Implement map_vec()

Fixes #435

NAMESPACE

@@ -160,6 +160,7 @@ export(map_if)
 export(map_int)
 export(map_lgl)
 export(map_raw)
+export(map_vec)
 export(modify)
 export(modify2)
 export(modify_at)
@@ -213,4 +214,7 @@ export(zap)
 import(rlang)
 import(vctrs)
 importFrom(magrittr,"%>%")
+importFrom(vctrs,vec_c)
+importFrom(vctrs,vec_ptype_common)
+importFrom(vctrs,vec_size)
 useDynLib(purrr, .registration = TRUE)

R/map.R

@@ -11,6 +11,9 @@
 #' * `map_lgl()`, `map_int()`, `map_dbl()` and `map_chr()` return an
 #'   atomic vector of the indicated type (or die trying).
 #'
+#' * `map_vec()` simplifies to the common type of the output. It works with
+#'   most types of simple vectors like Date, POSIXct, factors, etc.
+#'
 #' * `map_dfr()` and `map_dfc()` return a data frame created by
 #'   row-binding and column-binding respectively. They require dplyr
 #'   to be installed. `map_df()` is an alias for `map_dfr()`.
@@ -218,6 +221,27 @@ map_raw <- function(.x, .f, ...) {
   .Call(map_impl, environment(), ".x", ".f", "raw")
 }
 
+
+#' @rdname map
+#' @param .ptype If `NULL`, the default, the output type is the common type
+#'   of the elements of the result. Otherwise, supply a "prototype" giving
+#'   the desired type of output.
+#' @importFrom vctrs vec_c vec_size vec_ptype_common
+#' @export
+map_vec <- function(.x, .f, ..., .ptype = NULL) {
+  out <- map(.x, .f, ...)
+
+  .ptype <- vec_ptype_common(!!!out, .ptype = .ptype)
+  for (i in seq_along(out)) {
+    if (vec_size(out[[i]]) != 1L) {
+      stop_bad_element_vector(out[[i]], i, .ptype, 1L, what = "Result")
+    }
+  }
+
+  vec_c(!!!out, .ptype = .ptype)
+}
+
+
 #' @rdname map
 #' @param .id Either a string or `NULL`. If a string, the output will contain
 #'   a variable with that name, storing either the name (if `.x` is named) or

tests/testthat/_snaps/map.md

@@ -0,0 +1,24 @@
+# requires output be length 1
+
+    Code
+      map_vec(1:2, ~ rep(1, .x))
+    Condition
+      Error in `stop_bad_type()`:
+      ! Result 2 must be a single double, not a double vector of length 2
+
+# requires common type of output
+
+    Code
+      map_vec(1:2, ~ if (.x == 1) factor("x") else 1)
+    Condition
+      Error in `map_vec()`:
+      ! Can't combine `..1` <factor<bf275>> and `..2` <double>.
+
+# can enforce .ptype
+
+    Code
+      map_vec(1:2, ~ factor("x"), .ptype = integer())
+    Condition
+      Error:
+      ! Can't convert <factor<bf275>> to <integer>.
+

tests/testthat/test-map.R

@@ -170,3 +170,32 @@ test_that("map() with empty input copies names", {
   expect_identical(map_chr(named_list, identity), named(chr()))
   expect_identical(map_raw(named_list, identity), named(raw()))
 })
+
+
+# map_vec -----------------------------------------------------------------
+
+test_that("still iterates using [[", {
+  df <- data.frame(x = 1, y = 2, z = 3)
+  expect_equal(map_vec(df, length), c(x = 1, y = 1, z = 1))
+})
+
+test_that("requires output be length 1", {
+  expect_snapshot(error = TRUE, {
+    map_vec(1:2, ~ rep(1, .x))
+  })
+})
+
+test_that("requires common type of output", {
+  out <- map_vec(1:2, ~ factor("x"))
+  expect_equal(out, factor(c("x", "x")))
+
+  expect_snapshot(error = TRUE, {
+    map_vec(1:2, ~ if (.x == 1) factor("x") else 1)
+  })
+})
+
+test_that("can enforce .ptype", {
+  expect_snapshot(error = TRUE, {
+    map_vec(1:2, ~ factor("x"), .ptype = integer())
+  })
+})