# Canonical Structures¶

Authors: | Assia Mahboubi and Enrico Tassi |
---|

This chapter explains the basics of canonical structures and how they can be used to overload notations and build a hierarchy of algebraic structures. The examples are taken from [MT13]. We invite the interested reader to refer to this paper for all the details that are omitted here for brevity. The interested reader shall also find in [GZND11] a detailed description of another, complementary, use of canonical structures: advanced proof search. This latter papers also presents many techniques one can employ to tune the inference of canonical structures.

## Notation overloading¶

We build an infix notation == for a comparison predicate. Such notation will be overloaded, and its meaning will depend on the types of the terms that are compared.

- Module EQ.
- Interactive Module EQ started
- Record class (T : Type) := Class { cmp : T -> T -> Prop }.
- class is defined cmp is defined
- Structure type := Pack { obj : Type; class_of : class obj }.
- type is defined obj is defined class_of is defined
- Definition op (e : type) : obj e -> obj e -> Prop := let 'Pack _ (Class _ the_cmp) := e in the_cmp.
- op is defined
- Check op.
- op : forall e : type, obj e -> obj e -> Prop
- Arguments op {e} x y : simpl never.
- Arguments Class {T} cmp.
- Module theory.
- Interactive Module theory started
- Notation "x == y" := (op x y) (at level 70).
- End theory.
- Module theory is defined
- End EQ.
- Module EQ is defined

We use Coq modules as namespaces. This allows us to follow the same
pattern and naming convention for the rest of the chapter. The base
namespace contains the definitions of the algebraic structure. To
keep the example small, the algebraic structure `EQ.type`

we are
defining is very simplistic, and characterizes terms on which a binary
relation is defined, without requiring such relation to validate any
property. The inner theory module contains the overloaded notation `==`

and will eventually contain lemmas holding all the instances of the
algebraic structure (in this case there are no lemmas).

Note that in practice the user may want to declare `EQ.obj`

as a
coercion, but we will not do that here.

The following line tests that, when we assume a type `e`

that is in
theEQ class, we can relate two of its objects with `==`

.

- Import EQ.theory.
- Check forall (e : EQ.type) (a b : EQ.obj e), a == b.
- forall (e : EQ.type) (a b : EQ.obj e), a == b : Prop

Still, no concrete type is in the `EQ`

class.

- Fail Check 3 == 3.
- The command has indeed failed with message: The term "3" has type "nat" while it is expected to have type "EQ.obj ?e".

We amend that by equipping `nat`

with a comparison relation.

- Definition nat_eq (x y : nat) := Nat.compare x y = Eq.
- nat_eq is defined
- Definition nat_EQcl : EQ.class nat := EQ.Class nat_eq.
- nat_EQcl is defined
- Canonical Structure nat_EQty : EQ.type := EQ.Pack nat nat_EQcl.
- nat_EQty is defined
- Check 3 == 3.
- 3 == 3 : Prop
- Eval compute in 3 == 4.
- = Lt = Eq : Prop

This last test shows that Coq is now not only able to type check `3 == 3`

,
but also that the infix relation was bound to the `nat_eq`

relation.
This relation is selected whenever `==`

is used on terms of type nat.
This can be read in the line declaring the canonical structure
`nat_EQty`

, where the first argument to `Pack`

is the key and its second
argument a group of canonical values associated to the key. In this
case we associate to nat only one canonical value (since its class,
`nat_EQcl`

has just one member). The use of the projection `op`

requires
its argument to be in the class `EQ`

, and uses such a member (function)
to actually compare its arguments.

Similarly, we could equip any other type with a comparison relation,
and use the `==`

notation on terms of this type.

### Derived Canonical Structures¶

We know how to use `==`

on base types, like `nat`

, `bool`

, `Z`

. Here we show
how to deal with type constructors, i.e. how to make the following
example work:

- Fail Check forall (e : EQ.type) (a b : EQ.obj e), (a, b) == (a, b).
- The command has indeed failed with message: In environment e : EQ.type a : EQ.obj e b : EQ.obj e The term "(a, b)" has type "(EQ.obj e * EQ.obj e)%type" while it is expected to have type "EQ.obj ?e".

The error message is telling that Coq has no idea on how to compare pairs of objects. The following construction is telling Coq exactly how to do that.

