Improved Error Message for Assigning NULL to List Column Items

NEWS.md

@@ -186,6 +186,8 @@ This feature resolves [#4387](https://github.com/Rdatatable/data.table/issues/43
 
 24. The internal `init()` function in `fread.c` module has been marked as `static`, [#6328](https://github.com/Rdatatable/data.table/pull/6328). This is to avoid name collisions, and the resulting segfaults, with other libraries that might expose the same symbol name, and be already loaded by the R process. This was observed in Cray HPE environments where the `libsci` library providing LAPACK to R already has an `init` symbol. Thanks to @rtobar for the report and fix.
 
+25. `set()` and `:=` now provide an improved error message when attempting to directly assign `NULL` to a list column item. The new message guides users to use `list(list(NULL))` instead, enhancing clarity and preventing common mistakes. Previously, this operation would throw a generic error, but now the message is more informative and helpful. Thanks to @Nj221102 for the implementation and @MichaelChirico for the suggestion.
+
 ## TRANSLATIONS
 
 1. Fix a typo in a Mandarin translation of an error message that was hiding the actual error message, [#6172](https://github.com/Rdatatable/data.table/issues/6172). Thanks @trafficfan for the report and @MichaelChirico for the fix.

inst/tests/tests.Rraw

@@ -5578,8 +5578,8 @@ test(1352.8, DT[,sum(b),by=.(a%%2,c)], DT[,sum(b),by=list(a%%2,c)])
 
 # that :=NULL together with i is now an error
 DT = data.table(a=1:3, b=1:6)
-test(1353.1, DT[2, b:=NULL], error="When deleting columns, i should not be provided")
-test(1353.2, DT[2, c("a","b"):=list(42, NULL)], error="When deleting columns, i should not be provided")
+test(1353.1, DT[2, b:=NULL], error=".*Direct assignment of NULL to a list column item is not supported.*")
+test(1353.2, DT[2, c("a","b"):=list(42, NULL)], error=".*Direct assignment of NULL to a list column item is not supported.*")
 
 # order optimisation caused trouble due to chaining because of 'substitute(x)' usage in [.data.table.
 set.seed(1L)
@@ -7243,7 +7243,7 @@ if (test_bit64) {
 # fix for #1082
 dt1 = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9, key=c("x", "y"))
 dt2 = copy(dt1)
-test(1502.1, dt1["a", z := NULL], error="When deleting columns, i should not be provided")
+test(1502.1, dt1["a", z := NULL], error=".*Direct assignment of NULL to a list column item is not supported.*")
 # this shouldn't segfault on 'dt1[...]'
 test(1502.2, dt1["a", z := 42L], dt2["a", z := 42L])
 
@@ -13235,7 +13235,7 @@ test(1946, unique(DT, by="B"), data.table(A=1:3, B=1:3, foo=7:9))
 
 # corruption when deleting a missing column and providing i too, #3089
 DT = data.table(A=1:5)
-test(1947.1, DT[A<0, c('A','B'):=.(NULL, A)], error="When deleting columns, i should not be provided")
+test(1947.1, DT[A<0, c('A','B'):=.(NULL, A)], error=".*Direct assignment of NULL to a list column item is not supported.*")
 test(1947.2, DT, data.table(A=1:5))
 
 ## tests for backticks and spaces in column names of on=, #2931

src/assign.c

@@ -458,7 +458,7 @@ SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values)
     coln--;
     SEXP thisvalue = RHS_list_of_columns ? VECTOR_ELT(values, i) : values;
     vlen = length(thisvalue);
-    if (isNull(thisvalue) && !isNull(rows)) error(_("When deleting columns, i should not be provided"));  // #1082, #3089
+    if (isNull(thisvalue) && !isNull(rows)) error(_("Direct assignment of NULL to a list column item is not supported. If you intend to assign NULL to an item in the list column, use list(list(NULL)) in your query."));  // #1082, #3089
     if (coln+1 <= oldncol) colnam = STRING_ELT(names,coln);
     else colnam = STRING_ELT(newcolnames,coln-length(names));
     if (coln+1 <= oldncol && isNull(thisvalue)) continue;  // delete existing column(s) afterwards, near end of this function