如何在Agda中定义除法运算符? [英] How to define division operator in Agda?
问题描述
_ / _:N - > N - > frac
m / one = m / one
(suc m)/ n = ??我不知道该写什么。
Please help。
这是@gallais说的,你可以明确地使用有根据的递归,但我不喜欢这种方法,因为它是完全不可读的。数据类型
record是{α} {A:Setα}(x:A):Setαwhere $ b $b¡ = x
打开是
! :∀{α} {A:Setα} - > (x:A)→>是x
! _ = _
允许将值提升到类型级别,例如,您可以定义类型安全 pred
函数:
pred + +:∀{n} - >是(成功) - > ℕ
pred + = pred∘$
然后
test-1:pred +(!1)≡0
test-1 = refl
typechecks,而
失败:pred +(!0) ≡0
fail = refl
没有。可以用相同的方式定义减数与正减数(以确保充分发现):
_- +::∀ {m} - > ℕ - >是(成功) - > ℕ
n-+ im = n∸im im
然后使用我描述的这里,你可以重复减去另一个数字,直到差值小于第二个数字:
lem:∀{nm} {im:Is(suc m)} - > m < n - > n + 1 im
lem {suc n} {m}(s≤s_)=s≤'s(≤⇒≤'(n∸m≤mnn))
iter-sub:∀{m} - > ℕ - >是(成功) - >列表ℕ
iter-sub n im = calls(λn - > n-+ im)< -well-founded lem(_≤?_(im))n
$ b例如
test-1 :iter-sub 10(!3)≡10∷7∷4∷[]
test-1 = refl
test-2:iter-sub 16(!4)≡16∷16∷ 12∷8∷4∷[]
test-2 = refl
div +
然后就是
_div +_:∀{m} - > ℕ - >是(成功) - > ℕ
n div + im = length(iter-sub n im)
和版本类似到
Data.Nat.DivMod
模块中的模块(仅包含Mod
部分):_div_:ℕ - > (m:ℕ){_:False(m≟0)} - > ℕ
n div 0 =λ{()}
n div(suc m)= n div +(!(suc m))
一些测试:
test-3:map(λn - > n b $ b(0∷1∷2∷3∷4∷5∷6∷7∷8∷9∷[])
≡(0∷0∷0∷1∷1∷1∷ 2∷2∷2∷3∷[])
test-3 = refl
注但是,标准库中的版本还包含健全性证明:
属性:dividend≡toℕremaining + quotient * divisor
整个代码。
I want to divide two natural number. I have made function like this
_/_ : N -> N -> frac m / one = m / one (suc m) / n = ?? I dont know what to write here.
Please help.
解决方案As @gallais says you can use well-founded recursion explicitly, but I don't like this approach, because it's totally unreadable.
This datatype
record Is {α} {A : Set α} (x : A) : Set α where ¡ = x open Is ! : ∀ {α} {A : Set α} -> (x : A) -> Is x ! _ = _
allows to lift values to the type level, for example you can define a type-safe
pred
function:pred⁺ : ∀ {n} -> Is (suc n) -> ℕ pred⁺ = pred ∘ ¡
Then
test-1 : pred⁺ (! 1) ≡ 0 test-1 = refl
typechecks, while
fail : pred⁺ (! 0) ≡ 0 fail = refl
doesn't. It's possible to define subtraction with positive subtrahend (to ensure well-foundness) in the same way:
_-⁺_ : ∀ {m} -> ℕ -> Is (suc m) -> ℕ n -⁺ im = n ∸ ¡ im
Then using stuff that I described here, you can repeatedly subtract one number from another until the difference is smaller than the second number:
lem : ∀ {n m} {im : Is (suc m)} -> m < n -> n -⁺ im <′ n lem {suc n} {m} (s≤s _) = s≤′s (≤⇒≤′ (n∸m≤n m n)) iter-sub : ∀ {m} -> ℕ -> Is (suc m) -> List ℕ iter-sub n im = calls (λ n -> n -⁺ im) <-well-founded lem (_≤?_ (¡ im)) n
For example
test-1 : iter-sub 10 (! 3) ≡ 10 ∷ 7 ∷ 4 ∷ [] test-1 = refl test-2 : iter-sub 16 (! 4) ≡ 16 ∷ 12 ∷ 8 ∷ 4 ∷ [] test-2 = refl
div⁺
then is simply_div⁺_ : ∀ {m} -> ℕ -> Is (suc m) -> ℕ n div⁺ im = length (iter-sub n im)
And a version similar to the one in the
Data.Nat.DivMod
module (only without theMod
part):_div_ : ℕ -> (m : ℕ) {_ : False (m ≟ 0)} -> ℕ n div 0 = λ{()} n div (suc m) = n div⁺ (! (suc m))
Some tests:
test-3 : map (λ n -> n div 3) (0 ∷ 1 ∷ 2 ∷ 3 ∷ 4 ∷ 5 ∷ 6 ∷ 7 ∷ 8 ∷ 9 ∷ []) ≡ (0 ∷ 0 ∷ 0 ∷ 1 ∷ 1 ∷ 1 ∷ 2 ∷ 2 ∷ 2 ∷ 3 ∷ []) test-3 = refl
Note however, that the version in the standard library also contains the soundness proof:
property : dividend ≡ toℕ remainder + quotient * divisor
The whole code.
这篇关于如何在Agda中定义除法运算符?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!