# syntax: syntactic model for types structures (monoid operations)

Require Export positives comparisons.
Require Import Eqdep.
Require Import monoid.
Set Implicit Arguments.
Set Asymmetric Patterns.

# Free syntactic model

Section s.

Notation I := positive.
Variable A : Set.
Variables s t: A I.

residuated Kleene allegory expressions over a set A of variables, the variables being typed according to the functions s (source) and t (target).
The indexing types (I) are fixed to be positive numbers:
• this is convenient and efficient in practice, for reification
• we need a decidable type to get the untyping theorems without axioms
Note that we include constructors for flat operations (cup,cap,bot,top,neg): it is not convenient to reuse lsyntax.expr here since we would need to restrict the alphabet under those nodes to something like A_(n,m) = { a: A / s a = n t a = m }.

Inductive expr: I I Type :=
| e_zer: n m, expr n m
| e_top: n m, expr n m
| e_one: n, expr n n
| e_pls: n m, expr n m expr n m expr n m
| e_cap: n m, expr n m expr n m expr n m
| e_neg: n m, expr n m expr n m

| e_dot: n m p, expr n m expr m p expr n p
| e_itr: n, expr n n expr n n
| e_str: n, expr n n expr n n
| e_cnv: n m, expr n m expr m n
| e_ldv: n m p, expr n m expr n p expr m p
| e_rdv: n m p, expr m n expr p n expr p m
| e_var: a, expr (s a) (t a).

level of an expression: the set of operations that appear in that expression
Fixpoint e_level n m (x: expr n m): level :=
match x with
| e_zer _ _BOT
| e_top _ _TOP
| e_one _MIN
| e_pls _ _ x yCUP + e_level x + e_level y
| e_cap _ _ x yCAP + e_level x + e_level y
| e_neg _ _ xBL + e_level x

| e_dot _ _ _ x ye_level x + e_level y
| e_itr _ xSTR + e_level x
| e_str _ xSTR + e_level x
| e_cnv _ _ xCNV + e_level x
| e_ldv _ _ _ x y
| e_rdv _ _ _ x yDIV + e_level x + e_level y
| e_var aMIN
end%level.

Section e.
Context {X: ops} {f': I ob X}.
Variable f: a, X (f' (s a)) (f' (t a)).

interpretation of an expression into an arbitray structure, given an assignation f of the variables (and a interpretation function f' for the types)

Fixpoint eval n m (x: expr n m): X (f' n) (f' m) :=
match x with
| e_zer _ _ ⇒ 0
| e_top _ _top
| e_one _ ⇒ 1
| e_pls _ _ x yeval x + eval y
| e_cap _ _ x yeval x eval y
| e_neg _ _ x! eval x

| e_dot _ _ _ x yeval x eval y
| e_itr _ x(eval x)^+
| e_str _ x(eval x)^*
| e_cnv _ _ x(eval x
| e_ldv _ _ _ x yeval x -o eval y
| e_rdv _ _ _ x yeval y o- eval x
| e_var af a
end.
End e.

Section l.
Variable l: level.

# (In)equality of syntactic expressions.

Like in lsyntax, we use an impredicative encoding to define (in)equality in the free syntactic model, and we parametrise all definition with the level l at which we want to interpret the given expressions.

Definition e_leq n m (x y: expr n m) :=
X (L:laws l X) f' (f: a, X (f' (s a)) (f' (t a))), eval f x eval f y.
Definition e_weq n m (x y: expr n m) :=
X (L:laws l X) f' (f: a, X (f' (s a)) (f' (t a))), eval f x eval f y.

by packing syntactic expressions and the above predicates into a canonical structure for flat operations, and another one for the other operations, we get all notations for free

Canonical Structure expr_lattice_ops n m := {|
car := expr n m;
leq := @e_leq n m;
weq := @e_weq n m;
cup := @e_pls n m;
cap := @e_cap n m;
neg := @e_neg n m;
bot := @e_zer n m;
top := @e_top n m
|}.

