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195 lines
3.8 KiB
Markdown
195 lines
3.8 KiB
Markdown
# 模式匹配
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[](https://gezf7g7pd5.execute-api.ap-northeast-1.amazonaws.com/default/source_up_to_date?owner=erg-lang&repos=erg&ref=main&path=doc/EN/syntax/27_pattern_matching.md&commit_hash=e959b3e54bfa8cee4929743b0193a129e7525c61)
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## Erg 中可用的模式
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### 變量模式
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```python
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# 基本任務
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i = 1
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# 有類型
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i: Int = 1
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# 匿名類型
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i: {1, 2, 3} = 2
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# 功能
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fn x = x + 1
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# 等于
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fn x: Add(Int) = x + 1
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# (匿名)函數
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fn = x -> x + 1
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fn: Int -> Int = x -> x + 1
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# 高階類型
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a: [Int; 4] = [0, 1, 2, 3]
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# or
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a: Array Int, 4 = [0, 1, 2, 3]
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```
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### 文字字面量
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```python
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# 如果在編譯時無法確定`i`為 1，則引發TypeError
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# 省略 `_: {1} = i`
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1 = i
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# 簡單的模式匹配
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match x:
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1 -> "1"
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2 -> "2"
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_ -> "other"
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# 斐波那契函數
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fib0 = 0
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fib1 = 1
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fibn: Nat = fibn-1 + fibn-2
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```
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### 常量模式
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```python
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cond=False
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match! cond:
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True => print! "cond is True"
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_ => print! "cond is False"
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PI = 3.141592653589793
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E = 2.718281828459045
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num = PI
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name = match num:
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PI -> "pi"
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E -> "e"
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_ -> "unnamed"
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```
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### 篩子圖案
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```python,checker_ignore
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# 這兩個是一樣的
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Array(T, N: {N | N >= 3})
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Array(T, N | N >= 3)
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f M, N | M >= 0, N >= 1 = ...
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f(1, 0) # 類型錯誤: N(第二個參數)必須為 1 或更多
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```
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### 丟棄(通配符)模式
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```python
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_ = 1
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_: Int = 1
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zero_ = 0
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right(_, r) = r
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```
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### 可變長度模式
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它與稍后描述的元組/數組/記錄模式結合使用
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```python
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[i, *j] = [1, 2, 3, 4]
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assert j == [2, 3, 4]
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first|T|(fst: T, *rest: T) = fst
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assert first(1, 2, 3) == 1
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```
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### 元組模式
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```python
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(i, j) = (1, 2)
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((k, l), _) = ((1, 2), (3, 4))
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# 如果不嵌套，() 可以省略(1, 2 被視為(1, 2))
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m, n = 1, 2
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f(x, y) = ...
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```
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### 數組模式
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```python
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[i, j] = [1, 2]
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[[k, l], _] = [[1, 2], [3, 4]]
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length [] = 0
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length [_, *rest] = 1 + length rest
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```
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#### record 模式
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```python
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record = {i = 1; j = 2; k = 3}
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{j;} = record # i, k 將被釋放
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{sin; cos; tan;} = import "math"
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{*} = import "math" # import all
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person = {name = "John Smith"; age = 20}
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age = match person:
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{name = "Alice"; _} -> 7
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{_; age} -> age
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f {x: Int; y: Int} = ...
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```
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### 數據類模式
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```python
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Point = Inherit {x = Int; y = Int}
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p = Point::{x = 1; y = 2}
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Point::{x; y} = p
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Nil T = Class Impl := Phantom T
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Cons T = Inherit {head = T; rest = List T}
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List T = Enum Nil(T), Cons(T)
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List T.
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first self =
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match self:
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Cons::{head; ...} -> x
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_ -> ...
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second self =
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match self:
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Cons::{rest=Cons::{head; ...}; ...} -> head
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_ -> ...
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```
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### 枚舉模式
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* 其實只是枚舉類型
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```python
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match x:
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i: {1, 2} -> "one or two: \{i}"
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_ -> "other"
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```
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### Range 模式
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* 實際上，它只是一個區間類型
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```python
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# 0 < i < 1
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i: 0<..<1 = 0.5
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# 1 < j <= 2
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_: {[I, J] | I, J: 1<..2} = [1, 2]
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# 1 <= i <= 5
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match i
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i: 1..5 -> ...
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```
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### 不是模式的東西，不能被模式化的東西
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模式是可以唯一指定的東西。在這方面，模式匹配不同于普通的條件分支
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條件規格不是唯一的。例如，要檢查數字 `n` 是否為偶數，正統是 `n % 2 == 0`，但也可以寫成 `(n / 2).round() == n / 2`
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非唯一形式無論是正常工作還是等效于另一個條件都不是微不足道的
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#### Set
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沒有固定的模式。因為集合沒有辦法唯一地檢索元素
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您可以通過迭代器檢索它們，但不能保證順序
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<p align='center'>
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<a href='./26_object_system.md'>上一頁</a> | <a href='./28_comprehension.md'>下一頁</a>
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</p>
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