type check formula input to fit.model_spec() (tidymodels#952)

R/fit.R

@@ -116,6 +116,22 @@ fit.model_spec <-
     control <- condense_control(control, control_parsnip())
     check_case_weights(case_weights, object)
 
+    if (!inherits(formula, "formula")) {
+      msg <- "The {.arg formula} argument must be a formula, but it is a \\
+              {.cls {class(formula)[1]}}."
+
+      if (inherits(formula, "recipe")) {
+        msg <-
+          c(
+            msg,
+            "i" = "To fit a model with a recipe preprocessor, please use a \\
+                 {.help [workflow](workflows::workflow)}."
+          )
+      }
+
+      cli::cli_abort(msg)
+    }
+
     dots <- quos(...)
 
     if (length(possible_engines(object)) == 0) {

tests/testthat/_snaps/fit_interfaces.md

@@ -1,3 +1,20 @@
+# misspecified formula argument
+
+    Code
+      fit(linear_reg(), rec, mtcars)
+    Condition
+      Error in `fit()`:
+      ! The `formula` argument must be a formula, but it is a <recipe>.
+      i To fit a model with a recipe preprocessor, please use a workflow (`?workflows::workflow()`).
+
+---
+
+    Code
+      fit(linear_reg(), "boop", mtcars)
+    Condition
+      Error in `fit()`:
+      ! The `formula` argument must be a formula, but it is a <character>.
+
 # No loaded engines
 
     ! parsnip could not locate an implementation for `cubist_rules` model specifications.

tests/testthat/test_fit_interfaces.R

@@ -74,6 +74,16 @@ test_that('unknown modes', {
   )
 })
 
+test_that("misspecified formula argument", {
+  rec <- structure(list(), class = "recipe")
+  expect_snapshot(error = TRUE,
+    fit(linear_reg(), rec, mtcars)
+  )
+  expect_snapshot(error = TRUE,
+    fit(linear_reg(), "boop", mtcars)
+  )
+})
+
 test_that("elapsed time parsnip mods", {
   lm1 <-
     linear_reg() %>%