Agda：返回列表的最后一个元素

如何解决Agda：返回列表的最后一个元素

当我签入 ********************** * Accellerated build * ********************** running bdist_wheel running build running build_py creating build creating build\lib.win-amd64-3.9 creating build\lib.win-amd64-3.9\yarl copying yarl\_quoting.py -> build\lib.win-amd64-3.9\yarl copying yarl\_quoting_py.py -> build\lib.win-amd64-3.9\yarl copying yarl\_url.py -> build\lib.win-amd64-3.9\yarl copying yarl\__init__.py -> build\lib.win-amd64-3.9\yarl running egg_info writing yarl.egg-info\PKG-INFO writing dependency_links to yarl.egg-info\dependency_links.txt writing requirements to yarl.egg-info\requires.txt writing top-level names to yarl.egg-info\top_level.txt reading manifest file 'yarl.egg-info\SOURCES.txt' reading manifest template 'MANIFEST.in' warning: no previously-included files matching '*.pyc' found anywhere in distribution warning: no previously-included files matching '*.cache' found anywhere in distribution warning: no previously-included files found matching 'yarl\*.html' warning: no previously-included files found matching 'yarl\*.so' warning: no previously-included files found matching 'yarl\*.pyd' no previously-included directories found matching 'docs\_build' writing manifest file 'yarl.egg-info\SOURCES.txt' copying yarl\__init__.pyi -> build\lib.win-amd64-3.9\yarl copying yarl\_quoting_c.c -> build\lib.win-amd64-3.9\yarl copying yarl\_quoting_c.pyi -> build\lib.win-amd64-3.9\yarl copying yarl\_quoting_c.pyx -> build\lib.win-amd64-3.9\yarl copying yarl\py.typed -> build\lib.win-amd64-3.9\yarl running build_ext building 'yarl._quoting_c' extension error: Microsoft Visual C++ 14.0 or greater is required. Get it with "Microsoft C++ Build Tools": https://visualstudio.microsoft.com/visual-cpp-build-tools/ ---------------------------------------- ERROR: Failed building wheel for yarl Failed to build multidict yarl ERROR: Could not build wheels for multidict,yarl which use PEP 517 and cannot be installed directly 时，我发现没有agda-stdlib/src/Data/List/Base.agda函数，但是我在last中看到了agda-stdlib/src/Data/Vec/Base.agda的函数。

当我尝试使用它时，尽管出现一些不确定的类型错误，但我不确定。

这就是我要称呼它的方式： last

其中

last (fromList todos)

和

todos : List Todo

我得到的错误是

record Todo : Set where
  field
    text      : String
    completed : Bool
    id        : ℕ

我猜想这与Data.List.foldr (λ _ → suc) 0 todos != suc _n_31 of type ℕ when checking that the expression fromList todos has type Data.Vec.Vec Todo (1 + _n_31) 具有此签名的事实有关：

last

但是我很困惑，因为当我在last : ∀ {n} → Vec A (1 + n) → A 上执行以下操作时：

C-c C-l

我实现了这个目标

last (fromList ?)

我以为?0 : List Todo 满意。

我应该在此处进行哪些更改以使其通过此错误？ 还是有另一种方式获取todos的最后一个元素？

谢谢！

编辑

我尝试了另一条路线，并决定将List替换为Vec。但是，当我尝试这样做时，我遇到了另一个我不太了解的错误

List

  Todo.completed (last (todos ∷ʳ record { text = text ; completed = false ; id = 1 }))
≡⟨⟩
  Todo.completed (record { text = text ; completed = false ; id = 1 })

我不确定错误消息在这里说什么。

编辑

我试图进一步简化问题

false !=
Todo.completed
(last
 (todos ∷ʳ record { text = text ; completed = false ; id = 1 })
 | Data.Vec.initLast
   (todos ∷ʳ record { text = text ; completed = false ; id = 1 }))
of type Bool
when checking that the inferred type of an application
  false ≡ _y_45
matches the expected type
  Todo.completed
  (last
   (todos ∷ʳ record { text = text ; completed = false ; id = 1 }))
  ≡ false

但我仍然收到类似的错误：

AddNat : ∀ {n : ℕ} → (Vec ℕ n) → (Vec ℕ (1 + n))
AddNat numbers = numbers ∷ʳ 1

AddNatLastAddedElementIsNat :
  ∀ {n : ℕ} (numbers : Vec ℕ (1 + n)) →
    last (AddNat numbers) ≡ 1
AddNatLastAddedElementIsNat numbers =
  begin
    last (AddNat numbers)
  ≡⟨⟩
    last (numbers ∷ʳ 1)
  ≡⟨⟩
    1
  ∎

为什么1 != (last (numbers ∷ʳ 1) | Data.Vec.initLast (numbers ∷ʳ 1)) of type ℕ when checking that the expression 1 ∎ has type last (numbers ∷ʳ 1) ≡ 1 显示为last (numbers ∷ʳ 1)类型？ (last (numbers ∷ʳ 1) | Data.Vec.initLast (numbers ∷ʳ 1))是否表示它是求和类型，并且我需要在两种情况下都进行模式匹配？谢谢！

解决方法

fromList ?0中的目标确实具有类型List Todo，但是由于fromList ?0具有类型Vec _ (length ?0)，如果类型检查器得出结论认为不能否n使得length ?0（在定义上）等于1 + n。 就您而言，一旦您说出?0 := todos，类似?0 := x ∷ todos这样的事情就会发生。

通常，像last这样的函数需要知道列表或向量是非空的，另一种方法是定义返回Maybe A而不是A的函数。