Discuss type consistency

Fixes #1109

logicals.qmd

@@ -346,7 +346,7 @@ flights |>
 In most cases, however, `any()` and `all()` are a little too crude, and it would be nice to be able to get a little more detail about how many values are `TRUE` or `FALSE`.
 That leads us to the numeric summaries.
 
-### Numeric summaries of logical vectors
+### Numeric summaries of logical vectors {#sec-numeric-summaries-of-logicals}
 
 When you use a logical vector in a numeric context, `TRUE` becomes 1 and `FALSE` becomes 0.
 This makes `sum()` and `mean()` very useful with logical vectors because `sum(x)` will give the number of `TRUE`s and `mean(x)` the proportion of `TRUE`s.
@@ -545,6 +545,32 @@ flights |>
   )
 ```
 
+### Compatible types
+
+Note that both `if_else()` and `case_when()` require **compatible** types in the output.
+If they're not compatible, you'll see errors like this:
+
+```{r}
+#| error: true
+
+if_else(TRUE, "a", 1)
+
+case_when(
+  x < -1 ~ TRUE,  
+  x > 0  ~ lubridate::now()
+)
+```
+
+Overall, relatively few types are compatible, because automatically converting one type of vector to another is a common source of errors.
+Here are the most important cases that are compatible:
+
+-   Numeric and logical vectors are compatible, as we discussed in @sec-numeric-summaries-of-logicals.
+-   Strings and factors (@sec-factors) are compatible, because you can think of a factor as a string with a restricted set of values.
+-   Dates and date-times, which we'll discuss in @sec-dates-and-times, are compatible because you can think of a date as a special case of date-time.
+-   `NA`, which is technically a logical vector, is compatible with everything because every vector has some way of representing a missing value.
+
+We don't expect you to memorize these rules, but they should become second nature over time because they are applied consistently throughout the tidyverse.
+
 ## Summary
 
 The definition of a logical vector is simple because each value must be either `TRUE`, `FALSE`, or `NA`.