enhanced error message

Author: venom1204

NEWS.md

@@ -46,6 +46,8 @@
 
 10. `data.table()` and `as.data.table()` with `keep.rownames=TRUE` now extract row names from named vectors, matching `data.frame()` behavior. Names from the first named vector in the input are used to create the row names column (default name `"rn"` or custom name via `keep.rownames="column_name"`), [#1916](https://github.com/Rdatatable/data.table/issues/1916). Thanks to @richierocks for the feature request and @Mukulyadav2004 for the implementation.
 
+11. Assigning a raw function to a new column, e.g. `DT[, new_col := mean]`, now throws an informative error. This prevents a common mistake and guides the user to wrap the function in a `list()`, e.g. `DT[, new_col := list(mean)]`, if the intent is to create a list-column of functions. Thanks to @tdhock for the report and @venom1204 for the fix.
+
 ### BUG FIXES
 
 1. `fread()` no longer warns on certain systems on R 4.5.0+ where the file owner can't be resolved, [#6918](https://github.com/Rdatatable/data.table/issues/6918). Thanks @ProfFancyPants for the report and PR.

R/data.table.R

@@ -1411,7 +1411,13 @@ replace_dot_alias = function(e) {
     }
 
     if (!is.null(lhs)) {
-      # TODO?: use set() here now that it can add new columns. Then remove newnames and alloc logic above.
+      newnames = setdiff(lhs, names(x))
+      if (length(newnames) > 0) {
+        if (is.function(jval)) {
+          stopf("RHS of `:=` is a function. To create a new column of functions, it must be a list column (e.g., wrap the function in `list()`).")
+        }
+      }
+      # TODO?: use set() here now that it can add new columns.Then remove newnames and alloc logic above.
       .Call(Cassign,x,irows,cols,newnames,jval)
       return(suppPrint(x))
     }

inst/tests/tests.Rraw

@@ -21426,3 +21426,8 @@ test(2330.7, as.data.table(list(z), keep.rownames=TRUE), data.table(rn=rep("", 3
 
 M <- matrix(1:6, nrow=3, dimnames=list(rep("", 3), c("V1", "V2")))   #  test of list(M) for empty-rowname'd matrix input
 test(2330.8, as.data.table(list(M), keep.rownames=TRUE), data.table(rn=rep("", 3), V1=1:3, V2=4:6))
+
+#5829 checking the RHS of :=
+DT = data.table(x = 1:3)
+test(2331.1, DT[, y := mean], error = "RHS of `:=` is a function")
+test(2331.2, DT[, y := list(mean)], error = "RHS of `:=` is a length-1 list containing a function")