- Definition pair_eq (e1 e2 : EQ.type) (x y : EQ.obj e1 * EQ.obj e2) := fst x == fst y /\ snd x == snd y.
- pair_eq is defined
- Definition pair_EQcl e1 e2 := EQ.Class (pair_eq e1 e2).
- pair_EQcl is defined
- Canonical Structure pair_EQty (e1 e2 : EQ.type) : EQ.type := EQ.Pack (EQ.obj e1 * EQ.obj e2) (pair_EQcl e1 e2).
- pair_EQty is defined
- Check forall (e : EQ.type) (a b : EQ.obj e), (a, b) == (a, b).
- forall (e : EQ.type) (a b : EQ.obj e), (a, b) == (a, b) : Prop
- Check forall n m : nat, (3, 4) == (n, m).
- forall n m : nat, (3, 4) == (n, m) : Prop

Thanks to the `pair_EQty`

declaration, Coq is able to build a comparison
relation for pairs whenever it is able to build a comparison relation
for each component of the pair. The declaration associates to the key `*`

(the type constructor of pairs) the canonical comparison
relation `pair_eq`

whenever the type constructor `*`

is applied to two
types being themselves in the `EQ`

class.

## Hierarchy of structures¶

To get to an interesting example we need another base class to be
available. We choose the class of types that are equipped with an
order relation, to which we associate the infix `<=`

notation.

- Module LE.
- Interactive Module LE started
- Record class T := Class { cmp : T -> T -> Prop }.
- class is defined cmp is defined
- Structure type := Pack { obj : Type; class_of : class obj }.
- type is defined obj is defined class_of is defined
- Definition op (e : type) : obj e -> obj e -> Prop := let 'Pack _ (Class _ f) := e in f.
- op is defined
- Arguments op {_} x y : simpl never.
- Arguments Class {T} cmp.
- Module theory.
- Interactive Module theory started
- Notation "x <= y" := (op x y) (at level 70).
- End theory.
- Module theory is defined
- End LE.
- Module LE is defined

As before we register a canonical `LE`

class for `nat`

.

- Import LE.theory.
- Definition nat_le x y := Nat.compare x y <> Gt.
- nat_le is defined
- Definition nat_LEcl : LE.class nat := LE.Class nat_le.
- nat_LEcl is defined
- Canonical Structure nat_LEty : LE.type := LE.Pack nat nat_LEcl.
- nat_LEty is defined

And we enable Coq to relate pair of terms with `<=`

.

- Definition pair_le e1 e2 (x y : LE.obj e1 * LE.obj e2) := fst x <= fst y /\ snd x <= snd y.
- pair_le is defined
- Definition pair_LEcl e1 e2 := LE.Class (pair_le e1 e2).
- pair_LEcl is defined
- Canonical Structure pair_LEty (e1 e2 : LE.type) : LE.type := LE.Pack (LE.obj e1 * LE.obj e2) (pair_LEcl e1 e2).
- pair_LEty is defined
- Check (3,4,5) <= (3,4,5).
- (3, 4, 5) <= (3, 4, 5) : Prop

At the current stage we can use `==`

and `<=`

on concrete types, like
tuples of natural numbers, but we can’t develop an algebraic theory
over the types that are equipped with both relations.

- Check 2 <= 3 /\ 2 == 2.
- 2 <= 3 /\ 2 == 2 : Prop
- Fail Check forall (e : EQ.type) (x y : EQ.obj e), x <= y -> y <= x -> x == y.
- The command has indeed failed with message: In environment e : EQ.type x : EQ.obj e y : EQ.obj e The term "x" has type "EQ.obj e" while it is expected to have type "LE.obj ?e".
- Fail Check forall (e : LE.type) (x y : LE.obj e), x <= y -> y <= x -> x == y.
- The command has indeed failed with message: In environment e : LE.type x : LE.obj e y : LE.obj e The term "x" has type "LE.obj e" while it is expected to have type "EQ.obj ?e".

We need to define a new class that inherits from both `EQ`

and `LE`

.

- Module LEQ.
- Interactive Module LEQ started
- Record mixin (e : EQ.type) (le : EQ.obj e -> EQ.obj e -> Prop) := Mixin { compat : forall x y : EQ.obj e, le x y /\ le y x <-> x == y }.
- mixin is defined compat is defined
- Record class T := Class { EQ_class : EQ.class T; LE_class : LE.class T; extra : mixin (EQ.Pack T EQ_class) (LE.cmp T LE_class) }.
- class is defined EQ_class is defined LE_class is defined extra is defined
- Structure type := _Pack { obj : Type; #[canonical(false)] class_of : class obj }.
- type is defined obj is defined class_of is defined
- Arguments Mixin {e le} _.
- Arguments Class {T} _ _ _.

The mixin component of the `LEQ`

class contains all the extra content we
are adding to `EQ`

and `LE`

. In particular it contains the requirement
that the two relations we are combining are compatible.

The `class_of`

projection of the `type`

structure is annotated as *not canonical*;
it plays no role in the search for instances.

Unfortunately there is still an obstacle to developing the algebraic theory of this new class.

