Add Model Type

Add a new model class to tidypredict

This covers adding support for an R model class that tidypredict does not yet parse: either a new function (like quantreg::rq) or a not-yet-supported model type (svm_linear(), naive_Bayes(), pls(), etc.). tidypredict turns a fitted model into a Tidy Eval formula; supporting a new class means implementing a small set of S3 generics plus, for novel algorithms, new parsing and formula-building logic. (A tracking issue of candidate model types may exist, historically tidymodels/tidypredict#232.)

First: which family does the model fit?

The amount of new logic depends on whether an existing formula builder already produces the shape you need.

• Linear / additive-in-coefficients (lm, glm, earth, glmnet, rq). Prediction is a weighted sum of terms. These reuse parse_model_lm() and build_fit_formula() almost verbatim — adding one is mostly S3 wiring. R/model-rq.R is the smallest such file; R/model-lm.R and R/model-earth.R show fuller versions.
• Tree / ensemble (rpart, ranger, randomForest, xgboost, lightgbm, catboost, cubist, partykit). Prediction walks a tree into nested case_when() (parsed-model version 3). Substantial; study the closest existing R/model-*.R.
• Genuinely new structure (SVM, naive Bayes, PLS, discriminant, MLP). No existing builder fits. You must design a parser that captures the model's parameters and a builder that emits a Tidy Eval expression using only SQL-translatable operations (arithmetic, case_when, exp/log, comparisons). This is real design work — start by writing, by hand, the prediction formula for a tiny fitted example and confirming it matches predict(), then codify it.

If the family is unclear, ask the user which existing model is most similar, or spike the by-hand formula first.

The three S3 generics

Every model dispatches on its S3 class. Define all of these (each #' @export) in a new R/model-<name>.R, with comment-banner sections matching the existing model files (# Predict, # Parse model).

1. tidypredict_fit.<class> — usually parse_model(model) then hand the parsed model to a builder. For linear models: build_fit_formula(parse_model(model)).
2. parse_model.<class> — extract the model into a serializable parsed structure. For linear models this is just parse_model_lm(model). New algorithms build a list with a $general block (model, type, version) plus algorithm-specific fields. parse_model (R/parsemodel.R) documents the parsed-model contract and the pm_* type/subclass system used for dispatch of tidypredict_fit.
3. acceptable_formula.<class> — reuse acceptable_lm(model) for formula-based models; otherwise implement the checks that reject formula features you cannot translate (unsupported contrasts, inline functions).

Minimal linear example

# Predict ---------------------------------------
#' @export
tidypredict_fit.myclass <- function(model) {
  parsedmodel <- parse_model(model)
  build_fit_formula(parsedmodel)
}

# Parse model --------------------------------------
#' @export
parse_model.myclass <- function(model) parse_model_lm(model)

#' @export
acceptable_formula.myclass <- function(model) acceptable_lm(model)

If parse_model_lm() does not already reach your class, add your class where the existing dispatch branches (see the glm branch at R/model-lm.R:107). Read parse_model_lm before assuming it works — check it pulls coefficients and terms the way your class stores them.

tidypredict_test

tidypredict_test (R/tidypredict_test.R) has a .default method that works for any model with a standard predict(). Only add a tidypredict_test.<class> method if the model needs matrix input or non-standard prediction (see the xgboost/lightgbm/catboost methods). Multiple-output models (multiple quantiles, multiclass) return a named list of expressions from tidypredict_fit; mirror the rqs / catboost-multiclass handling.

Register, then document

Rscript -e "devtools::document()"

Confirm the new S3method(...) lines appear in NAMESPACE.

Tests

Create tests/testthat/test-model-<name>.R, modeled on test-model-rq.R (a simple linear model) or test-model-rpart.R / test-model-lightgbm.R (trees/boosting). The suite has a strong shared shape; follow it.

