Minor list tweaks

lists.Rmd

@@ -535,7 +535,12 @@ df %>% transpose() %>% str()
 
 When you do many operations on a list, sometimes one will fail. When this happens, you'll get an error message, and no output. This is annoying: why does one failure prevent you from accessing all the other successes? How do you ensure that one bad apple doesn't ruin the whole barrel?
 
-In this section you'll learn how to deal this situation with a new function: `safely()`. `safely()` is an adverb: it takes a function and returns a modified function. In this case, the modified function returns a list with elements `result` (the original result) and `error` (the text of the error if it occured). For any given run, one will always be `NULL`. 
+In this section you'll learn how to deal this situation with a new function: `safely()`. `safely()` is an adverb: it takes a function modifies it. In this case, the modified function never throws an error and always succeeds. Instead, it returns a list with two elements:
+
+1. `result`: the original result. If there was an error, this will be `NULL`.
+
+1. `error`: the text of the error if it occured. If the operation was 
+   successful this will be `NULL`.
 
 (You might be familiar with the `try()` function in base R. It's similar, but because it sometimes returns the original result and it sometimes returns an error object it's more difficult to work with.)
 
@@ -572,6 +577,8 @@ x[!is_ok]
 y$result[is_ok] %>% flatten_dbl()
 ```
 
+(Note that you should always check that the error is null, not that the result is not-null. Sometimes the correct response is `NULL`.)
+
 Other related functions:
 
 *   `possibly()`: if you don't care about the error message, and instead
@@ -770,7 +777,7 @@ Purrr comes with a number of predicate functions built-in:
 | `is_vector()`    |  x  |  x  |  x  |  x  |  x   |      |
 | `is_null()`      |     |     |     |     |      | x    |
 
-Compared to the base R functions, they only inspect the type of object, not its attributes. This means they tend to be less suprising: 
+Compared to the base R functions, they only inspect the type of the object, not its attributes. This means they tend to be less suprising: 
 
 ```{r}
 is.atomic(NULL)