Coq學習筆記(一)
OMG,沒學離散數學之前覺得這就是門數學,學了之後被邏輯和編程同時吊打,這裏根據一些資料把之前學的coq相關規則回顧一遍,畢竟要準備期末考試複習了,這學期東西又難又是在線上教學,一定要好好加油鴨。(鬼姐姐根本沒在怕的)
所用參考書目爲《Logic Foundations》
BASICS
函數編程
函數編程強調讓我們瞭解如何將輸入映射到輸出,同時將函數(方法)作爲一級值(類似於數字,集合都屬於數學中某一級別的值),也就是將函數作爲數據來處理。
函數式編程思想也包括我們在OOP中涉及到的多態,代碼重用,但是函數式編程是通過函數將程序模塊化。
枚舉類型
coq內置特性非常少,並沒有像高級語言一樣已經定義了常用的數據類型,而是提供了一種強大的機制來從頭定義新的數據類型。
當然coq標準庫提供了原始的數據類型,但我們這裏選擇顯式地重述定義。
引例:Days of the week(定義一個類型)
defining a new set of data values-a type
Coq < Inductive day:Type:=
Coq < |monday
Coq < |tuesday
Coq < |wednesday
Coq < |thursday
Coq < |friday
Coq < |saturday
Coq < |sunday.
day is defined
day_rect is defined
day_ind is defined
day_rec is defined
- coq代碼中經常會用數據:類型的格式展現一個數據和其相應的類型
- “:=”在coq中表示等於,用"."結束語句
- 這裏定義了一個新的類型,名字是day,有七個成員
用coq定義一個函數對這些數據進行操作
Definition next_weekday (d:day) : day :=
match d with
| monday => tuesday
| tuesday => wednesday
| wednesday => thursday
| thursday => friday
| friday => monday
| saturday => monday
| sunday => monday
end.
1 定義函數 Definition fun_name (輸入):(返回),注意這裏的輸入輸出依然採用數據名稱:數據類型的格式,但是一旦命名的數據名稱必須在環境中使用(比如d),對於輸入,一般都需要使用一個數據,需要說明數據,但是類型可有可無,對於輸出,必須說明其類型(coq可以進行類型判斷,但是我們一般會包括他們,使閱讀更加容易)
2 在定義每個情況的輸出類型時,必須定義所有可能的情況,比如這裏需要列舉所有星期對應的返回值。
定義好一個函數後,我們使用Compute命令對結果進行查詢。
Compute (next_weekday friday).
Compute (next_weekday (next_weekday saturday)).
調用函數的時候和高級語言格式很接近。
執行結果:
一些基礎語法定義
Type
每個表達式都有一個類型
- New Types from Old
就像前面定義的Days of week一樣,我們定義的都是枚舉類型的例子
Check命令
確定輸入內容的類型
- 簡單的類型
Coq < Check True.
True
: Prop
Coq < Check 3.
3
: nat
Coq < Check 3+2.
3 + 2
: nat
- 有序數對
Coq < Check (2,3=5).
(2, 3 = 5)
: nat * Prop
- 函數類型
1 使用關鍵字fun構造一個新函數,它將替換λlambda微積分(之前的博客有提到)和類似理論的符號。
they are written with arrows.
Coq < Check (fun x:nat=>x=3).
fun x : nat => x = 3
: nat -> Prop
Coq < Check (forall x:nat,x<3\/(exists y:nat,x=y+3)).
forall x : nat, x < 3 \/ (exists y : nat, x = y + 3)
: Prop
2 let的使用(爲函數提供一個臨時的名字,之前的博客提到過)
Check (let f := fun x => (x * 3,x) in f 3).
let f := fun x : nat => (x * 3, x) in f 3 : nat * nat
3 構造枚舉類函數時其成員也可以定義成一個函數(constructor)
Inductive reb:Type :=
|red
|green
|blue.
Inductive color :Type:=
|black
|white
|primary (p:reb).
Check primary red.
Check primary blue.
多元組
多個參數的單個構造函數用於創建元組類型
比如我們在用二進制編碼時,可以將某一位定義是取值爲0或1的函數,那樣我們表示一個字節時相當於八位的元組,很像在高級語言中的數組
Inductive bit:Type :=
|B0
|B1.
Inductive byte:Type :=
| bits(b0 b1 b2 b3 b4 b5 b6 b7:bit).
Check (bits B0 B1 B0 B1 B0 B1 B0 B1).
下劃線的使用,我們用下劃線代替未定義的變量,比如我們要檢查定義的字節是否各位爲全0
Definition all_zero (input:byte):bool:=
match input with
|(bits B0 B0 B0 B0 B0 B0 B0 B0)=>true
|(bits _ _ _ _ _ _ _ _)=>false
end.