The canonical first test. Nearly every model file opens with a test named "returns the right output" (boosting files call it "parse_model returns correct structure") asserting the full bundle:

• tidypredict_fit(model) returns "language" (expect_type(tf, "language")).
• parse_model(model) is a list of length 2 (general + one payload slot): expect_s3_class(pm, "list"), expect_equal(length(pm), 2).
• pm$general$model equals the recorded model string, and pm$general$version equals the parser version (linear-family models are v2, trees v3, glmnet v1).
• Snapshot the fit expression: expect_snapshot(rlang::expr_text(tf)).

Correctness vs predict(). Either snapshot tidypredict_test(model, df), or evaluate the fit expression directly (rlang::eval_tidy(tf, df) / dplyr::mutate(df, pred = !!tf)) and expect_equal to predict(model, ...). Include a factor/character predictor to exercise contrasts and categorical splits.

Cover the arguments that change the fitted model. Read the model function's signature and identify every argument that alters coefficients, tree structure, or the prediction, then fit with non-default values for each and assert both tidypredict_fit() works and tidypredict_test() matches predict(). Do not just test defaults. Examples: test-model-rq.R sweeps tau, method, weights and multiple quantiles; test-model-lightgbm.R has a "... predictions match native predict" test per objective; test-model-earth.R sweeps pmethod, degree, and the formula-vs-XY interface.

Conventions to match:

• skip_if_not_installed("<pkg>") in each test_that for any Suggests-only package.
• Numeric stability: coefficient snapshots are unstable across OS. Round model$coefficients before snapshotting (see lm/glm/earth), or use the round_print() helper in helper-printing.R (see glmnet). Ranger tests additionally skip_on_cran() and skip_on_os(c("windows", "linux")).
• Error paths: use expect_snapshot(..., error = TRUE) for unsupported configurations (bad objective/family, unsupported multi-output). A regression-only model must error on classification in both tidypredict_fit and parse_model (see rf/ranger).
• Save/reload roundtrip: add a "Model can be saved and re-loaded" test that writes parse_model() to YAML and back (yaml::write_yaml() -> yaml::read_yaml() -> as_parsed_model()) and asserts the fit is unchanged. Present in lm, glm, cubist, earth, glmnet, and the boosting files.
• Embed the issue number in the test description when a test guards a specific fix (e.g. "Gamma family works (#200)"), matching the existing tests.

Classification, multiclass, multi-output. Classification models compare against predict(model, type = "class"|"prob"|"response"). Multiclass and multi-output models return a named list of expressions from tidypredict_fit (see rqs quantiles, lightgbm/catboost multiclass): assert expect_named, that each element is a language object, that probabilities sum to 1, and note that tidypredict_test intentionally errors on multiclass.

Tree-specific tests (if applicable): a "produced case_when uses .default" test grepping the expr text for \.default, a stump test (no splits -> fit returns the bare mean), and the nested-case_when builder tests in test-tree-nested.R.

Other test files to touch:

• tests/testthat/test-acceptable.R: add a case for acceptable_formula.<class> if you implemented it (it should reject inline formula functions and accept valid syntax).
• tests/testthat/test-sql.R or inline: confirm SQL translation. Minimally tidypredict_sql(model, dbplyr::simulate_dbi()) returns class "sql". Tree/boosting models additionally round-trip through a real SQLite DB (guarded by skip_if_not_installed on DBI/RSQLite/dbplyr) comparing DB-computed predictions to predict().
• If your parser's format later changes, backwards compatibility is covered by frozen .rds fixtures in tests/testthat/backwards-compat/ with a generate-*.R script; a brand-new class needs no fixture at creation.

Rscript -e "devtools::test_active_file('R/model-<name>.R')"
Rscript -e "testthat::snapshot_review('model-<name>')"

