From b6df808b8ff4795f4523c39860070d6153cfad5d Mon Sep 17 00:00:00 2001
From: Adam Chlipala <adam@chlipala.net>
Date: Fri, 5 Dec 2014 19:41:27 -0500
Subject: Move code from last changeset, to improve performance

---
 src/elaborate.sml | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

(limited to 'src/elaborate.sml')

diff --git a/src/elaborate.sml b/src/elaborate.sml
index c55dec01..749bd2f1 100644
--- a/src/elaborate.sml
+++ b/src/elaborate.sml
@@ -3020,6 +3020,25 @@ and subSgn' counterparts env strLoc sgn1 (sgn2 as (_, loc2)) =
 
       | (L'.SgnConst sgis1, L'.SgnConst sgis2) =>
         let
+            (* This reshuffling was added to avoid some unfortunate unification behavior.
+             * In particular, in sub-signature checking, constraints might be unified,
+             * even when we don't expect them to be unifiable, deciding on bad values
+             * for unification variables and dooming later unification.
+             * By putting all the constraints _last_, we allow all the other unifications
+             * to happen first, hoping that no unification variables survive to confuse
+             * constraint unification. *)
+
+            val sgis2 =
+                let
+                    val (constraints, others) = List.partition
+                                                   (fn (L'.SgiConstraint _, _) => true
+                                                   | _ => false) sgis2
+                in
+                    case constraints of
+                        [] => sgis2
+                      | _ => others @ constraints
+                end
+
             (*val () = prefaces "subSgn" [("sgn1", p_sgn env sgn1),
                                         ("sgn2", p_sgn env sgn2),
                                         ("sgis1", p_sgn env (L'.SgnConst sgis1, loc2)),
-- 
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