Ruby 實用技巧集錦：區塊序列調用、陣列解構、Hash#fetch、預設值 Hash、sort_by、present? 等進階語法

1. 區塊序列調用與多重賦值

def touch_down
  yield [3, 7]  # 傳遞陣列給區塊
  puts "touchdown!"
end

# 區塊參數用括號接收陣列元素
touch_down do |(first_down, second_down)|
  puts "#{first_down} yards on the run"
  puts "#{second_down} yards passed"
end
# 輸出:
# 3 yards on the run
# 7 yards passed
# touchdown!

2. 陣列解構：首元素與剩餘元素

args = [1, 2, 3]
first, *rest = args
first  # => 1
rest   # => [2, 3]

# 巢狀解構
(a, b), c = [[1,2], 3]
a  # => 1
b  # => 2
c  # => 3

# 忽略不需要的值
_, second, *tail = [1,2,3,4]
second  # => 2
tail    # => [3,4]

3. Hash#fetch：安全取值與預設值

items = { apples: 2, oranges: 3 }

items.fetch(:apples)           # => 2
items.fetch(:bananas)          # => KeyError (拋出異常)
items.fetch(:bananas, 0)       # => 0 (預設值)
items.fetch(:bananas) { |k| "No #{k}!" }  # => "No bananas!" (區塊形式)

# vs []
items[:bananas]      # => nil (靜默失敗)
items.fetch(:bananas) # => KeyError (明確失敗)

# 實務：必要設定檢查
config = { host: 'localhost' }
port = config.fetch(:port) { raise "Missing required config: port" }

4. 預設值 Hash：Hash.new { |h,k| ... }

# 區塊形式：鍵不存在時自動執行區塊並儲存結果
smash = Hash.new { |hash, key| hash[key] = "a #{key} just got SMASHED!" }

smash[:plum] = "cannot smash."
smash[:plum]          # => "cannot smash." (已存在，不執行區塊)
smash[:watermelon]    # => "a watermelon just got SMASHED!" (執行區塊並儲存)
smash[:watermelon]    # => "a watermelon just got SMASHED!" (直接回傳快取)

# 費波納契數列快取（經典範例）
fib = Hash.new { |h, n| h[n] = n < 2 ? n : h[n-1] + h[n-2] }
fib[10]  # => 55
fib[20]  # => 6765 (極快，動態規劃)

5. Array#sort_by：依區塊回傳值排序

cars = %w[beetle volt camry]

# 依長度排序
cars.sort_by { |car| car.size }  # => ["volt", "camry", "beetle"]

# 多重排序鍵：先長度，再字母序
cars.sort_by { |car| [car.size, car] }

# 結合 Symbol#to_proc (Ruby 1.9+)
cars.sort_by(&:size)
cars.sort_by(&:length)

# 降序
cars.sort_by { |car| -car.size }
cars.sort_by(&:size).reverse

6. String#present? / blank? (ActiveSupport)

# 需 require 'active_support/all' 或在 Rails 專案中

"brain".present?  # => true
"".present?       # => false
"   ".present?    # => false (空白也視為 blank)
nil.present?      # => false

# 相反
"brain".blank?  # => false
"".blank?       # => true

# 物件通用
[].blank?       # => true
{}.blank?       # => true
0.blank?        # => false (數字 0 不為 blank)
false.blank?    # => true

7. 相關查詢方法比較

8. 更多實用技巧

tap：鏈式操作中檢查中間值

User.create(name: "Alice").tap do |user|
  puts "Created user: #{user.id}"
  user.update!(last_login: Time.current)
end
# 回傳 user 物件，可繼續鏈式

then / yield_self：管線操作

# Ruby 2.5+ yield_self, 2.6+ then
"hello".then { |s| s.upcase }.then { |s| s + "!" }  # => "HELLO!"

# 實務：條件轉換
value = get_value
result = value.then { |v| v * 2 if v > 10 }

dig：深層 Hash/陣列安全存取

data = { user: { profile: { settings: { theme: 'dark' } } } }
data.dig(:user, :profile, :settings, :theme)  # => "dark"
data.dig(:user, :profile, :missing, :key)     # => nil (不拋出錯誤)

# 陣列也支援
matrix = [[1,2], [3,4]]
matrix.dig(1, 0)  # => 3

to_h：枚舉物件轉 Hash

[[:a, 1], [:b, 2]].to_h  # => {a: 1, b: 2}
{a: 1, b: 2}.to_h        # => {a: 1, b: 2} (Hash 回傳自己)

# 結合 map
%i[a b c].map { |k| [k, k.to_s.upcase] }.to_h  # => {:a=>"A", :b=>"B", :c=>"C"}

values_at：一次取多個鍵/索引

{a: 1, b: 2, c: 3}.values_at(:a, :c)  # => [1, 3]
%w[a b c d e].values_at(0, 2, 4)      # => ["a", "c", "e"]

fetch_values：取多個鍵，缺失拋出異常

{a: 1, b: 2}.fetch_values(:a, :b)  # => [1, 2]
{a: 1}.fetch_values(:a, :c)        # => KeyError

& 運算子：Symbol#to_proc 簡寫

%w[apple banana].map(&:upcase)      # => ["APPLE", "BANANA"]
[1,2,3].select(&:odd?)              # => [1, 3]
users.map(&:name)                   # 呼叫每個物件的 name 方法

9. 效能小撇步

• Hash.new { |h,k| ... } 避免重複計算（Memoization）
• sort_by 比 sort { |a,b| ... } 快（Schwartzian transform）
• &:method 比 { |x| x.method } 略快且簡潔
• fetch 比 [] + || 更明確語意

總結

這些技巧展現了 Ruby 表達力的核心：區塊語法、解構賦值、安全存取、函數式鏈式操作。掌握 Hash#fetch、預設值 Hash、sort_by、dig、tap/then、&:symbol 等慣用寫法，能寫出更安全、簡潔、高效的 Ruby 程式碼。ActiveSupport 的 blank?/present? 是 Rails 專案標準用法，純 Ruby 專案可自行實作或引入 active_support/core_ext。