <- r"(
#[extendr(use_try_from = {use_try_from})]
fn {fun_name}(x: &str) {{
    println!("{{}}", x)
}}
)"

  rextendr::rust_source(
    code = glue::glue(code),
    patch.crates_io = list(`extendr-api` = list(git = "https://github.com/yutannihilation/extendr", branch = "fix/issue-102-improve-string-scalar-error")),
    env = globalenv()
  )
}

# Use try_from impl
gen_fun("f1", TRUE)
#> ℹ build directory: '/tmp/Rtmp9mx0FQ/file2b2fb60e771c1'
#> ✓ Writing '/tmp/Rtmp9mx0FQ/file2b2fb60e771c1/target/extendr_wrappers.R'.
# Use from_robj impl
gen_fun("f2", FALSE)
#> ℹ build directory: '/tmp/Rtmp9mx0FQ/file2b2fb60e771c1'
#> ✓ Writing '/tmp/Rtmp9mx0FQ/file2b2fb60e771c1/target/extendr_wrappers.R'.

f1(letters)
#> Error in f1(letters): Expected Scalar, got String
f2(letters)
#> Error in f2(letters): Not a string object.

^{Created on 2021-07-18 by the reprex package (v2.0.0)}