Compute all_zero(bits B1 B1 B1 B1 B1 B1 B1 B1).
Modules
本課程中涉及很少
Compute命令
定義一個新常量
Coq < Definition example:=fun x=>x*x.
example is defined
Coq < Compute example 1.
= 1
: nat
Coq < Compute example 3.
= 9
: nat
BOOLEANS
布爾表達式的構造
相關定義
- 構建布爾集合,歸納定義布爾集合
Coq < Inductive bool:Type:=
Coq < |true
Coq < |false.
bool is defined
bool_rect is defined
bool_ind is defined
bool_rec is defined
- 定義布爾運算
Coq < Definition negb b:=
Coq < match b with
Coq < |true=>false
Coq < |false=>true
Coq < end.
negb is defined
Coq < Definition andb n m:=
Coq < match n with
Coq < |true=>m
Coq < |false=>false
Coq < end.
andb is defined
Coq < Definition orb n m :=
Coq < match n with
Coq < |true=>true
Coq < |false=>m
Coq < end.
orb is defined
Coq < Compute orb true false.
= true
: bool
Coq < Compute negb true.
= false
: bool
插入標記
Coq < Notation "n | m" := (andb n m)(at level 85,right associativity).
Coq < Notation "n & m":=(andb n m)(at level 80,right associativity).
Coq < Notation "\ n":=(negb n)(at level 70,right associativity).
Coq < Check true & false | \true.
(true & false) & \ true
: bool
Coq < Compute true & false | \true.
= false
: bool
定義計算級別和結合順序
布爾表達式的相關運算律
用Example檢查計算結果是否符合預期
Coq < Example test1: orb true false=false.
test1 < Proof. simpl. reflexivity. Qed.
課堂實例
自然數和鏈表的遞歸構造
自然數
- 自然數的構造
1 自然數的內部表示
Inductive nat:Type:=
|o
|S(n:nat).
(*O和S只是一種表達方式,O是歸納基礎,S是構造算子,我們甚至可以這樣定義*)
Inductive nat':Type:=
|Lilghost
|darling (d:nat').
Check darling(darling(darling Lilghost)).
遞歸定義:O是歸納基礎,S是構造算子
Coq < Check 1.
1
: nat
Coq < Check S(S(S O)).
3
: nat
coq內部用一進製表示自然數(上邊那個是O不是0)
我們使用命令
Print nat.
可以輸出nat的內部表示
Coq < Print nat.
Inductive nat : Set := O : nat | S : nat -> nat
For S: Argument scope is [nat_scope]
Unset Printing Notations.
Set Printing Notations.
表示打開其內部表示
Coq < Unset Printing Notations.
Coq < Check 3.
S (S (S O))
: nat
2 一些關於自然數的函數
- 前置函數
- 利用遞歸函數確定給定自然數是否爲偶數
3 定義自然數的加法
加法的定義:取m的n次後繼
定義遞歸函數時用Fixpoint命令
Fixpoint plus n m :=
match n with
| O => m
| S n' => S (plus n' m)
end.
Compute plus 2 3.
Notation "x + y":=(plus x y).
Compute 2+3.
使用Notation能用符號表示函數
- 練習:使用遞歸定義自然數乘法,減法
練習答案:
Coq < Fixpoint plus n m:=
Coq < match n with
Coq < |O=>m
Coq < |S n'=>S (plus n' m)
Coq < end.
plus is defined
plus is recursively defined (decreasing on 1st argument)
Coq < Fixpoint mulit n m:=
Coq < match n with
Coq < |1=>m
Coq < |S n'=>plus m (mulit n' m)
Coq < |O=>O
Coq < end.
mulit is defined
mulit is recursively defined (decreasing on 1st argument)
Coq < Compute mulit 3 5.
= 15
: nat
Coq < Notation "n * m":=(mulit n m).
Coq < Compute 3*0.
= 0
: nat
Coq < Compute 5*9.
= 45
: nat
Coq < Fixpoint sub n m :=
Coq < match n,m with
Coq < |O,_=>O
Coq < |_,O=> n
Coq < |S x',S y'=>sub x' y'
Coq < end.
sub is defined
sub is recursively defined (decreasing on 1st argument)
Coq < Compute sub 5 2.
= 3
: nat
遞歸是函數式編程的基本理念,保證函數的終止性,沒有傳統過程語言的循環結構
鏈表的構造
Inductive natlist : Set :=
| nil : natlist
| cons : nat -> natlist -> natlist.
Definition l1 := cons 2 (cons 1 nil).
