模式匹配 ↑

模式匹配是一种允许对结构化值进行深度匹配的功能：检查结构并将匹配的部分绑定到局部变量。

Ruby 中的模式匹配使用 case/in 表达式实现

case <expression>
in <pattern1>
  ...
in <pattern2>
  ...
in <pattern3>
  ...
else
  ...
end

（请注意，in 和 when 分支不能在一个 case 表达式中混合使用。）

或者使用 => 运算符和 in 运算符，它们可以在独立的表达式中使用

<expression> => <pattern>

<expression> in <pattern>

case/in 表达式是穷尽的：如果表达式的值与 case 表达式的任何分支都不匹配（并且缺少 else 分支），则会引发 NoMatchingPatternError。

因此，case 表达式可以用于条件匹配和解包

config = {db: {user: 'admin', password: 'abc123'}}

case config
in db: {user:} # matches subhash and puts matched value in variable user
  puts "Connect with user '#{user}'"
in connection: {username: }
  puts "Connect with user '#{username}'"
else
  puts "Unrecognized structure of config"
end
# Prints: "Connect with user 'admin'"

而当预先知道预期的数据结构时，=> 运算符最有用，仅用于解包其中的一部分

config = {db: {user: 'admin', password: 'abc123'}}

config => {db: {user:}} # will raise if the config's structure is unexpected

puts "Connect with user '#{user}'"
# Prints: "Connect with user 'admin'"

<表达式> in <模式> 与 case <表达式>; in <模式>; true; else false; end 相同。当您只想知道是否已匹配模式时，可以使用它

users = [{name: "Alice", age: 12}, {name: "Bob", age: 23}]
users.any? {|user| user in {name: /B/, age: 20..} } #=> true

请参阅下文，了解更多示例和语法说明。

模式 ↑

模式可以是

任何模式都可以嵌套在指定 <子模式> 的数组/查找/哈希模式中。

Array 模式和查找模式匹配数组，或响应 deconstruct 的对象（请参阅下面关于后者的内容）。Hash 模式匹配哈希，或响应 deconstruct_keys 的对象（请参阅下面关于后者的内容）。请注意，哈希模式仅支持符号键。

数组模式和哈希模式行为之间的一个重要区别是，数组仅匹配整个数组

case [1, 2, 3]
in [Integer, Integer]
  "matched"
else
  "not matched"
end
#=> "not matched"

而即使存在除了指定部分之外的其他键，哈希也会匹配

case {a: 1, b: 2, c: 3}
in {a: Integer}
  "matched"
else
  "not matched"
end
#=> "matched"

{} 是此规则的唯一例外。 仅当给定空哈希时，它才匹配

case {a: 1, b: 2, c: 3}
in {}
  "matched"
else
  "not matched"
end
#=> "not matched"

case {}
in {}
  "matched"
else
  "not matched"
end
#=> "matched"

还可以通过 **nil 指定匹配的哈希中不应存在除模式显式指定的键之外的其他键

case {a: 1, b: 2}
in {a: Integer, **nil} # this will not match the pattern having keys other than a:
  "matched a part"
in {a: Integer, b: Integer, **nil}
  "matched a whole"
else
  "not matched"
end
#=> "matched a whole"

数组和哈希模式都支持“rest”规范

case [1, 2, 3]
in [Integer, *]
  "matched"
else
  "not matched"
end
#=> "matched"

case {a: 1, b: 2, c: 3}
in {a: Integer, **}
  "matched"
else
  "not matched"
end
#=> "matched"

两种模式周围的括号都可以省略

 case [1, 2]
 in Integer, Integer
   "matched"
 else
   "not matched"
 end
 #=> "matched"

 case {a: 1, b: 2, c: 3}
 in a: Integer
   "matched"
 else
   "not matched"
 end
 #=> "matched"

[1, 2] => a, b
[1, 2] in a, b

{a: 1, b: 2, c: 3} => a:
{a: 1, b: 2, c: 3} in a:

Find 模式类似于数组模式，但它可用于检查给定对象是否具有与该模式匹配的任何元素

case ["a", 1, "b", "c", 2]
in [*, String, String, *]
  "matched"
else
  "not matched"
end