Canonical Structure expr_ops := {|
ob := I;
mor := expr_lattice_ops;
dot := e_dot;
one := e_one;
itr := e_itr;
str := e_str;
cnv := e_cnv;
ldv := e_ldv;
rdv := e_rdv
|}.

we easily show that we get a model so that we immediately benefit from all lemmas about the various structures

Global Instance expr_lattice_laws n m: lattice.laws l (expr_lattice_ops n m).
Proof.
constructor; try right. constructor.
intros x X L u f. reflexivity.
intros x y z H H' X L u f. transitivity (eval f y); auto.
intros x y. split.
intro H. split; intros X L u f. now apply weq_leq, H. now apply weq_geq, H.
intros [H H'] X L u f. apply antisym; auto.
intros Hl x y z. split.
intro H. split; intros X L u f; specialize (H X L u f); simpl in H; hlattice.
intros [H H'] X L u f. simpl. apply cup_spec; auto.
intros Hl x y z. split.
intro H. split; intros X L u f; specialize (H X L u f); simpl in H; hlattice.
intros [H H'] X L u f. simpl. apply cap_spec; auto.
intros x X L u f. apply leq_bx.
intros x X L u f. apply leq_xt.
intros Hl x y z X L u f. apply cupcap_.
intros Hl x X L u f. apply capneg.
intros Hl x X L u f. apply cupneg.
Qed.

Global Instance expr_laws: laws l expr_ops.
Proof.
constructor; repeat right; repeat intro; simpl.
apply expr_lattice_laws.
apply dotA.
apply dot1x.
apply dotx1.
apply dot_leq; auto.
now rewrite dotplsx.
now rewrite dotxpls.
now rewrite dot0x.
now rewrite dotx0.
apply cnvdot_.
apply cnv_invol.
apply cnv_leq. now refine (H0 _ _ _ _).
apply cnv_ext.
apply str_refl.
apply str_cons.
apply str_ind_l. now refine (H0 _ _ _ _).
apply str_ind_r. now refine (H0 _ _ _ _).
apply itr_str_l.
apply capdotx.
split; intros E X L f' f; intros; simpl; apply ldv_spec, (E X L f' f).
split; intros E X L f' f; intros; simpl; apply rdv_spec, (E X L f' f).
Qed.

End l.

# Testing for particular constants

Definition is_zer n m (x: expr n m) :=
match x with e_zer _ _true | _false end.

Definition is_top n m (x: expr n m) :=
match x with e_top _ _true | _false end.

Inductive is_case n m (k x: expr n m): expr n m bool Prop :=
| is_case_true: x=k is_case k x k true
| is_case_false: xk is_case k x x false.

Lemma is_zer_spec n m (x: expr n m): is_case (e_zer n m) x x (is_zer x).
Proof. destruct x; constructor; discriminate || reflexivity. Qed.

Lemma is_top_spec n m (x: expr n m): is_case (e_top n m) x x (is_top x).
Proof. destruct x; constructor; discriminate || reflexivity. Qed.

casting the type of an expression
Definition cast n m n' m' (Hn: n=n') (Hm: m=m') (x: expr n m): expr n' m' :=
eq_rect n (fun nexpr n m') (eq_rect m (expr n) x _ Hm) _ Hn.

End s.
Arguments e_var [A s t] a.
Arguments e_one [A s t] n.
Arguments e_zer [A s t] n m.
Arguments e_top [A s t] n m.

Declare Scope ast_scope.
Bind Scope ast_scope with expr.
Delimit Scope ast_scope with ast.

additional notations, to specify explicitly at which level expressions are considered, or to work directly with the bare constructors (by opposition with the encapsulated ones, through monoid.ops)

Notation expr_ l s t n m := (expr_ops s t l n m).
Notation "x <==_[ l ] y" := (@leq (expr_ops _ _ l _ _) x y) (at level 79): ra_scope.
Notation "x ==_[ l ] y" := (@weq (expr_ops _ _ l _ _) x y) (at level 79): ra_scope.