Docs, NEWS, DESCRIPTION

• DESCRIPTION: add the modeling package to Suggests. If it is a headline model type, add it to the Description: supported-models list.
• NEWS.md: one bullet, alphabetical by function name, mention the function and issue/PR number, no line wrapping.
• Vignettes: a small addition (like rq) can join the nearest existing vignette (vignettes/lm.Rmd). A brand-new headline model type gets its own vignettes/<name>.Rmd, plus a _pkgdown.yml navbar entry under "Model list" and an entry in the articles: section (the index is explicit, so an unlisted vignette makes pkgdown error).
• vignettes/models.Rmd is the supported-model list of record. Add a row to the right table (Model, Fit with, parsnip spec and engine, link to the model's article). Leave the parsnip cell blank if no test covers that route.
• Counts: vignettes/models.Rmd and README.md both state an exact class/package count. Recompute it from NAMESPACE (parse_model() methods minus model_fit, plus the three tidypredict_fit()-only H2O classes) and update both, then Rscript -e "devtools::build_readme()".
• The README keeps only a one-line-per-category summary. Add the package to the matching bullet; do not reintroduce a full list there.
• If you add a new exported doc topic, add it to _pkgdown.yml and run Rscript -e "pkgdown::check_pkgdown()".

Orbital helpers

tidypredict does some of the heavy lifting for the orbital package, which builds its own predictions from tidypredict's parsing internals rather than from the final Tidy Eval formula. Many supported models export a small set of helper functions specifically for orbital.

Whether you need them depends on the family. Simple linear-family classes usually do not add their own helpers: R/model-rq.R has no # For {orbital} section at all, because orbital consumes it through the shared linear path (the .build_linear_pred extractor lives in R/model-glmnet.R). Trees/forests and glmnet-style models, on the other hand, each export dedicated helpers, and for those a new class is not complete without them. If your class reuses parse_model_lm() and build_fit_formula() verbatim like rq, you can likely skip this section; if it introduces new tree or coefficient structure, add helpers matching the closest existing model.

The convention (grep # For {orbital} to see every example):

• A # For {orbital} banner section in R/model-<name>.R.
• Helper names are dot-prefixed (.extract_xgb_trees, .rpart_tree_info_full, .extract_rf_classprob, .build_linear_pred, .extract_glmnet_multiclass). The dot keeps them out of tab-completion while still exported.
• Roxygen with both @keywords internal and @export (exported so orbital can call them, but kept out of the reference index).
• They expose the raw structure orbital needs, not a formula. Established shapes:
  • Linear models: a linear-predictor builder / coefficient extractor (see .build_linear_pred, .extract_glmnet_multiclass in R/model-glmnet.R).
  • Trees / forests: tree-info extractors and, for classification, class-probability extractors (.extract_rf_trees / .extract_rf_classprob, .rpart_tree_info_full / .extract_rpart_classprob, and the shared .build_nested_case_when_tree in R/tree-nested.R).

Match the shape used by the closest existing model so orbital can consume your class the same way.

Test each helper directly (see the # Tests for .extract_rf_classprob() and # Tests for .extract_rf_trees() blocks in test-model-rf.R) with this three-part pattern:

• Structure: expect_type(result, "list"), expect_length() to the tree/class count, expect_named() by class level for classprob/multiclass, and testthat::expect_all_true(vapply(result, is.language, logical(1))) that every element is a language object (as rf/ranger do).
• Correctness by reconstruction: rlang::eval_tidy each expression, then combine the way the model does (average trees for regression, sum votes / n_trees for probabilities, softmax for glmnet multiclass) and expect_equal to predict(model, type = "prob"|"response").
• Error guards via expect_snapshot(..., error = TRUE): wrong model class, wrong task (classprob on a regression model, trees on a classification model), and any structural precondition (e.g. ranger needs probability = TRUE). Also cover edge counts: binary classification and a single-tree forest.

parsnip engine

If the class is reachable through a parsnip engine, finish by exposing and testing that path with the add-model-engine skill (add a test-tidymodels.R test; model_fit delegation usually needs no new code).

Wrap up

air format .
Rscript -e "devtools::test()"
Rscript -e "devtools::document()"

Review that DESCRIPTION, NEWS.md, NAMESPACE, tests, and docs are consistent. To see the full file set a real model-adding change touches, find a recent one in the history and inspect it, e.g. git log --oneline --grep="[Aa]dd support" then git show --stat <sha>. If a supported-models tracking issue is open (historically tidymodels/tidypredict#232), note in the PR which checkbox to check.