What is meaning of first tilde in purrr::map

I was looking at this example that uses map. Here it is:

mtcars %>%
  split(.$cyl) %>% # from base R
  map(~ lm(mpg ~ wt, data = .))

What is the meaning of the first tilde in map(~ lm...? That is, how does R interpret the first tilde? (I understand that the second tilde indicates a function...). Another way of asking is, why doesn't the following work?

mtcars %>%
  split(.$cyl) %>% # from base R
  map(lm(mpg ~ wt, data = .))