- Module theory.
- Interactive Module theory started
- Fail Check forall (le : type) (n m : obj le), n <= m -> n <= m -> n == m.
- The command has indeed failed with message: In environment le : type n : obj le m : obj le The term "n" has type "obj le" while it is expected to have type "LE.obj ?e".

The problem is that the two classes `LE`

and `LEQ`

are not yet related by
a subclass relation. In other words Coq does not see that an object of
the `LEQ`

class is also an object of the `LE`

class.

The following two constructions tell Coq how to canonically build the
`LE.type`

and `EQ.type`

structure given an `LEQ.type`

structure on the same
type.

- Definition to_EQ (e : type) : EQ.type := EQ.Pack (obj e) (EQ_class _ (class_of e)).
- to_EQ is defined
- Canonical Structure to_EQ.
- Definition to_LE (e : type) : LE.type := LE.Pack (obj e) (LE_class _ (class_of e)).
- to_LE is defined
- Canonical Structure to_LE.

We can now formulate out first theorem on the objects of the `LEQ`

structure.

- Lemma lele_eq (e : type) (x y : obj e) : x <= y -> y <= x -> x == y.
- 1 subgoal e : type x, y : obj e ============================ x <= y -> y <= x -> x == y
- now intros; apply (compat _ _ (extra _ (class_of e)) x y); split.
- No more subgoals.
- Qed.
- Arguments lele_eq {e} x y _ _.
- End theory.
- Module theory is defined
- End LEQ.
- Module LEQ is defined
- Import LEQ.theory.
- Check lele_eq.
- lele_eq : forall x y : LEQ.obj ?e, x <= y -> y <= x -> x == y where ?e : [ |- LEQ.type]

Of course one would like to apply results proved in the algebraic setting to any concrete instate of the algebraic structure.

- Example test_algebraic (n m : nat) : n <= m -> m <= n -> n == m.
- 1 subgoal n, m : nat ============================ n <= m -> m <= n -> n == m
- Fail apply (lele_eq n m).
- The command has indeed failed with message: In environment n, m : nat The term "n" has type "nat" while it is expected to have type "LEQ.obj ?e".
- Abort.
- Example test_algebraic2 (l1 l2 : LEQ.type) (n m : LEQ.obj l1 * LEQ.obj l2) : n <= m -> m <= n -> n == m.
- 1 subgoal l1, l2 : LEQ.type n, m : LEQ.obj l1 * LEQ.obj l2 ============================ n <= m -> m <= n -> n == m
- Fail apply (lele_eq n m).
- The command has indeed failed with message: In environment l1, l2 : LEQ.type n, m : LEQ.obj l1 * LEQ.obj l2 The term "n" has type "(LEQ.obj l1 * LEQ.obj l2)%type" while it is expected to have type "LEQ.obj ?e".
- Abort.

Again one has to tell Coq that the type `nat`

is in the `LEQ`

class, and
how the type constructor `*`

interacts with the `LEQ`

class. In the
following proofs are omitted for brevity.

- Lemma nat_LEQ_compat (n m : nat) : n <= m /\ m <= n <-> n == m.
- 1 subgoal n, m : nat ============================ n <= m /\ m <= n <-> n == m
- Admitted.
- nat_LEQ_compat is declared
- Definition nat_LEQmx := LEQ.Mixin nat_LEQ_compat.
- nat_LEQmx is defined
- Lemma pair_LEQ_compat (l1 l2 : LEQ.type) (n m : LEQ.obj l1 * LEQ.obj l2) : n <= m /\ m <= n <-> n == m.
- 1 subgoal l1, l2 : LEQ.type n, m : LEQ.obj l1 * LEQ.obj l2 ============================ n <= m /\ m <= n <-> n == m
- Admitted.
- pair_LEQ_compat is declared
- Definition pair_LEQmx l1 l2 := LEQ.Mixin (pair_LEQ_compat l1 l2).
- pair_LEQmx is defined

The following script registers an `LEQ`

class for `nat`

and for the type
constructor `*`

. It also tests that they work as expected.

Unfortunately, these declarations are very verbose. In the following subsection we show how to make them more compact.

