map_*

The map() function will return its output as a list. However, there are several different map() functions; you can use map_*() functions to tell purrr the type of output you want. The * in map_*() represents different R data types. For instance, you might want the output to be a vector of numbers so that we can put it inside a data frame. So, unless you want something to be returned as a list, you need to determine what you want the output to be before you write your map() function.