Infix "+" := e_pls: ast_scope.
Infix "∩" := e_cap: ast_scope.
Infix "⋅" := e_dot: ast_scope.
Notation "1" := (e_one _): ast_scope.
Notation "0" := (e_zer _ _): ast_scope.
Notation top := (e_top _ _).
Notation "x ^+" := (e_itr x): ast_scope.
Notation "x °" := (e_cnv x): ast_scope.
Notation "x ^*" := (e_str x): ast_scope.
Notation "! x" := (e_neg x): ast_scope.
Notation "x -o y" := (e_ldv x y) (right associativity, at level 60): ast_scope.
Notation "y o- x" := (e_rdv x y) (left associativity, at level 61): ast_scope.

# weakening (in)equations

any equation holding at some level holds at all higher levels

Lemma e_leq_weaken {h k} {Hl: h k} A s t n m (x y: @expr A s t n m):
x <==_[h] y x <==_[k] y.
Proof. intros H X L f' f. eapply @H, lower_laws. Qed.

Lemma e_weq_weaken {h k} {Hl: h k} A s t n m (x y: @expr A s t n m):
x ==_[h] y x ==_[k] y.
Proof. intros H X L f' f. eapply @H, lower_laws. Qed.

# comparing expressions syntactically

Section expr_cmp.

Notation I := positive.
Context {A : cmpType}.
Variables s t: A I.
Notation expr := (expr s t).

we need to generalise the comparison function to expressions of distinct types because of Coq's dependent types
Fixpoint expr_compare n m (x: expr n m) p q (y: expr p q) :=
match x,y with
| e_zer _ _, e_zer _ _
| e_top _ _, e_top _ _
| e_one _, e_one _Eq
| e_var a, e_var bcmp a b
| e_pls _ _ x x', e_pls _ _ y y'
| e_cap _ _ x x', e_cap _ _ y y'
lex (expr_compare x y) (expr_compare x' y')
| e_dot _ u _ x x', e_dot _ v _ y y'
| e_ldv u _ _ x x', e_ldv v _ _ y y'
| e_rdv u _ _ x x', e_rdv v _ _ y y'
lex (cmp u v) (lex (expr_compare x y) (expr_compare x' y'))
| e_neg _ _ x, e_neg _ _ y
| e_itr _ x, e_itr _ y
| e_str _ x, e_str _ y
| e_cnv _ _ x, e_cnv _ _ yexpr_compare x y
| e_zer _ _, _Lt
| _, e_zer _ _Gt
| e_one _, _Lt
| _, e_one _Gt
| e_top _ _, _Lt
| _, e_top _ _Gt
| e_var _, _Lt
| _, e_var _Gt
| e_itr _ _, _Lt
| _, e_itr _ _Gt
| e_str _ _, _Lt
| _, e_str _ _Gt
| e_cnv _ _ _, _Lt
| _, e_cnv _ _ _Gt
| e_ldv _ _ _ _ _, _Lt
| _, e_ldv _ _ _ _ _Gt
| e_rdv _ _ _ _ _, _Lt
| _, e_rdv _ _ _ _ _Gt
| e_pls _ _ _ _, _Lt
| _, e_pls _ _ _ _Gt
| e_cap _ _ _ _, _Lt
| _, e_cap _ _ _ _Gt
| e_neg _ _ _, _Lt
| _, e_neg _ _ _Gt
end.

since A is decidable, cast provably acts as an identity
Lemma cast_eq n m (x: expr n m) (H: n=n) (H': m=m): cast H H' x = x.
Proof. unfold cast. now rewrite 2 cmp_eq_rect_eq. Qed.

auxiliary lemma for expr_compare_spec below, using dependent equality
Lemma expr_compare_eq_dep n m (x: expr n m): p q (y: expr p q),
expr_compare x y = Eq n=p m=q
eq_dep (I×I) (fun xexpr (fst x) (snd x)) (n,m) x (p,q) y.
Proof.
induction x; intros ? ? z C; destruct z; try discriminate C;
try (intros <- <- || intros <- _); try reflexivity;
simpl expr_compare in C.

