Equivalent of `break` in purrr::map

There is no equivalent. In fact, one thing that makes map (and similar functions) so superior to general loops in terms of readability is that they have absolutely predicable behaviour: they will execute the function exactly once for each element, no exceptions (except, uh, if there’s an exception: you could raise a condition via stop to short-circuit execution, but this is very rarely advisable).

Instead, your case doesn’t call for map, it calls for something along the lines of purrr::keep or purrr::reduce.

Think of it this way: map, reduce, etc. are abstractions which correspond to specific special cases of the more general for loop. Their purpose is to make clear which special case is being handled. As a programmer, your task then becomes to find the right abstraction.

In your particular case I would probably completely rewrite the statement using dplyr so giving a “best” purrr solution is hard: the best solution is not to use purrr. That said, you could use purrr::detect as follows:

names(iris) %>%
    detect(`==`, 'Sepal.Width') %>%
    `[`(iris, .) %>%
    head(2)

Or

seq_along(iris) %>%
    detect(~ names(iris[.x]) == 'Sepal.Width') %>%
    `[`(iris, .) %>%
    head(2)

… but really, here’s dplyr for comparison:

iris %>%
    select(Sepal.Width) %>%
    head(2)

1) callCC can be used to get this effect:

callCC(function(k) {
  fun2 <- function(x) {
    print(x) # just to show that x = 5 is never run
    df <- iris %>% select(x) %>% head(2)
    if (names(df) == "Petal.Width") k(df)
  }
  map_df(1:5, fun2)
})

giving:

[1] 1
[1] 2
[1] 3
[1] 4
  Petal.Width
1         0.2
2         0.2

1a) If it is important to use fun without change then try this instead:

callCC(function(k) map_df(1:5, ~ if (!is.null(df <- fun(.x))) k(df)))

2) purrr::reduce An alternative is to use reduce from purrr (or Reduce from base R):

f <- function(x, y) if (is.null(x)) fun(y) else x
reduce(1:5, f, .init = NULL)

This is not as good as (1) and (1a) from the viewpoint that it will still involve iterating over each element of 1:5 but will only invoke fun for 1:4. In contrast (1) and (1a) actually return after running fun or fun2 on 4.