Best Practices for Yardstick Source Development

Context: This guide is for source development - contributing to the yardstick package directly.

Key principle: ✅ You CAN use internal functions - you’re developing the package, so internals are available.

For extension development (creating new packages), see Best Practices (Extension).

Using Internal Functions in Yardstick

When to Use Internal Functions

✅ Use internal functions when:

  • Shared logic exists between multiple metrics

  • Complex calculations are already implemented

  • Consistency with existing metrics is needed

  • Avoiding code duplication

❌ Don’t use internal functions when:

  • Simple logic can be written inline

  • The internal function doesn’t quite fit your needs

  • It would make code less readable

Finding Existing Internal Functions

# List all objects in yardstick (including internals)
ls("package:yardstick", all.names = TRUE)

# Search for specific internal functions
apropos("yardstick_", where = TRUE)

# View internal function source
yardstick:::yardstick_mean

# Search in package directory
# grep -r "yardstick_" R/

Common Internal Helpers

yardstick_mean() - Weighted Mean

Use for calculating weighted or unweighted means:

# In your implementation function
mae_impl <- function(truth, estimate, case_weights = NULL) {
  errors <- abs(truth - estimate)

  # Use internal helper
  yardstick_mean(errors, case_weights = case_weights)
}

Why use it:

  • Handles case weights consistently

  • Converts hardhat weights automatically

  • Matches behavior of other metrics

finalize_estimator_internal() - Estimator Selection

Use for multiclass metrics with estimator variants:

# In your data frame method
accuracy.data.frame <- function(data, truth, estimate,
                                estimator = NULL, na_rm = TRUE,
                                case_weights = NULL, ...) {
  estimator <- finalize_estimator_internal(
    estimator,
    metric_class = "accuracy",
    call = rlang::caller_env()
  )

  # Rest of implementation
  # ...
}

What it does:

  • Auto-detects binary vs multiclass

  • Validates estimator parameter

  • Provides consistent error messages

yardstick_remove_missing() - NA Handling

Use for consistent NA removal:

mae_vec <- function(truth, estimate, na_rm = TRUE, case_weights = NULL, ...) {
  check_numeric_metric(truth, estimate, case_weights)

  if (na_rm) {
    result <- yardstick_remove_missing(truth, estimate, case_weights)
    truth <- result$truth
    estimate <- result$estimate
    case_weights <- result$case_weights
  } else if (yardstick_any_missing(truth, estimate, case_weights)) {
    return(NA_real_)
  }

  mae_impl(truth, estimate, case_weights)
}

yardstick_any_missing() - Check for NAs

if (yardstick_any_missing(truth, estimate, case_weights)) {
  return(NA_real_)
}

Validation Functions

# For numeric metrics
check_numeric_metric(truth, estimate, case_weights)

# For class metrics
check_class_metric(truth, estimate, case_weights)

# For probability metrics
check_prob_metric(truth, estimate, case_weights)

These provide consistent validation and error messages.

File Naming Conventions

Yardstick organizes code by metric type:

Source File Names

  • Numeric metrics: R/num-[name].R

    • Examples: num-mae.R, num-rmse.R, num-huber_loss.R

  • Class metrics: R/class-[name].R

    • Examples: class-accuracy.R, class-precision.R, class-recall.R

  • Probability metrics: R/prob-[name].R

    • Examples: prob-roc_auc.R, prob-mn_log_loss.R, prob-brier_class.R

  • Survival metrics: R/surv-[name].R

    • Examples: surv-concordance_survival.R, surv-brier_survival.R

  • Quantile metrics: R/quant-[name].R

    • Examples: quant-weighted_interval_score.R

Test File Names Match Source

  • R/num-mae.R → tests/testthat/test-num-mae.R

  • R/class-accuracy.R → tests/testthat/test-class-accuracy.R

Documentation Patterns

Yardstick uses templates extensively for consistent documentation.