apply compare_lex_eq in C as [C1 C2].
apply IHx1, cmp_eq_dep_eq in C1; trivial.
apply IHx2, cmp_eq_dep_eq in C2; trivial.
subst. reflexivity.

apply compare_lex_eq in C as [C1 C2].
apply IHx1, cmp_eq_dep_eq in C1; trivial.
apply IHx2, cmp_eq_dep_eq in C2; trivial.
subst. reflexivity.

apply IHx, cmp_eq_dep_eq in C; trivial. subst. reflexivity.

apply compare_lex_eq in C as [C C']. apply cmp_eq in C. subst.
apply compare_lex_eq in C' as [C1 C2].
apply IHx1, cmp_eq_dep_eq in C1; trivial.
apply IHx2, cmp_eq_dep_eq in C2; trivial.
subst. reflexivity.

apply IHx, cmp_eq_dep_eq in C; trivial. subst. reflexivity.

apply IHx, cmp_eq_dep_eq in C; trivial. subst. reflexivity.

apply IHx, cmp_eq_dep_eq in C; trivial. subst. reflexivity.

apply compare_lex_eq in C as [C C']. apply cmp_eq in C. subst.
apply compare_lex_eq in C' as [C1 C2].
apply IHx1, cmp_eq_dep_eq in C1; trivial.
apply IHx2, cmp_eq_dep_eq in C2; trivial.
subst. reflexivity.

apply compare_lex_eq in C as [C C']. apply cmp_eq in C. subst.
apply compare_lex_eq in C' as [C1 C2].
apply IHx1, cmp_eq_dep_eq in C1; trivial.
apply IHx2, cmp_eq_dep_eq in C2; trivial.
subst. reflexivity.

apply cmp_eq in C. subst. reflexivity.
Qed.

Lemma expr_compare_eq n m (x y: expr n m): x=y expr_compare x y = Eq.
Proof.
intros <-. induction x; simpl expr_compare; trivial;
rewrite ?compare_lex_eq, ?cmp_refl; tauto.
Qed.

final specification of the comparison function
Lemma expr_compare_spec n m (x y: expr n m): compare_spec (x=y) (expr_compare x y).
Proof.
generalize (fun Hexpr_compare_eq_dep x y H eq_refl eq_refl) as H.
generalize (@expr_compare_eq _ _ x y) as H'.
case (expr_compare x y); intros H' H; constructor.
now apply cmp_eq_dep_eq in H.
intro E. now discriminate H'.
intro E. now discriminate H'.
Qed.

packaging as a cmpType
Definition expr_compare_ n m (x y: expr n m) := expr_compare x y.
Canonical Structure cmp_expr n m := mk_simple_cmp (@expr_compare_ n m) (@expr_compare_spec n m).

End expr_cmp.

# Packages for typed reification

according to the interpretation function eval, (typed) reification has to provide a set A for indexing variables, and four maps:
• f': I ob X, to interpret types as written in the expressions ()
• s and t: A -> I, to specify the source and target types of each variable
• f: a: A, expr s t (s a) (t a), to interpret each variable
A is also fixed to be positive, so that we can use simple and efficient positive maps, but the fact that f has a dependent type is not convenient.
This is why we introduce the additionnal layer, to ease the definition of such maps: thanks to the definitions below, it suffices to provide the map f', and a map f: A Pack X f'.

Record Pack (X: ops) (f': positive ob X) := pack {
src: positive;
tgt: positive;
val: X (f' src) (f' tgt)
}.

Definition packed_eval (X: ops) (f': positive ob X) (f: positive Pack X f') :=
eval (fun aval (f a)) : n m, expr _ _ n m X (f' n) (f' m) .

these projections are used to build the reified expressions
Definition src_ (X: ops) (f': positive ob X) (f: positive Pack X f') a := src (f a).
Definition tgt_ (X: ops) (f': positive ob X) (f: positive Pack X f') a := tgt (f a).