Error messages for incorrect mode+engine combination

[hfrick]

Not all engines are available for all modes of a model. It would be nice if an unavailable combination of mode+engine would throw an informative error.

Currently, you can specify such an incorrect combination (the order of set_engine() and set_mode() doesn't matter) and then fit() errors with a message that doesn't refer to the mode+engine combination.

library(parsnip)
data(mtcars)

# engine C5.0 is only available for classification

# neither of the two specifications error
dt_reg_spec <- decision_tree() %>% 
  set_mode("regression") %>% 
  set_engine("C5.0")

dt_reg_spec <- decision_tree() %>% 
  set_engine("C5.0") %>% 
  set_mode("regression")

# fit() errors but the message doesn't point to the actual problem
dt_reg_spec %>% fit(mpg ~ ., data = mtcars)
#> Error: formula_ is unknown.

^{Created on 2021-07-14 by the reprex package (v2.0.0)}

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