From 2846764a93d3317b0934c61d078f54baf3a18599 Mon Sep 17 00:00:00 2001
From: Jay McCarthy <jay@racket-lang.org>
Date: Fri, 18 Apr 2014 10:09:15 -0600
Subject: [PATCH] Clarify Datalog/Racket interaction re Matthias

---
 pkgs/datalog/scribblings/racket.scrbl | 10 +++++++++-
 pkgs/datalog/tests/racket.rkt         |  5 +++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/pkgs/datalog/scribblings/racket.scrbl b/pkgs/datalog/scribblings/racket.scrbl
index 3b298fa674..c66517cdbb 100644
--- a/pkgs/datalog/scribblings/racket.scrbl
+++ b/pkgs/datalog/scribblings/racket.scrbl
@@ -61,6 +61,8 @@ The Datalog database can be directly used by Racket programs through this API.
 
           (datalog family
                    (? (add1 1 :- X)))
+          (datalog family
+                   (? (add1 X :- 2)))
           (datalog family
                    (? (#,(λ (x) (+ x 1)) 1 :- X)))]
 
@@ -107,7 +109,13 @@ for a variable symbol, or @RACKET[#,expr] where @racket[expr]
 evaluates to a constant datum. Bound identifiers in terms are treated
 as the datum they are bound to.
 
-External queries invalidate Datalog's guaranteed termination. For example, this program does not terminate:
+External queries fail if any logic variable is not fully resolved to a
+datum on the Datalog side. In other words, unbound logic variables
+never flow to Racket.
+
+External queries invalidate Datalog's guaranteed termination. For
+example, this program does not terminate:
+
 @racketblock[
  (datalog (make-theory)
           (! (:- (loop X)
diff --git a/pkgs/datalog/tests/racket.rkt b/pkgs/datalog/tests/racket.rkt
index 9f578d5dea..8d1ab3ac38 100644
--- a/pkgs/datalog/tests/racket.rkt
+++ b/pkgs/datalog/tests/racket.rkt
@@ -76,6 +76,11 @@
  =>
  (list (hasheq 'X 2))
 
+ (datalog parent
+          (? (add1 X :- 2)))
+ =>
+ (list)
+
  (datalog parent
           (? (#,(λ (x) (+ x 1)) 1 :- X)))
  =>