lean-theorem-proving/inductive-theorem.lean at main · Lslightly-courses/lean-theorem-proving · GitHub

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
-- Lean has first-class functions.
-- `twice` takes two arguments `f` and `a` where
-- `f` is a function from natural numbers to natural numbers, and
-- `a` is a natural number.
def twice (f : Nat → Nat) (a : Nat) :=
  f (f a)

-- `fun` is used to declare anonymous functions
#eval twice (fun x => x + 2) 10

-- You can prove theorems about your functions.
-- The following theorem states that for any natural number `a`,
-- adding 2 twice produces a value equal to `a + 4`.
theorem twiceAdd2 (a : Nat) : twice (fun x => x + 2) a = a + 4 :=
  -- The proof is by reflexivity. Lean "symbolically" reduces both sides of the equality
  -- until they are identical.
  rfl

-- `(· + 2)` is syntax sugar for `(fun x => x + 2)`. The parentheses + `·` notation
-- is useful for defining simple anonymous functions.
#eval twice (· + 2) 10

-- Enumerated types are a special case of inductive types in Lean,
-- which we will learn about later.
-- The following command creates a new type `Weekday`.
inductive Weekday where
  | sunday    : Weekday
  | monday    : Weekday
  | tuesday   : Weekday
  | wednesday : Weekday
  | thursday  : Weekday
  | friday    : Weekday
  | saturday  : Weekday

-- `Weekday` has 7 constructors/elements.
-- The constructors live in the `Weekday` namespace.
-- Think of `sunday`, `monday`, …, `saturday` as being distinct elements of `Weekday`,
-- with no other distinguishing properties.
-- The command `#check` prints the type of a term in Lean.
#check Weekday.sunday
#check Weekday.monday

-- The `open` command opens a namespace, making all declarations in it accessible without
-- qualification.
open Weekday
#check sunday
#check tuesday

-- You can define functions by pattern matching.
-- The following function converts a `Weekday` into a natural number.
def natOfWeekday (d : Weekday) : Nat :=
  match d with
  | sunday    => 1
  | monday    => 2
  | tuesday   => 3
  | wednesday => 4
  | thursday  => 5
  | friday    => 6
  | saturday  => 7

#eval natOfWeekday tuesday

def isMonday : Weekday → Bool :=
  -- `fun` + `match`  is a common idiom.
  -- The following expression is syntax sugar for
  -- `fun d => match d with | monday => true | _ => false`.
  fun
    | monday => true
    | _      => false

#eval isMonday monday
#eval isMonday sunday

-- Lean has support for type classes and polymorphic methods.
-- The `toString` method converts a value into a `String`.
#eval toString 10
#eval toString (10, 20)

-- The method `toString` converts values of any type that implements
-- the class `ToString`.
-- You can implement instances of `ToString` for your own types.
instance : ToString Weekday where
  toString (d : Weekday) : String :=
    match d with
    | sunday    => "Sunday"
    | monday    => "Monday"
    | tuesday   => "Tuesday"
    | wednesday => "Wednesday"
    | thursday  => "Thursday"
    | friday    => "Friday"
    | saturday  => "Saturday"

#eval toString (sunday, 10)

def Weekday.next (d : Weekday) : Weekday :=
  match d with
  | sunday    => monday
  | monday    => tuesday
  | tuesday   => wednesday
  | wednesday => thursday
  | thursday  => friday
  | friday    => saturday
  | saturday  => sunday

#eval Weekday.next Weekday.wednesday
-- Since the `Weekday` namespace has already been opened, you can also write
#eval next wednesday

-- Matching on a parameter like in the previous definition
-- is so common that Lean provides syntax sugar for it. The following
-- function uses it.
def Weekday.previous : Weekday -> Weekday
  | sunday    => saturday
  | monday    => sunday
  | tuesday   => monday
  | wednesday => tuesday
  | thursday  => wednesday
  | friday    => thursday
  | saturday  => friday

#eval next (previous wednesday)

-- We can prove that for any `Weekday` `d`, `next (previous d) = d`
theorem Weekday.nextOfPrevious (d : Weekday) : next (previous d) = d :=
  match d with
  | sunday    => rfl
  | monday    => rfl
  | tuesday   => rfl
  | wednesday => rfl
  | thursday  => rfl
  | friday    => rfl
  | saturday  => rfl -- rlf reflective

-- You can automate definitions such as `Weekday.nextOfPrevious`
-- using metaprogramming (or "tactics").
theorem Weekday.nextOfPrevious' (d : Weekday) : next (previous d) = d := by -- constructs a term of the expected type by running the tactic(s) tac.
  cases d       -- A proof by case distinction
  all_goals rfl  -- Each case is solved using `rfl`