72 lines
2.8 KiB
Text
72 lines
2.8 KiB
Text
import macros
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using Nat
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-- In this example, we prove two simple theorems about even/odd numbers.
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-- First, we define the predicates even and odd.
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definition even (a : Nat) := ∃ b, a = 2*b
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definition odd (a : Nat) := ∃ b, a = 2*b + 1
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-- Prove that the sum of two even numbers is even.
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--
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-- Notes: we use the macro
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-- obtain [bindings] ',' 'from' [expr]_1 ',' [expr]_2
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--
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-- It is syntax sugar for existential elimination.
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-- It expands to
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--
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-- exists_elim [expr]_1 (fun [binding], [expr]_2)
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--
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-- It is defined in the file macros.lua.
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--
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-- We also use the calculational proof style.
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-- See doc/lean/calc.md for more information.
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--
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-- We use the first two obtain-expressions to extract the
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-- witnesses w1 and w2 s.t. a = 2*w1 and b = 2*w2.
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-- We can do that because H1 and H2 are evidence/proof for the
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-- fact that 'a' and 'b' are even.
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--
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-- We use a calculational proof 'calc' expression to derive
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-- the witness w1+w2 for the fact that a+b is also even.
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-- That is, we provide a derivation showing that a+b = 2*(w1 + w2)
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theorem EvenPlusEven {a b : Nat} (H1 : even a) (H2 : even b) : even (a + b)
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:= obtain (w1 : Nat) (Hw1 : a = 2*w1), from H1,
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obtain (w2 : Nat) (Hw2 : b = 2*w2), from H2,
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exists_intro (w1 + w2)
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(calc a + b = 2*w1 + b : { Hw1 }
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... = 2*w1 + 2*w2 : { Hw2 }
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... = 2*(w1 + w2) : symm (distributer 2 w1 w2))
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-- In the following example, we omit the arguments for add_assoc, refl and distribute.
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-- Lean can infer them automatically.
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--
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-- refl is the reflexivity proof. (refl a) is a proof that two
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-- definitionally equal terms are indeed equal.
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-- "definitionally equal" means that they have the same normal form.
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-- We can also view it as "Proof by computation".
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-- The normal form of (1+1), and 2*1 is 2.
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--
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-- Another remark: '2*w + 1 + 1' is not definitionally equal to '2*w + 2*1'.
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-- The gotcha is that '2*w + 1 + 1' is actually '(2*w + 1) + 1' since +
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-- is left associative. Moreover, Lean normalizer does not use
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-- any theorems such as + associativity.
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theorem OddPlusOne {a : Nat} (H : odd a) : even (a + 1)
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:= obtain (w : Nat) (Hw : a = 2*w + 1), from H,
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exists_intro (w + 1)
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(calc a + 1 = 2*w + 1 + 1 : { Hw }
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... = 2*w + (1 + 1) : symm (add_assoc _ _ _)
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... = 2*w + 2*1 : refl _
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... = 2*(w + 1) : symm (distributer _ _ _))
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-- The following command displays the proof object produced by Lean after
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-- expanding macros, and infering implicit/missing arguments.
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print environment 2
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-- By default, Lean does not display implicit arguments.
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-- The following command will force it to display them.
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set_option pp::implicit true
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print environment 2
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-- As an exercise, prove that the sum of two odd numbers is even,
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-- and other similar theorems.
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