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Miscellaneous extensions

Program derivation

Coq comes with an extension called Derive, which supports program derivation. Typically in the style of Bird and Meertens or derivations of program refinements. To use the Derive extension it must first be required with Require Coq.derive.Derive. When the extension is loaded, it provides the following command:

Command Derive ident SuchThat term As ident

The first ident can appear in term. This command opens a new proof presenting the user with a goal for term in which the name ident is bound to an existential variable ?x (formally, there are other goals standing for the existential variables but they are shelved, as described in shelve).

When the proof ends two constants are defined:

• The first one is named using the first ident and is defined as the proof of the shelved goal (which is also the value of ?x). It is always transparent.
• The second one is named using the second ident. It has type term, and its body is the proof of the initially visible goal. It is opaque if the proof ends with Qed, and transparent if the proof ends with Defined.

Example

Require Coq.derive.Derive.

[Loading ML file derive_plugin.cmxs ... done]

Require Import Coq.Numbers.Natural.Peano.NPeano.

Section P.

Variables (n m k:nat).

n is declared m is declared k is declared

Derive p SuchThat ((k*n)+(k*m) = p) As h.

1 focused subgoal (shelved: 1) n, m, k : nat p := ?Goal : nat ============================ k * n + k * m = p

Proof.

rewrite <- Nat.mul_add_distr_l.

1 focused subgoal (shelved: 1) n, m, k : nat p := ?Goal : nat ============================ k * (n + m) = p

subst p.

1 focused subgoal (shelved: 1) n, m, k : nat ============================ k * (n + m) = ?Goal

reflexivity.

No more subgoals.

Qed.

End P.

Print p.

p = fun n m k : nat => k * (n + m) : nat -> nat -> nat -> nat Argument scopes are [nat_scope nat_scope nat_scope]

Check h.

h : forall n m k : nat, k * n + k * m = p n m k

Any property can be used as term, not only an equation. In particular, it could be an order relation specifying some form of program refinement or a non-executable property from which deriving a program is convenient.