feat : added condition to check the RHS of :=

R/data.table.R

@@ -1403,7 +1403,19 @@ 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.
+      if (length(cols) == 1L) {
+        is_new_col <- cols > ncol(x)
+        if (is_new_col || !is.list(x[[cols]])) {
+          if (is.function(jval)) {
+            stopf("RHS of `:=` is a function. To create a new column of functions or assign to a non-list-column, it must be wrapped in a list(), e.g., `:= list(myfun)`.")
+          }
+          if (is.list(jval) && length(jval) == 1L && is.function(jval[[1L]]) && nrow(x) > 1L) {
+            stopf("RHS of `:=` is a length-1 list containing a function. `data.table` does not automatically recycle lists. To create a list-column, use `rep(list(myfun), .N)`.")
+          }
+        }
+      }
+      newnames = setdiff(lhs, names(x))
+      # 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

@@ -21157,3 +21157,13 @@ test(2317.6, DT1[DF1, on='a', .(d = x.a + i.d)]$d, 5)
 test(2317.7, DT1[DF2, on='a', e := i.e]$e, 5)
 test(2317.8, DT1[DF2, on='a', e2 := x.a + i.e]$e2, 6)
 test(2317.9, DT1[DF2, on='a', .(e = x.a + i.e)]$e, 6)
+
+DT = data.table(x = 1:3)
+
+test(2318.1, DT[, y := mean], error = "RHS of `:=` is a function")
+test(2318.2, DT[, y := list(mean)], error = "RHS of `:=` is a length-1 list containing a function")
+f = mean
+test(2318.3, DT[, y := f], error = "RHS of `:=` is a function")
+test(2318.4, DT[, y := identity(mean)], error = "RHS of `:=` is a function")
+test(2318.5, data.table(x = 1)[, y := mean], error = "RHS of `:=` is a function")
+test(2318.6, data.table(x = 1:2)[, y := mean], error = "RHS of `:=` is a function")