变量绑定 ↑

除了深度结构检查之外，模式匹配的一个非常重要的功能是将匹配的部分绑定到局部变量。绑定的基本形式是在匹配的（子）模式之后指定 => variable_name（可能会发现这类似于在 rescue ExceptionClass => var 子句中将异常存储在局部变量中）

case [1, 2]
in Integer => a, Integer
  "matched: #{a}"
else
  "not matched"
end
#=> "matched: 1"

case {a: 1, b: 2, c: 3}
in a: Integer => m
  "matched: #{m}"
else
  "not matched"
end
#=> "matched: 1"

如果不需要额外的检查，仅需要将数据的一部分绑定到变量，则可以使用更简单的形式

case [1, 2]
in a, Integer
  "matched: #{a}"
else
  "not matched"
end
#=> "matched: 1"

case {a: 1, b: 2, c: 3}
in a: m
  "matched: #{m}"
else
  "not matched"
end
#=> "matched: 1"

对于哈希模式，甚至存在更简单的形式：仅键规范（没有任何子模式）也将局部变量与键的名称绑定

case {a: 1, b: 2, c: 3}
in a:
  "matched: #{a}"
else
  "not matched"
end
#=> "matched: 1"

Binding 也适用于嵌套模式

case {name: 'John', friends: [{name: 'Jane'}, {name: 'Rajesh'}]}
in name:, friends: [{name: first_friend}, *]
  "matched: #{first_friend}"
else
  "not matched"
end
#=> "matched: Jane"

模式的“rest”部分也可以绑定到变量

case [1, 2, 3]
in a, *rest
  "matched: #{a}, #{rest}"
else
  "not matched"
end
#=> "matched: 1, [2, 3]"

case {a: 1, b: 2, c: 3}
in a:, **rest
  "matched: #{a}, #{rest}"
else
  "not matched"
end
#=> "matched: 1, {b: 2, c: 3}"

Binding 目前不适用于与 | 连接的可选模式

case {a: 1, b: 2}
in {a: } | Array
  "matched: #{a}"
else
  "not matched"
end
# SyntaxError (illegal variable in alternative pattern (a))

以 _ 开头的变量是此规则的唯一例外

case {a: 1, b: 2}
in {a: _, b: _foo} | Array
  "matched: #{_}, #{_foo}"
else
  "not matched"
end
# => "matched: 1, 2"

但是，不建议重用绑定的值，因为此模式的目标是表示已丢弃的值。

变量固定 ↑

由于变量绑定功能，现有的局部变量不能直接用作子模式

expectation = 18

case [1, 2]
in expectation, *rest
  "matched. expectation was: #{expectation}"
else
  "not matched. expectation was: #{expectation}"
end
# expected: "not matched. expectation was: 18"
# real: "matched. expectation was: 1" -- local variable just rewritten

对于这种情况，可以使用固定运算符 ^，告诉 Ruby “仅将此值用作模式的一部分”

expectation = 18
case [1, 2]
in ^expectation, *rest
  "matched. expectation was: #{expectation}"
else
  "not matched. expectation was: #{expectation}"
end
#=> "not matched. expectation was: 18"

变量固定的一个重要用途是指定相同的值应在模式中多次出现

jane = {school: 'high', schools: [{id: 1, level: 'middle'}, {id: 2, level: 'high'}]}
john = {school: 'high', schools: [{id: 1, level: 'middle'}]}

case jane
in school:, schools: [*, {id:, level: ^school}] # select the last school, level should match
  "matched. school: #{id}"
else
  "not matched"
end
#=> "matched. school: 2"

case john # the specified school level is "high", but last school does not match
in school:, schools: [*, {id:, level: ^school}]
  "matched. school: #{id}"
else
  "not matched"
end
#=> "not matched"

除了固定局部变量之外，还可以固定实例变量、全局变量和类变量

$gvar = 1
class A
  @ivar = 2
  @@cvar = 3
  case [1, 2, 3]
  in ^$gvar, ^@ivar, ^@@cvar
    "matched"
  else
    "not matched"
  end
  #=> "matched"
end

