allow fit_xy() for GAMs with non-mgcv engines

NEWS.md

@@ -1,11 +1,13 @@
 # parsnip (development version)
 
-* Aligned `null_model()` with other model types; the model type now has an
-  engine argument that defaults to `"parsnip"` and is checked with the same
-  machinery that checks other model types in the package (#1083).
+
+* `fit_xy()` currently raises an error for `gen_additive_mod()` model specifications as the default engine (`"mgcv"`) specifies smoothing terms in model formulas. However, some engines specify smooths via additional arguments, in which case the restriction on `fit_xy()` is excessive. parsnip will now only raise an error when fitting a `gen_additive_mod()` with `fit_xy()` when using the `"mgcv"` engine (#775).
+
+* Aligned `null_model()` with other model types; the model type now has an engine argument that defaults to `"parsnip"` and is checked with the same machinery that checks other model types in the package (#1083).
 
 * New `extract_fit_time()` method has been added that returns the time it took to train the model (#853).
 
+
 # parsnip 1.2.1
 
 * Added a missing `tidy()` method for survival analysis glmnet models (#1086).

R/gen_additive_mod.R

@@ -94,7 +94,7 @@ translate.gen_additive_mod <- function(x, engine = x$engine, ...) {
 fit_xy.gen_additive_mod <- function(object, ...) {
   trace <- rlang::trace_back()
 
-  if ("workflows" %in% trace$namespace) {
+  if ("workflows" %in% trace$namespace & identical(object$engine, "mgcv")) {
     cli::cli_abort(
       c("!" = "When working with generalized additive models, please supply the
                model specification to {.fun workflows::add_model} along with a \\
@@ -104,9 +104,15 @@ fit_xy.gen_additive_mod <- function(object, ...) {
     )
   }
 
-  cli::cli_abort(c(
-    "!" = "Please use {.fun fit} rather than {.fun fit_xy} to train \\
-           generalized additive models.",
-    "i" = "See {.help model_formula} to learn more."
-  ))
+  if (identical(object$engine, "mgcv")) {
+    cli::cli_abort(c(
+      "!" = "Please use {.fun fit} rather than {.fun fit_xy} to train \\
+           generalized additive models with the {.val mgcv} engine.",
+      "i" = "See {.help model_formula} to learn more."
+    ))
+  }
+
+  # allow fitting GAMs that specify smooths via other arguments to use
+  # `fit_xy()` (#775)
+  NextMethod()
 }