Control Flow, Nil Safety และ Exceptions

user&.dig(:profile, :email)

user&.dig(:profile, :email)

return 0 unless user

class MissingConfigError < StandardError; end

raise MissingConfigError, "missing config: #{key}"

# EXAMPLE CODE
# Topic: topic_05_control_nil_exceptions
#
# Purpose:
# - This file demonstrates reference implementation for the concept.
# - It should pass tests from the beginning.
# - Read and understand it before solving exercises.

class ProfileEmail
  def extract(user)
    user&.dig(:profile, :email)
  end
end

# STUDENT TASK (BASIC)
# Topic: topic_05_control_nil_exceptions
#
# What to do:
# - Implement or improve the class/methods in this file.
# - Read tests in tests/topic_05_control_nil_exceptions_spec.rb under the "basic exercise" examples.
# - Make tests pass without breaking the example/advanced sections.
#
# Expected outcome:
# - You can run this topic tests and see all examples green after implementation.

class DiscountPolicy
  def discount_for(user)
    return 0 unless user
    return 20 if user[:vip]

    5
  end
end

# STUDENT TASK (ADVANCED)
# Topic: topic_05_control_nil_exceptions
#
# Academic purpose:
# - Distinguish optional data access from mandatory configuration lookup.
# - Practice encoding domain meaning in an exception class rather than raising generic errors.
#
# Real-world use case:
# - Application config often includes API keys, region names, feature flags, or service endpoints.
# - If one of those settings is missing, the system should fail loudly and explain why.
# - This is a useful counterexample to the earlier nil-safe lookup behavior.
#
# Why Ruby is beautiful here:
# - A custom exception is easy to define and gives the failure a clear name.
# - The method can stay short while still communicating a strong failure policy.
# - Guard-style control flow keeps the successful path obvious.
#
# What to do:
# - Complete the challenge behavior requested by the guide.
# - Be able to explain why this case should not silently return `nil`.
# - Use tests in tests/topic_05_control_nil_exceptions_spec.rb under the "advanced exercise" examples.
#
# Expected outcome:
# - Advanced tests pass and you can justify the fail-fast design.

class MissingConfigError < StandardError; end

class ConfigFetcher
  def fetch!(config, key)
    return config[key] if config.key?(key)

    raise MissingConfigError, "missing config: #{key}"
  end
end

# ANSWER KEY (BASIC)
# Topic: topic_05_control_nil_exceptions
#
# Solution idea:
# - Use guard clauses to separate cases clearly.
# - No user means no discount.
# - VIP users get the larger discount.
# - Everyone else gets the standard fallback.

class DiscountPolicy
  def discount_for(user)
    return 0 unless user
    return 20 if user[:vip]

    5
  end
end

# ANSWER KEY (ADVANCED)
# Topic: topic_05_control_nil_exceptions
#
# Solution idea:
# - Missing required configuration is treated as an error, not a normal nil case.
# - A custom exception gives the failure a domain-specific meaning.

class MissingConfigError < StandardError; end

class ConfigFetcher
  def fetch!(config, key)
    return config[key] if config.key?(key)

    raise MissingConfigError, "missing config: #{key}"
  end
end

# This spec is your learning companion for topic_05_control_nil_exceptions.
#
# How to use this file:
# 1) Run tests and observe failures/successes.
# 2) Keep EXAMPLE specs green from the beginning.
# 3) Implement BASIC exercise until BASIC specs pass.
# 4) Implement ADVANCED exercise and pass edge cases.
#
# Expected final result:
# - All examples in this file pass.
# - You understand both the concept and the implementation tradeoffs.

require_relative "../example"
require_relative "../basic_exercise"
require_relative "../adv_exercise"

RSpec.describe "topic_05_control_nil_exceptions" do
  describe "EXAMPLE purpose: understand the reference implementation" do
      it "extracts profile emails safely" do
        e = ProfileEmail.new
        expect(e.extract({ profile: { email: "a@b.com" } })).to eq("a@b.com")
        expect(e.extract(nil)).to eq(nil)
      end
  end

  describe "BASIC EXERCISE purpose: implement the comparable task" do
      it "applies discount rules" do
        d = DiscountPolicy.new
        expect(d.discount_for(nil)).to eq(0)
        expect(d.discount_for(vip: true)).to eq(20)
        expect(d.discount_for(vip: false)).to eq(5)
      end
  end

  describe "ADVANCED EXERCISE purpose: solve challenge and edge cases" do
      it "raises custom error for missing config" do
        f = ConfigFetcher.new
        expect(f.fetch!({ timeout: 30 }, :timeout)).to eq(30)
        expect { f.fetch!({}, :timeout) }.to raise_error(MissingConfigError, /timeout/)
      end
  end
end

#!/usr/bin/env bash
set -euo pipefail

SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
ROOT_DIR="$(cd "${SCRIPT_DIR}/.." && pwd)"

exec "${ROOT_DIR}/run_tests.sh" "$(basename "${SCRIPT_DIR}")"