Writing Affordances

Writing Affordances

Extract PORO wrappers that group 3+ operations around a noun/concept, keeping parent model focused.

Announce: "I'm using the writing-affordances skill to evaluate this for affordance extraction."

Detection Triggers

• 3+ methods share prefix: entropy_calculate, entropy_remind, entropy_cleanup
• Methods coordinate 2+ associations (assignments + watches + events)
• Fat model with mixed conceptual domains
• Related methods but connection non-obvious

Don't use for: Single trait (use concern) | Single association (use extension) | 1-2 methods (use plain method)

Decision Heuristics

Ask yourself:

1. "Can I pass this around?" - Would notify(description) be clearer than notify(event)?
2. "Do I call it or does Rails?" - Explicit calls = Affordance | Callbacks/validations = Concern
3. "Prefix cluster?" - entropy_* methods = Extract entropy affordance (prefix = noun)
4. "Call site clarity?" - card.entropy.auto_clean_at vs card.entropy_auto_clean_at
5. "Need infrastructure + operations?" - Concern provides associations/callbacks, affordance provides API

See @detecting.md for complete decision framework.

Pattern Comparison

Critical Constraints

Violating these = wrong pattern:

• 3+ methods minimum - Don't create single-method affordances
• PORO only - No ActiveRecord inheritance
• Parent is ONLY dependency - Card::Entropy.new(card), not new(card, user)
• Don't memoize parameterized - entropy(as_of: date) should NOT use ||=

Quick Pattern

# Before: Methods scattered in model or prefix smell
card.entropy_auto_clean_at
card.entropy_days_before_reminder
card.entropy_reminder_sent?

# After: Affordance
card.entropy.auto_clean_at
card.entropy.days_before_reminder
card.entropy.reminder_sent?

# Entry point in concern
module Card::Entropic
  def entropy
    Card::Entropy.for(self)  # Factory pattern
  end

  def entropic?
    entropy.present?
  end
end

# PORO affordance class
class Card::Entropy
  attr_reader :card, :auto_clean_period

  class << self
    def for(card)
      return unless card.last_active_at
      new(card, card.auto_postpone_period)
    end
  end

  def initialize(card, auto_clean_period)
    @card = card
    @auto_clean_period = auto_clean_period
  end

  def auto_clean_at
    card.last_active_at + auto_clean_period
  end

  def days_before_reminder
    (auto_clean_period * 0.25).seconds.in_days.round
  end
end

Verification

Non-obvious checks before completion:

• 3+ related methods (not single-method wrapper)
• Parameterized affordances NOT memoized
• Class documentation with usage examples
• Tests: both isolated and integration

Sub-documents

• @detecting.md - Full decision framework with clarifying questions
• @implementing.md - File organization, naming, concerns + affordances patterns