From 35fba70509d9fb219b2a88f8e7bd246b2671b39b Mon Sep 17 00:00:00 2001
From: Gaëtan Gilbert <gaetan.gilbert@skyskimmer.net>
Date: Thu, 9 Nov 2017 17:27:58 +0100
Subject: Simplification: cumulativity information is variance information.

Since cumulativity of an inductive type is the universe constraints
which make a term convertible with its universe-renamed copy, the only
constraints we can get are between a universe and its copy.

As such we do not need to be able to represent arbitrary constraints
between universes and copied universes in a double-sized ucontext,
instead we can just keep around an array describing whether a bound
universe is covariant, invariant or irrelevant (CIC has no
contravariant conversion rule).

Printing is fairly obtuse and should be improved: when we print the
CumulativityInfo we add marks to the universes of the instance: = for
invariant, + for covariant and * for irrelevant. ie

         Record Foo@{i j k} := { foo : Type@{i} -> Type@{j} }.
         Print Foo.

gives

        Cumulative Record Foo : Type@{max(i+1, j+1)} := Build_Foo
          { foo : Type@{i} -> Type@{j} }
          (* =i +j *k |=  *)
---
 kernel/reduction.ml | 27 +++++++--------------------
 1 file changed, 7 insertions(+), 20 deletions(-)

(limited to 'kernel/reduction.ml')

diff --git a/kernel/reduction.ml b/kernel/reduction.ml
index dc46ac01b..083692e91 100644
--- a/kernel/reduction.ml
+++ b/kernel/reduction.ml
@@ -219,26 +219,13 @@ let convert_instances ~flex u u' (s, check) =
   (check.compare_instances ~flex u u' s, check)
 
 let get_cumulativity_constraints cv_pb cumi u u' =
-  let length_ind_instance =
-    Univ.AUContext.size (Univ.ACumulativityInfo.univ_context cumi)
-  in
-  let ind_subtypctx = Univ.ACumulativityInfo.subtyp_context cumi in
-  if not ((length_ind_instance = Univ.Instance.length u) &&
-          (length_ind_instance = Univ.Instance.length u')) then
-     anomaly (Pp.str "Invalid inductive subtyping encountered!")
-  else
-    let comp_cst =
-      let comp_subst = (Univ.Instance.append u u') in
-      Univ.AUContext.instantiate comp_subst ind_subtypctx
-    in
-    match cv_pb with
-    | CONV ->
-      let comp_cst' =
-        let comp_subst = (Univ.Instance.append u' u) in
-        Univ.AUContext.instantiate comp_subst ind_subtypctx
-      in
-      Univ.Constraint.union comp_cst comp_cst'
-    | CUMUL -> comp_cst
+  match cv_pb with
+  | CONV ->
+    Univ.Variance.eq_constraints (Univ.ACumulativityInfo.variance cumi)
+      u u' Univ.Constraint.empty
+  | CUMUL ->
+    Univ.Variance.leq_constraints (Univ.ACumulativityInfo.variance cumi)
+      u u' Univ.Constraint.empty
 
 let convert_inductives_gen cmp_instances cmp_cumul cv_pb (mind,ind) nargs u1 u2 s =
   match mind.Declarations.mind_universes with
-- 
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