- Module Add_instance_attempt.
- Interactive Module Add_instance_attempt started
- Canonical Structure nat_LEQty : LEQ.type := LEQ._Pack nat (LEQ.Class nat_EQcl nat_LEcl nat_LEQmx).
- nat_LEQty is defined
- Canonical Structure pair_LEQty (l1 l2 : LEQ.type) : LEQ.type := LEQ._Pack (LEQ.obj l1 * LEQ.obj l2) (LEQ.Class (EQ.class_of (pair_EQty (to_EQ l1) (to_EQ l2))) (LE.class_of (pair_LEty (to_LE l1) (to_LE l2))) (pair_LEQmx l1 l2)).
- pair_LEQty is defined
- Example test_algebraic (n m : nat) : n <= m -> m <= n -> n == m.
- 1 subgoal n, m : nat ============================ n <= m -> m <= n -> n == m
- now apply (lele_eq n m).
- No more subgoals.
- Qed.
- Example test_algebraic2 (n m : nat * nat) : n <= m -> m <= n -> n == m.
- 1 subgoal n, m : nat * nat ============================ n <= m -> m <= n -> n == m
- now apply (lele_eq n m).
- No more subgoals.
- Qed.
- End Add_instance_attempt.
- Module Add_instance_attempt is defined

Note that no direct proof of `n <= m -> m <= n -> n == m`

is provided by
the user for `n`

and m of type `nat * nat`

. What the user provides is a
proof of this statement for `n`

and `m`

of type `nat`

and a proof that the
pair constructor preserves this property. The combination of these two
facts is a simple form of proof search that Coq performs automatically
while inferring canonical structures.

### Compact declaration of Canonical Structures¶

We need some infrastructure for that.

- Require Import Strings.String.
- [Loading ML file newring_plugin.cmxs ... done]
- Module infrastructure.
- Interactive Module infrastructure started
- Inductive phantom {T : Type} (t : T) : Type := Phantom.
- phantom is defined phantom_rect is defined phantom_ind is defined phantom_rec is defined phantom_sind is defined
- Definition unify {T1 T2} (t1 : T1) (t2 : T2) (s : option string) := phantom t1 -> phantom t2.
- unify is defined
- Definition id {T} {t : T} (x : phantom t) := x.
- id is defined
- Notation "[find v | t1 ~ t2 ] p" := (fun v (_ : unify t1 t2 None) => p) (at level 50, v ident, only parsing).
- Notation "[find v | t1 ~ t2 | s ] p" := (fun v (_ : unify t1 t2 (Some s)) => p) (at level 50, v ident, only parsing).
- Notation "'Error : t : s" := (unify _ t (Some s)) (at level 50, format "''Error' : t : s").
- Open Scope string_scope.
- End infrastructure.
- Module infrastructure is defined

To explain the notation `[find v | t1 ~ t2]`

let us pick one of its
instances: `[find e | EQ.obj e ~ T | "is not an EQ.type" ]`

. It should be
read as: “find a class e such that its objects have type T or fail
with message "T is not an EQ.type"”.

The other utilities are used to ask Coq to solve a specific unification problem, that will in turn require the inference of some canonical structures. They are explained in more details in [MT13].

We now have all we need to create a compact “packager” to declare
instances of the `LEQ`

class.

- Import infrastructure.
- Definition packager T e0 le0 (m0 : LEQ.mixin e0 le0) := [find e | EQ.obj e ~ T | "is not an EQ.type" ] [find o | LE.obj o ~ T | "is not an LE.type" ] [find ce | EQ.class_of e ~ ce ] [find co | LE.class_of o ~ co ] [find m | m ~ m0 | "is not the right mixin" ] LEQ._Pack T (LEQ.Class ce co m).
- packager is defined
- Notation Pack T m := (packager T _ _ m _ id _ id _ id _ id _ id).

The object `Pack`

takes a type `T`

(the key) and a mixin `m`

. It infers all
the other pieces of the class `LEQ`

and declares them as canonical
values associated to the `T`

key. All in all, the only new piece of
information we add in the `LEQ`

class is the mixin, all the rest is
already canonical for `T`

and hence can be inferred by Coq.

`Pack`

is a notation, hence it is not type checked at the time of its
declaration. It will be type checked when it is used, an in that case `T`

is
going to be a concrete type. The odd arguments `_`

and `id`

we pass to the
packager represent respectively the classes to be inferred (like `e`

, `o`

,
etc) and a token (`id`

) to force their inference. Again, for all the details
the reader can refer to [MT13].

The declaration of canonical instances can now be way more compact:

- Canonical Structure nat_LEQty := Eval hnf in Pack nat nat_LEQmx.
- nat_LEQty is defined
- Canonical Structure pair_LEQty (l1 l2 : LEQ.type) := Eval hnf in Pack (LEQ.obj l1 * LEQ.obj l2) (pair_LEQmx l1 l2).
- pair_LEQty is defined

Error messages are also quite intelligible (if one skips to the end of the message).

- Fail Canonical Structure err := Eval hnf in Pack bool nat_LEQmx.
- The command has indeed failed with message: The term "id" has type "phantom (EQ.obj ?e) -> phantom (EQ.obj ?e)" while it is expected to have type "'Error:bool:"is not an EQ.type"".