Using @template

Templates are defined in man-roxygen/ directory:

#' Mean Absolute Error
#'
#' @family numeric metrics
#' @family accuracy metrics
#' @templateVar fn mae
#' @template return
#' @template event_first
#'
#' @inheritParams rmse
#'
#' @export
mae <- function(data, ...) {
  UseMethod("mae")
}

Common Templates

@template return - Standard return value documentation @template event_first - Event level documentation for class metrics @template multiclass - Multiclass metric documentation

Using @templateVar

Define template variables before using templates:

#' @templateVar fn mae
#' @templateVar metric_fn mae
#' @template return

Inheriting Parameters

Use @inheritParams to avoid duplicating parameter documentation:

#' @inheritParams rmse
#' @param delta The delta parameter for Huber loss

This inherits truth, estimate, na_rm, case_weights, etc. from rmse.

Code Style Specific to Yardstick

Function Organization

Each metric should have three functions:

# 1. Generic (always exported)
#' @export
mae <- function(data, ...) {
  UseMethod("mae")
}

# 2. Data frame method (exported)
#' @export
#' @rdname mae
mae.data.frame <- function(data, truth, estimate, na_rm = TRUE,
                           case_weights = NULL, ...) {
  numeric_metric_summarizer(
    name = "mae",
    fn = mae_vec,
    data = data,
    truth = !!enquo(truth),
    estimate = !!enquo(estimate),
    na_rm = na_rm,
    case_weights = !!enquo(case_weights)
  )
}

# 3. Vector method (exported)
#' @export
mae_vec <- function(truth, estimate, na_rm = TRUE, case_weights = NULL, ...) {
  check_numeric_metric(truth, estimate, case_weights)

  if (na_rm) {
    result <- yardstick_remove_missing(truth, estimate, case_weights)
    truth <- result$truth
    estimate <- result$estimate
    case_weights <- result$case_weights
  } else if (yardstick_any_missing(truth, estimate, case_weights)) {
    return(NA_real_)
  }

  mae_impl(truth, estimate, case_weights)
}

# 4. Implementation (NOT exported - internal)
mae_impl <- function(truth, estimate, case_weights = NULL) {
  errors <- abs(truth - estimate)
  yardstick_mean(errors, case_weights = case_weights)
}

Creating the Metric

Wrap the generic with new_*_metric():

mae <- new_numeric_metric(mae, direction = "minimize")

# For class metrics
accuracy <- new_class_metric(accuracy, direction = "maximize")

# For probability metrics
roc_auc <- new_prob_metric(roc_auc, direction = "maximize")

Use Metric Summarizers

Don’t implement the data frame method from scratch. Use the appropriate summarizer:

# For numeric metrics
numeric_metric_summarizer(name = "mae", fn = mae_vec, ...)

# For class metrics
class_metric_summarizer(name = "accuracy", fn = accuracy_vec, ...)

# For probability metrics
prob_metric_summarizer(name = "roc_auc", fn = roc_auc_vec, ...)

These handle:

  • NSE (non-standard evaluation)

  • Grouped data frames

  • Case weights

  • Error handling

Creating New Internal Helpers

When to Create Internal Helpers

Create a new internal helper when:

  • Logic is shared by 2+ metrics

  • Complex calculation that’s hard to understand inline

  • Abstraction improves code clarity

Naming Convention

Internal helpers are NOT exported and typically:

  • Start with yardstick_ for utility functions

  • Have descriptive names (e.g., yardstick_mean, yardstick_table)

  • Are documented with roxygen but use @keywords internal

Example Internal Helper

#' Calculate weighted mean
#'
#' Internal helper for calculating weighted or unweighted means.
#'
#' @param x Numeric vector
#' @param case_weights Optional case weights
#'
#' @return Numeric scalar
#' @keywords internal
#' @noRd
yardstick_mean <- function(x, case_weights = NULL) {
  if (is.null(case_weights)) {
    mean(x)
  } else {
    # Convert hardhat weights
    if (inherits(case_weights, c("hardhat_importance_weights",
                                 "hardhat_frequency_weights"))) {
      case_weights <- as.double(case_weights)
    }
    weighted.mean(x, w = case_weights)
  }
}

Don’t Export Internal Helpers

Internal helpers should:

  • Have @keywords internal

  • Use @noRd to skip documentation generation

  • NOT have @export

Error Messages

Use cli Functions

Yardstick uses cli for error messages:

if (invalid_input) {
  cli::cli_abort(
    "{.arg estimator} must be one of {.val binary}, {.val macro}, or {.val micro}, not {.val {estimator}}.",
    call = call
  )
}

