Improve unchop() error for incompatible ptype

NEWS.md

@@ -1,5 +1,8 @@
 # tidyr (development version)
 
+* `unchop()` produces a more helpful error message when columns cannot be cast
+  to `ptype` (@mgirlich, #1477).
+
 * `expand_grid()` gains a new `.vary` argument, allowing users to control
   whether the first column varies fastest or slowest (#1543, @JamesHWade).
 

R/chop.R

@@ -254,8 +254,14 @@ df_unchop <- function(x, ..., ptype = NULL, keep_empty = FALSE, error_call = cal
     col_sizes <- x_sizes[[i]]
 
     if (!col_is_list) {
+      # Optimize rare non list-cols
       if (!is_null(col_ptype)) {
-        col <- vec_cast(col, col_ptype, x_arg = col_name, call = error_call)
+        col <- vec_cast(
+          x = col,
+          to = col_ptype,
+          x_arg = col_name,
+          call = error_call
+        )
       }
       out_cols[[i]] <- vec_slice(col, out_loc)
       next
@@ -267,7 +273,12 @@ df_unchop <- function(x, ..., ptype = NULL, keep_empty = FALSE, error_call = cal
     row_recycle <- col_sizes != sizes
     col[row_recycle] <- map2(col[row_recycle], sizes[row_recycle], vec_recycle, call = error_call)
 
-    col <- list_unchop(col, ptype = col_ptype)
+    col <- list_unchop(
+      x = col,
+      ptype = col_ptype,
+      error_arg = col_name,
+      error_call = error_call
+    )
 
     if (is_null(col)) {
       # This can happen when both of these are true:

tests/testthat/_snaps/chop.md

@@ -11,6 +11,14 @@
       Error in `chop()`:
       ! `cols` is absent but must be supplied.
 
+# incompatible ptype mentions the column (#1477)
+
+    Code
+      unnest(df, data, ptype = list(data = integer()))
+    Condition
+      Error in `unnest()`:
+      ! Can't convert `data[[2]]` <character> to <integer>.
+
 # incompatible sizes are caught
 
     Code

tests/testthat/_snaps/unnest.md

@@ -31,7 +31,7 @@
       (expect_error(unnest(df, x)))
     Output
       <error/vctrs_error_ptype2>
-      Error in `list_unchop()`:
+      Error in `unnest()`:
       ! Can't combine `x[[1]]` <double> and `x[[2]]` <tbl_df>.
 
 # unnest() advises on outer / inner name duplication

tests/testthat/test-chop.R

@@ -228,6 +228,14 @@ test_that("`ptype = list()` uses list ptype", {
   )
 })
 
+test_that("incompatible ptype mentions the column (#1477)", {
+  df <- tibble(data = list(1, "2"))
+
+  expect_snapshot(error = TRUE, {
+    unnest(df, data, ptype = list(data = integer()))
+  })
+})
+
 test_that("unchopping a bare empty list results in unspecified()", {
   df <- tibble(x = integer(), y = list())
   expect <- tibble(x = integer(), y = unspecified())