From e6c4ecfc10b60d802400d8450fb73a37a605b70f Mon Sep 17 00:00:00 2001
From: Mark Thom <markjordanthom@gmail.com>
Date: Sat, 22 Jan 2022 12:22:50 -0700
Subject: [PATCH] lazily throw exceptions of dcg_body from goal-expanded
 phrase/{2,3} (#1244)

---
 src/lib/dcgs.pl | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/lib/dcgs.pl b/src/lib/dcgs.pl
index 7d6dd905..488c7a92 100644
--- a/src/lib/dcgs.pl
+++ b/src/lib/dcgs.pl
@@ -156,6 +156,9 @@ seqq([Es|Ess]) --> seq(Es), seqq(Ess).
 user:goal_expansion(phrase(GRBody, S, S0), GRBody1) :-
     load_context(GRBody, M, GRBody0),
     nonvar(GRBody0),
-    dcg_body(GRBody0, S, S0, GRBody1, M).
+    catch(dcgs:dcg_body(GRBody0, S, S0, GRBody1, M),
+          error(E, must_be/2),
+          (  GRBody1 = throw(error(E, must_be/2))  )
+         ).
 
 user:goal_expansion(phrase(GRBody, S), phrase(GRBody, S, [])).
-- 
2.54.0