Add an example of argument checking

Author: hadley

vignettes/custom-expectation.Rmd

@@ -122,6 +122,10 @@ Or imagine if you're checking to see if an object inherits from an S3 class. In
 
 ```{r}
 expect_s3_class <- function(object, class) {
+  if (!rlang::is_string(class)) {
+    rlang::abort("`class` must be a string.")
+  }
+
   act <- quasi_label(rlang::enquo(object), arg = "object")
 
   if (!is.object(act$val)) {
@@ -158,6 +162,13 @@ expect_s3_class(x2, "integer")
 expect_s3_class(x3, "integer")
 ```
 
+Note that I also check that the `class` argument must be a string. This is an error, not a failure, because it suggests you're using the function incorrectly.
+
+```{r}
+#| error: true
+expect_s3_class(x1, 1)
+```
+
 ## Repeated code
 
 As you write more expectations, you might discover repeated code that you want to extract out in to a helper. Unfortunately creating helper functions is not straightforward in testthat because every `fail()` captures the calling environment in order to give maximally useful tracebacks. Because getting this right is not critical (you'll just get a slightly suboptimal traceback in the case of failure), we don't recommend bothering. However, we document it here because it's important to get it right in testthat itself.