refactor: ♻️ use assert_string() to check character scalars (#180)

# Description

Instead of the `assert_character()` and `assert_scalar()` combo. See
https://mllg.github.io/checkmate/reference/checkString.html.

Needs no review.

## Checklist

- [X] Ran `just run-all`

Author: signekb

R/convert.R

@@ -103,8 +103,7 @@ convert_file <- function(
   # Initial checks.
   checkmate::assert_character(path)
   checkmate::assert_file_exists(path)
-  checkmate::assert_character(output_dir)
-  checkmate::assert_scalar(output_dir)
+  checkmate::assert_string(output_dir)
   checkmate::assert_int(chunk_size, lower = 10000L)
 
   # Prepare variables used in repeat below.

R/list.R

@@ -13,7 +13,7 @@
 list_sas_files <- function(path) {
   # Check input.
   checkmate::assert_directory(path)
-  checkmate::assert_scalar(path)
+  checkmate::assert_string(path)
 
   # List all SAS files in the directory and its subdirectories.
   # (?i) makes the regex case-insensitive.
@@ -47,7 +47,7 @@ list_sas_files <- function(path) {
 list_parquet_files <- function(path) {
   # Check input.
   checkmate::assert_directory(path)
-  checkmate::assert_scalar(path)
+  checkmate::assert_string(path)
 
   # List all Parquet files in the directory and its subdirectories.
   # (?i) makes the regex case-insensitive.

R/read.R

@@ -20,8 +20,7 @@ read_register <- function(
   path
 ) {
   # Check input.
-  checkmate::assert_character(path)
-  checkmate::assert_scalar(path)
+  checkmate::assert_string(path)
   checkmate::assert(
     checkmate::check_file_exists(path),
     checkmate::check_directory_exists(path)

tests/testthat/test-convert.R

@@ -71,7 +71,7 @@ test_that("convert_file() errors with incorrect input parameters", {
   # Incorrect output_dir type.
   expect_error(
     convert_file(path = sas_kontakter[[1]], output_dir = 1),
-    regexp = "character"
+    regexp = "string"
   )
   # output_dir must be scalar.
   expect_error(

tests/testthat/test-read.R

@@ -73,7 +73,7 @@ test_that("reading a non-existing Parquet register throws an error", {
 })
 
 test_that("incorrect input type throws an error", {
-  expect_error(read_register(123), regexp = "character")
+  expect_error(read_register(123), regexp = "string")
   expect_error(
     read_register(c("path1.parquet", "path2.parquet")),
     regexp = "length 1"