Return an error when length > 1 of vector is passed to a scaler argument

[yutannihilation]

Fix some part of #64

library(rextendr)

rust_function(
  code = "fn one_float(input: f64) -> f64 {
    5.*input
  }",
  patch.crates_io = c(
    'extendr-api = { path = "/home/yutani/repo/extendr/extendr-api" }',
    'extendr-macros = { git = "https://github.com/extendr/extendr" }'
  )
)
#> build directory: /tmp/RtmpVtIB3g/file7a70ee5a29f
#> Prebuild libR bindings are not available. Run `install_libR_bindings()` to improve future build times.

one_float(numeric(0))
#> Error in one_float(numeric(0)): zero length vector
one_float(numeric(1))
#> [1] 0
one_float(numeric(2))
#> Error in one_float(numeric(2)): >1 length vector

^{Created on 2020-12-08 by the reprex package (v0.3.0)}

(I'm yet to figure out how to write tests for this...)

[clauswilke]

Errors that are sent back to R probably have to be tested from R. This could eventually be part of a test suite that we put into the rextendr package.

[yutannihilation]

have to be tested from R

Makes sense, thanks. I included only minimum tests here, but let's test thoroughly from R in future.

[clauswilke]

Maybe we can improve the error messages a bit? How about:
"Vector of length zero given. Expected length 1."
"Vector of length >1 given. Expected length 1."

[yutannihilation]

I agree it's nice to improve the error messages. To follow the tidyverse style (c.f. https://style.tidyverse.org/error-messages.html#problem-statement), how about placing the problem state first:

"Input must be length 1. Vector of length ... given."

[clauswilke]

Sounds good to me. I suggest one minor edit: "Input must be of length 1."

[yutannihilation]

Thanks, I'll use it.