84 lines
2.8 KiB
Text
84 lines
2.8 KiB
Text
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Import macros.
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(*
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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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*)
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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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(*
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Prove that the sum of two even numbers is even.
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Notes: we use the macro
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Obtain [bindings] ',' 'from' [expr]_1 ',' [expr]_2
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It is syntax sugar for existential elimination.
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It expands to
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ExistsElim [expr]_1 (fun [binding], [expr]_2)
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It is defined in the file macros.lua.
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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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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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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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*)
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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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ExistsIntro (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 (Nat::Distribute 2 w1 w2)).
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(*
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In the following example, we omit the arguments for Nat::PlusAssoc, Refl and Nat::Distribute.
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Lean can infer them automatically.
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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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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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*)
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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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ExistsIntro (w + 1)
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(calc a + 1 = 2*w + 1 + 1 : { Hw }
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... = 2*w + (1 + 1) : Symm (Nat::PlusAssoc _ _ _)
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... = 2*w + 2*1 : Refl _
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... = 2*(w + 1) : Symm (Nat::Distribute _ _ _)).
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(*
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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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*)
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Show Environment 2.
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(*
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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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*)
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SetOption pp::implicit true.
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Show Environment 2.
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(*
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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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*)
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