(* syntax tree of l1 is:
cons
|
+-----+-----+
| |
2 cons
|
+---+---+
| |
1 nil
*)
Definition l2 := cons 3 nil.
Compute l1.
Notation "[ ]" := nil.
Notation "[ a ; .. ; b ]" := (cons a .. (cons b nil) .. ).
Compute l1.
Fixpoint plus s t := match s with
| [] => t
| cons a s' => cons a (plus s' t)
end.
Compute plus [3] [2;1].
Notation "x + y" := (plus x y).
Compute l2 + l1.
Fixpoint rev s := match s with
| [] => []
| cons a s' => (rev s') + [a]
end.
Compute rev (l2 + l1).
- 練習:計算鏈表長度,自然數鏈表元素求和。
Coq < Fixpoint plus n m:=
Coq < match n with
Coq < |O=>m
Coq < |S n'=>S(plus n' m)
Coq < end.
plus is defined
plus is recursively defined (decreasing on 1st argument)
Coq < Inductive list:Set:=
Coq < |nil:list
Coq < |cons :nat->list->list.
list is defined
list_rect is defined
list_ind is defined
list_rec is defined
Coq < Definition l1:=cons 2(cons 1(cons 3 nil)).
l1 is defined
Coq < Fixpoint nons s :=
Coq < match s with
Coq < |nil=>O
Coq < |cons a s'=>plus 1 (nons s')
Coq < end.
nons is defined
nons is recursively defined (decreasing on 1st argument)
Coq < Compute nons l1.
= 3
: nat
Coq < Fixpoint sum s :=
Coq < match s with
Coq < |nil=>O
Coq < |cons a s'=>plus a (sum s')
Coq < end.
sum is defined
sum is recursively defined (decreasing on 1st argument)
Coq < Compute sum l1.
= 6
: nat
參考書目課後題答案
- EX1:standard (nandb)
Definition negb b:=
match b with
|true=>false
|false=>true
end.
Definition nandb (b1:bool) (b2:bool) : bool :=
match b1 with
|false=>true
|true=>negb b2
end.
Example test_nandb1: (nandb true false) = true.
Proof. simpl. reflexivity. Qed.
Example test_nandb2: (nandb false false) = true.
Proof. simpl. reflexivity. Qed.
Example test_nandb3: (nandb false true) = true.
Proof. simpl. reflexivity. Qed.
Example test_nandb4: (nandb true true) = false.
Proof. simpl. reflexivity. Qed.
- EX2: standard (andb3)
Definition andb n m:=
match n with
|true=>m
|false=>false
end.
Definition andb3 (b1:bool) (b2:bool) (b3:bool) : bool :=
match b1 with
|false=>false
|true=>andb b2 b3
end.
Example test_andb31: (andb3 true true true) = true.
Proof. simpl. reflexivity. Qed.
Example test_andb32: (andb3 false true true) = false.
Proof. simpl. reflexivity. Qed.
Example test_andb33: (andb3 true false true) = false.
Proof. simpl. reflexivity. Qed.
Example test_andb34: (andb3 true true false) = false.
Proof. simpl. reflexivity. Qed.
- EX3 standard (factorial)
Fixpoint muti m n:=
match m with
|O=>O
|S m'=>(plus n (muti m' n))
end.
Fixpoint factorial (n:nat) : nat:=
match n with
|O=>1
|S n'=>(muti n (factorial n'))
end.
Example test_factorial1: (factorial 3) = 6.
Proof. simpl. reflexivity. Qed.
Example test_factorial2: (factorial 5) = (mult 10 12).
Proof. simpl. reflexivity. Qed.
Fixpoint eqb (n m:nat):bool:=
match n with
|O=>match m with
|O=>true
|S m'=>false
end
|S n'=>match m with
|O=>false
|S m'=>(eqb n' m')
end
end.
Compute (eqb 3 5).
- EX4
Fixpoint eqb (n m:nat):bool:=
match n with
|O=>match m with
|O=>true
|S m'=>false
end
|S n'=>match m with
|O=>false
|S m'=>(eqb n' m')
end
end.
Definition ltb (n m : nat) : bool:=
match (minus n m)with
|O=>(negb(eqb n m))
|_=>false
end.
Notation "x <? y" := (ltb x y) (at level 70) : nat_scope.
Example test_ltb1: (ltb 2 2) = false.
Proof. simpl. reflexivity. Qed.
Example test_ltb2: (ltb 2 4) = true.
Proof. simpl. reflexivity. Qed.
Example test_ltb3: (ltb 4 2) = false.
Proof. simpl. reflexivity. Qed.
終於結束第一部分了,接下來是coq獨特的證明功能,本篇代碼中間隱藏了數個小彩蛋,鬼姐姐們可以找找噢。