您还可以使用括号固定任意表达式的结果

a = 1
b = 2
case 3
in ^(a + b)
  "matched"
else
  "not matched"
end
#=> "matched"

匹配非原始对象：deconstruct 和 deconstruct_keys ↑

如上所述，除了字面数组和哈希之外，数组、查找和哈希模式将尝试匹配任何实现 deconstruct（对于数组/查找模式）或 deconstruct_keys（对于哈希模式）的对象。

class Point
  def initialize(x, y)
    @x, @y = x, y
  end

  def deconstruct
    puts "deconstruct called"
    [@x, @y]
  end

  def deconstruct_keys(keys)
    puts "deconstruct_keys called with #{keys.inspect}"
    {x: @x, y: @y}
  end
end

case Point.new(1, -2)
in px, Integer  # sub-patterns and variable binding works
  "matched: #{px}"
else
  "not matched"
end
# prints "deconstruct called"
"matched: 1"

case Point.new(1, -2)
in x: 0.. => px
  "matched: #{px}"
else
  "not matched"
end
# prints: deconstruct_keys called with [:x]
#=> "matched: 1"

keys 传递给 deconstruct_keys 以便在匹配的类中提供优化空间：如果计算完整的哈希表示很昂贵，则可以仅计算必要的子哈希。当使用 **rest 模式时，nil 作为 keys 值传递

case Point.new(1, -2)
in x: 0.. => px, **rest
  "matched: #{px}"
else
  "not matched"
end
# prints: deconstruct_keys called with nil
#=> "matched: 1"

此外，在匹配自定义类时，可以将预期的类指定为模式的一部分，并通过 === 进行检查

class SuperPoint < Point
end

case Point.new(1, -2)
in SuperPoint(x: 0.. => px)
  "matched: #{px}"
else
  "not matched"
end
#=> "not matched"

case SuperPoint.new(1, -2)
in SuperPoint[x: 0.. => px] # [] or () parentheses are allowed
  "matched: #{px}"
else
  "not matched"
end
#=> "matched: 1"

这些核心和库类实现了析构

守卫子句 ↑

当 case/in 表达式中的模式匹配时，可以使用 if 附加额外的条件（守卫子句）。此条件可以使用绑定的变量

case [1, 2]
in a, b if b == a*2
  "matched"
else
  "not matched"
end
#=> "matched"

case [1, 1]
in a, b if b == a*2
  "matched"
else
  "not matched"
end
#=> "not matched"

unless 也有效

case [1, 1]
in a, b unless b == a*2
  "matched"
else
  "not matched"
end
#=> "matched"

请注意，=> 和 in 运算符不能有守卫子句。以下示例将解析为带有修饰符 if 的独立表达式。

[1, 2] in a, b if b == a*2

附录 A. 模式语法 ↑

近似语法是

pattern: value_pattern
       | variable_pattern
       | alternative_pattern
       | as_pattern
       | array_pattern
       | find_pattern
       | hash_pattern

value_pattern: literal
             | Constant
             | ^local_variable
             | ^instance_variable
             | ^class_variable
             | ^global_variable
             | ^(expression)

variable_pattern: variable

alternative_pattern: pattern | pattern | ...

as_pattern: pattern => variable

array_pattern: [pattern, ..., *variable]
             | Constant(pattern, ..., *variable)
             | Constant[pattern, ..., *variable]

find_pattern: [*variable, pattern, ..., *variable]
            | Constant(*variable, pattern, ..., *variable)
            | Constant[*variable, pattern, ..., *variable]

hash_pattern: {key: pattern, key:, ..., **variable}
            | Constant(key: pattern, key:, ..., **variable)
            | Constant[key: pattern, key:, ..., **variable]

附录 B. 一些未定义行为示例 ↑

为了在未来留出优化的空间，规范包含一些未定义的行为。

在未匹配的模式中使用变量

case [0, 1]
in [a, 2]
  "not matched"
in b
  "matched"
in c
  "not matched"
end
a #=> undefined
c #=> undefined

deconstruct、deconstruct_keys 方法调用的次数

$i = 0
ary = [0]
def ary.deconstruct
  $i += 1
  self
end
case ary
in [0, 1]
  "not matched"
in [0]
  "matched"
end
$i #=> undefined