Error informatively if predictors or outcomes are missing

Closes #144

NEWS.md

@@ -1,5 +1,8 @@
 # workflows (development version)
 
+* Improved error message in `workflow_variables()` if either `outcomes` or
+  `predictors` are missing (#144).
+
 * Removed ellipsis dependency in favor of equivalent functions in rlang.
 
 * New `extract_parameter_set_dials()` and `extract_parameter_dials()` methods 

R/pre-action-variables.R

@@ -251,13 +251,24 @@ workflow_variables <- function(outcomes, predictors) {
   )
 }
 
-new_workflow_variables <- function(outcomes, predictors) {
+new_workflow_variables <- function(outcomes,
+                                   predictors,
+                                   ...,
+                                   call = caller_env()) {
+  check_dots_empty()
+
   if (!is_quosure(outcomes)) {
     abort("`outcomes` must be a quosure.", .internal = TRUE)
   }
   if (!is_quosure(predictors)) {
     abort("`predictors` must be a quosure.", .internal = TRUE)
   }
+  if (quo_is_missing(outcomes)) {
+    abort("`outcomes` can't be missing.", call = call)
+  }
+  if (quo_is_missing(predictors)) {
+    abort("`predictors` can't be missing.", call = call)
+  }
 
   data <- list(
     outcomes = outcomes,

tests/testthat/_snaps/pre-action-variables.md

@@ -14,6 +14,22 @@
       Error in `add_variables()`:
       ! Variables cannot be added when a formula already exists.
 
+# informative error if either `predictors` or `outcomes` aren't provided (#144)
+
+    Code
+      add_variables(workflow(), outcomes = mpg)
+    Condition
+      Error in `workflow_variables()`:
+      ! `predictors` can't be missing.
+
+---
+
+    Code
+      add_variables(workflow(), predictors = mpg)
+    Condition
+      Error in `workflow_variables()`:
+      ! `outcomes` can't be missing.
+
 # cannot add two variables
 
     Code

tests/testthat/test-pre-action-variables.R

@@ -34,6 +34,11 @@ test_that("cannot add variables if a formula already exist", {
   expect_snapshot(error = TRUE, add_variables(wf, y, x))
 })
 
+test_that("informative error if either `predictors` or `outcomes` aren't provided (#144)", {
+  expect_snapshot(error = TRUE, add_variables(workflow(), outcomes = mpg))
+  expect_snapshot(error = TRUE, add_variables(workflow(), predictors = mpg))
+})
+
 test_that("works with fit()", {
   mod <- parsnip::linear_reg()
   mod <- parsnip::set_engine(mod, "lm")