[GitHub] [arrow] jonkeane commented on a change in pull request #12277: ARROW-15480: [R] Expand on schema/colnames mismatch error messages

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jonkeane commented on a change in pull request #12277:
URL: https://github.com/apache/arrow/pull/12277#discussion_r794636116



##########
File path: r/R/dataset-format.R
##########
@@ -133,10 +133,36 @@ CsvFileFormat$create <- function(...,
   schema_names <- names(schema)
 
   if (!is.null(schema) & !identical(schema_names, column_names)) {
+    missing_from_schema <- setdiff(column_names, schema_names)
+    missing_from_colnames <- setdiff(schema_names, column_names)
+    message_colnames <- NULL
+    message_schema <- NULL
+    message_order <- NULL
+
+    if (length(missing_from_colnames) > 0) {
+      message_colnames <- paste(
+        oxford_paste(missing_from_colnames, quote_symbol = "`"),
+        "not present in `column_names`"
+      )
+    }

Review comment:
       We don't need to do this as part of this PR, but I've seen this pattern a few times now:
   
   ```
   missing_from <- setdiff(set_a, set_b)
   if (length(missing_rom) > 0) {
     # construct a message
     # sometimes also abort()
   }
   ```
   
   Maybe the message bits are too unique and there's a bit too many types of them that we couldn't do something like a function that `check_match(x = set_a, y = set_b, x_name = "column_names", y_name = "schema")` that would produce messages like "X, Y, and Z not present in `column_names`".
   
   If you think that's feasible, would you mind making a jira linking this code to it as an improvement we could make?




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