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=={{header|Coq}}== |
=={{header|Coq}}== |
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< |
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Inductive nat : Set := |
Inductive nat : Set := |
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| O : nat |
| O : nat |
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assumption. |
assumption. |
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Qed. |
Qed. |
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</ |
</pre> |
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=={{header|Omega}}== |
=={{header|Omega}}== |
Revision as of 01:51, 12 March 2008
You are encouraged to solve this task according to the task description, using any language you may know.
Define a type for natural numbers (0, 1, 2, 3, ...) and addition on them. Define a type of even numbers (0, 2, 4, 6, ...) then prove that the addition of any two even numbers is even.
Ada
Natural is a pre-defined subtype for Ada.
package Evens is type Even_Number is private; function "+"(Left, Right : Even_Number) return Even_Number; function "-"(Left, Right : Even_Number) return Even_Number; function "*"(Left, Right : Even_Number) return Even_Number; function "/"(Left, Right : Even_Number) return Natural; function Image(Item : Even_Number) return String; function To_Even(Item : Natural) return Even_Number; function To_Natural(Item : Even_Number) return Natural; Constraint_Error : exception; private type Even_Number is record Value : Natural := 0; end record; end Evens;
package body Evens is --------- -- "+" -- --------- function "+" (Left, Right : Even_Number) return Even_Number is Temp : Even_Number; begin Temp.Value := Left.Value + Right.Value; return Temp; end "+"; --------- -- "-" -- --------- function "-" (Left, Right : Even_Number) return Even_Number is Temp : Even_Number; begin if Right.Value > Left.Value then raise Constraint_Error; end if; Temp.Value := Left.Value - Right.Value; return Temp; end "-"; --------- -- "*" -- --------- function "*" (Left, Right : Even_Number) return Even_Number is Temp : Even_Number; begin Temp.Value := Left.Value * Right.Value; return Temp; end "*"; --------- -- "/" -- --------- function "/" (Left, Right : Even_Number) return Natural is Temp : Natural := Left.Value / Right.Value; begin return Temp; end "/"; ----------- -- Image -- ----------- function Image (Item : Even_Number) return String is begin return Natural'Image(Item.Value); end Image; ------------- -- To_Even -- ------------- function To_Even (Item : Natural) return Even_Number is Temp : Even_Number; begin if Item mod 2 /= 0 then raise Constraint_Error; end if; Temp.Value := Item; return Temp; end To_Even; ---------------- -- To_Natural -- ---------------- function To_Natural (Item : Even_Number) return Natural is begin return Item.Value; end To_Natural; end Evens;
Coq
Inductive nat : Set := | O : nat | S : nat -> nat. Fixpoint plus (n m:nat) {struct n} : nat := match n with | O => m | S p => S (p + m) end where "n + m" := (plus n m) : nat_scope. Inductive even : nat -> Set := | even_O : even O | even_SSn : forall n:nat, even n -> even (S (S n)). Theorem even_plus_even : forall n m:nat, even n -> even m -> even (n + m). Proof. intros n m H H0. elim H. trivial. intros. simpl. case even_SSn. intros. apply even_SSn; assumption. assumption. Qed.
Omega
data Even :: Nat ~> *0 where EZ:: Even Z ES:: Even n -> Even (S (S n)) plus:: Nat ~> Nat ~> Nat {plus Z m} = m {plus (S n) m} = S {plus n m} even_plus:: Even m -> Even n -> Even {plus m n} even_plus EZ en = en even_plus (ES em) en = ES (even_plus em en)
Agda2
<agda2> module Arith where
data Nat : Set where
zero : Nat suc : Nat -> Nat
_+_ : Nat -> Nat -> Nat zero + n = n suc m + n = suc (m + n)
data Even : Nat -> Set where
even_zero : Even zero even_suc_suc : {n : Nat} -> Even n -> Even (suc (suc n))
_even+_ : {m n : Nat} -> Even m -> Even n -> Even (m + n) even_zero even+ en = en even_suc_suc em even+ en = even_suc_suc (em even+ en) </agda2>
Twelf
<twelf> nat : type. z : nat. s : nat -> nat.
plus : nat -> nat -> nat -> type.
plus-z : plus z N2 N2.
plus-s : plus (s N1) N2 (s N3)
<- plus N1 N2 N3.
%% declare totality assertion
%mode plus +N1 +N2 -N3.
%worlds () (plus _ _ _).
%% check totality assertion %total N1 (plus N1 _ _).
even : nat -> type. even-z : even z. even-s : even (s (s N))
<- even N.
sum-evens : even N1 -> even N2 -> plus N1 N2 N3 -> even N3 -> type.
%mode sum-evens +D1 +D2 +Dplus -D3.
sez : sum-evens
even-z (DevenN2 : even N2) (plus-z : plus z N2 N2) DevenN2.
ses : sum-evens
( (even-s DevenN1') : even (s (s N1'))) (DevenN2 : even N2) ( (plus-s (plus-s Dplus)) : plus (s (s N1')) N2 (s (s N3'))) (even-s DevenN3') <- sum-evens DevenN1' DevenN2 Dplus DevenN3'.
%worlds () (sum-evens _ _ _ _). %total D (sum-evens D _ _ _). </twelf>