README.md

Locker

The lock and mlock commands let you lock definitions.

Example of lock

lock Definition x := 3.

is elaborated to

Lemma x_key_subproof : unit. Proof. exact: tt. Qed.
Definition x := locked_with x_key_subproof 3.
Canonical x_unlock_subterm := Unlockable ...

Here locked_with comes from ssreflect.v and protects the body of x under a match on x_key_subproof which is Qed opaque. Hence x is provably equal to 3, but not computationally equal to it.

Given the canonical structure registration, rewrite unlock will replace x by 3.

Example of mlock

mlock Definition x := 3.

is elaborated to

Module Type x_Locked.
Axiom body : nat.
Axiom unlock : body = 3.
End x_Locked.
Module x : x_Locked. ... End x.
Notation x := x.body.
Canonical x_unlock_subterm := Unlockable x.unlock.

Hence x (actually x.body) is a new symbol and x.unlock is its defining equation.

Given the canonical structure registration, rewrite unlock will replace x by 3.

Limitations

mlock uses a module based locking. The body is really sealed but this command cannot be used inside sections (since modules cannot be declared inside sections).

lock uses opaque key based locking. It can be used everywhere, even inside sections, but conversion (term comparison) may cross the lock (by congruence) and hence compare possibly large terms.

See also the section about locking in ssereflect.v.