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Minor list tweaks

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hadley 2015-12-01 12:46:38 +04:00
parent f26fdd729f
commit 0ddccf6350
1 changed files with 9 additions and 2 deletions
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lists.Rmd
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@ -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)