title: using purrr:map over multiple columns (more than 2)
body: <p>I have the following data --</p>
  
  <pre><code>structure(list(k = structure(c(17563, 17591, 17622, 17652, 17683
  ), class = "Date"), startdt = structure(c(17532, 17561, 17589, 
  17620, 17650), class = "Date"), enddt = structure(c(17560, 17588, 
  17619, 17649, 17652), class = "Date")), row.names = c(NA, -5L
  ), class = c("grouped_df", "tbl_df", "tbl", "data.frame"), .Names = c("k", 
  "startdt", "enddt"), vars = "k", drop = TRUE, indices = list(
  0L, 1L, 2L, 3L, 4L), group_sizes = c(1L, 1L, 1L, 1L, 1L), 
  biggest_group_size = 1L, labels = structure(list(
  k = structure(c(17563, 17591, 17622, 17652, 17683), class = "Date")), 
  row.names = c(NA, 
  -5L), class = "data.frame", vars = "k", drop = TRUE, .Names = "k"))
  </code></pre>
  
  <p>and I want to pipe it into a function. here's how I would use map3, which does not exist (hooray) --</p>
  
  <pre><code>dates %&gt;% map3_df(.x = $.k, .y = $.startdt, .z = $.enddt,
   .f = ~ fetchdata(startdate = .z, enddate = .z, k = .x)
  </code></pre>
  
  <p>Pmap may be an option? i'm just not too familiar, and an issue seems to be that i can't assign variables explicitly (ie .x, .y, .z) and the function i want to map to takes a ton of optional variables (and i only want to specify startdate, enddt, and k).</p>
  
  <p>Any help appreciated, thanks in advance.</p>