Pass call for Better Error Context

accuracy.data.frame <- function(data, truth, estimate,
                                estimator = NULL, ...,
                                call = rlang::caller_env()) {
  # Use call in error messages
  if (is_bad) {
    cli::cli_abort("Error message", call = call)
  }
}

Consistent Error Message Style

Follow existing patterns in yardstick:

# Good
cli::cli_abort("{.arg truth} must be a factor, not {.cls {class(truth)}}.")

# Good
cli::cli_abort("Found {length(extra)} unexpected column{?s}: {.var {extra}}.")

# Avoid
stop("truth must be a factor")

Multiclass Metrics

Supporting Multiple Estimators

For multiclass metrics, support macro, micro, and macro_weighted:

accuracy.data.frame <- function(data, truth, estimate,
                                estimator = NULL, na_rm = TRUE,
                                case_weights = NULL, ...,
                                call = rlang::caller_env()) {
  # Finalize estimator
  estimator <- finalize_estimator_internal(
    estimator,
    metric_class = "accuracy",
    call = call
  )

  class_metric_summarizer(
    name = "accuracy",
    fn = accuracy_vec,
    data = data,
    truth = !!enquo(truth),
    estimate = !!enquo(estimate),
    estimator = estimator,
    na_rm = na_rm,
    case_weights = !!enquo(case_weights),
    call = call
  )
}

Implement Binary and Estimator Variants

# Binary implementation
accuracy_binary <- function(truth, estimate, case_weights) {
  # Implementation for 2-class case
}

# Multiclass implementation
accuracy_estimator_impl <- function(truth, estimate, estimator, case_weights) {
  if (estimator == "macro") {
    # Per-class average
  } else if (estimator == "micro") {
    # Pool all observations
  } else if (estimator == "macro_weighted") {
    # Weighted by prevalence
  }
}

Working with Confusion Matrices

For class metrics based on confusion matrices:

# Create confusion matrix with weights
xtab <- yardstick_table(truth, estimate, case_weights)

# Extract values (for binary)
tp <- xtab[2, 2]  # True positives
tn <- xtab[1, 1]  # True negatives
fp <- xtab[1, 2]  # False positives
fn <- xtab[2, 1]  # False negatives

# Calculate metric
(tp + tn) / (tp + tn + fp + fn)

Consistency with Existing Metrics

Study Similar Metrics

Before implementing, study similar existing metrics:

  • For numeric metrics: Look at R/num-mae.R, R/num-rmse.R

  • For class metrics: Look at R/class-accuracy.R, R/class-precision.R

  • For probability metrics: Look at R/prob-roc_auc.R

Match Parameter Names and Order

Keep parameter names and order consistent:

# Standard order for numeric metrics
metric_vec <- function(truth, estimate, na_rm = TRUE, case_weights = NULL, ...)

# Standard order for class metrics
metric_vec <- function(truth, estimate, estimator = NULL, na_rm = TRUE,
                      case_weights = NULL, event_level = "first", ...)

# Standard order for probability metrics
metric_vec <- function(truth, estimate, estimator = NULL, na_rm = TRUE,
                      case_weights = NULL, event_level = "first", ...)

Use Standard Return Format

All metrics return a tibble with:

  • .metric: Character, metric name

  • .estimator: Character, estimator type

  • .estimate: Numeric, metric value

tibble::tibble(
  .metric = "mae",
  .estimator = "standard",
  .estimate = 0.5
)

Performance Considerations

Vectorization

Prefer vectorized operations:

# Good
errors <- abs(truth - estimate)
mean(errors)

# Avoid
sum(abs(truth - estimate)) / length(truth)

Avoid Unnecessary Copies

# Good - modify in place
if (na_rm) {
  result <- yardstick_remove_missing(truth, estimate, case_weights)
  truth <- result$truth
  estimate <- result$estimate
  case_weights <- result$case_weights
}

# Avoid - creates unnecessary copies
if (na_rm) {
  indices <- !is.na(truth) & !is.na(estimate)
  truth <- truth[indices]
  estimate <- estimate[indices]
  if (!is.null(case_weights)) {
    case_weights <- case_weights[indices]
  }
}

Next Steps