Clean error if summarising summarised variable

Fxies #75

R/step-subset.R

@@ -114,6 +114,7 @@ select.dtplyr_step <- function(.data, ...) {
 #' @export
 summarise.dtplyr_step <- function(.data, ...) {
   dots <- capture_dots(.data, ...)
+  check_summarise_vars(dots)
 
   if (length(dots) == 0) {
     if (length(.data$groups) == 0) {
@@ -261,3 +262,19 @@ simplify_names <- function(vars) {
   names(vars)[vars == names(vars)] <- ""
   vars
 }
+
+# For each expression, check if it uses any newly created variables
+check_summarise_vars <- function(dots) {
+  for (i in seq_along(dots)) {
+    used_vars <- all_names(get_expr(dots[[i]]))
+    cur_vars <- names(dots)[seq_len(i - 1)]
+
+    if (any(used_vars %in% cur_vars)) {
+      abort(paste0(
+        "`", names(dots)[[i]], "` ",
+        "refers to a variable created earlier in this summarise().\n",
+        "Do you need an extra mutate() step?"
+      ))
+    }
+  }
+}

tests/testthat/test-step-subset.R

@@ -145,6 +145,11 @@ test_that("empty summarise returns unique groups", {
   )
 })
 
+test_that("if for unsupported resummarise", {
+  dt <- lazy_dt(data.frame(x = 1:3, y = 1:3))
+  expect_error(dt %>% summarise(x = mean(x), x2 = sd(x)), "mutate")
+})
+
 # select/rename ------------------------------------------------------------------
 
 test_that("renames grouping vars", {