Elixir-Pattern matching

模式匹配可以用来解构复杂的数据结构：

= 等号操作符

• 解构时要求 = 左侧与右侧的数据类型相同
• 解构tuple,要求 = 左右的tuple中的元素个数相同

  # tuple 解构的常用场景
  iex(21)> {:ok, result} = {:ok, [1, 2, 3]}
  {:ok, [1, 2, 3]}
  iex(22)> 
  
  iex(10)> {a, b, c} = {:a, 1, "hello"}
  {:a, 1, "hello"}
  iex(11)> a 
  :a
  iex(12)> b
  1
  iex(13)> c
  "hello"
  iex(14)>
  iex(15)> {a, b, c, d} = {:a, 1, "hello"}
  ** (MatchError) no match of right hand side value: {:a, 1, "hello"}
  
  iex(15)> {a, b, c, d} = {:a, 1, "hello", :d, :e}
  ** (MatchError) no match of right hand side value: {:a, 1, "hello", :d, :e}
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• 解构list，与tuple相同，= 左右两侧的数据长度相同

  iex(1)> [a, b, c] = [:a, 2, "hello"]
  [:a, 2, "hello"]
  iex(2)> a
  :a
  iex(3)> b
  2
  iex(4)> c
  "hello"
  iex(5)> 
  # 匹配list的head和tail
  iex(5)> [head | tail] = [1, 2, 3, 4]
  [1, 2, 3, 4]
  iex(6)> head
  1
  iex(7)> tail
  [2, 3, 4]
  iex(8)> 
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^ pin操作符

使用了 ^ pin 操作符，在进行模式匹配时，只是进行比较，不会在给此变量重新付值

iex(1)> x = 1
1
iex(2)> x = 2
2
iex(3)> ^x = 3
** (MatchError) no match of right hand side value: 3

iex(3)> ^x = 2
2
iex(4)> 

iex(8)> {^x, a} = {2, "a"}
{2, "a"}
iex(9)> a
"a"
iex(10)> x
2
iex(11)> {^x, a} = {"x", "a"}
** (MatchError) no match of right hand side value: {"x", "a"}
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注意⚠️：模式匹配时，= 等式左侧不能是函数。

iex(2)> length([1, 2, 3]) = 3
** (CompileError) iex:2: cannot invoke remote function :erlang.length/1 inside a match
iex(2)> 3 = length([1, 2, 3])
3
iex(3)>
iex(12)> sum = fn a, b -> a + b end
#Function<41.3316493/2 in :erl_eval.expr/6>
iex(13)> sum.(1, 2) = 3
** (CompileError) iex:13: invalid pattern in match, anonymous call is not allowed in matches
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