From e5ab41faf611c61878ad792cbaaf0294cd5715dd Mon Sep 17 00:00:00 2001
From: Peter Mikkelsen <peter@pmikkelsen.com>
Date: Thu, 1 Jul 2021 19:55:40 +0000
Subject: Add exceptions :) implement catch/3 and throw/1

---
 eval.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

(limited to 'eval.c')

diff --git a/eval.c b/eval.c
index e28e5e6..ae34175 100644
--- a/eval.c
+++ b/eval.c
@@ -7,7 +7,6 @@
 Goal *addgoals(Goal *, Term *);
 Term *findclause(Term *, Term *, Binding **);
 int equalterms(Term *, Term *);
-void applybinding(Term *, Binding *);
 Goal *copygoals(Goal *);
 Builtin findbuiltin(Term *);
 
@@ -21,16 +20,18 @@ evalquery(Term *database, Term *query, Binding **resultbindings, Choicepoint **r
 
 	if(choicestack == nil){
 	/*
-		The goal stack has the original query at the very bottom, protected by a goal there the ->goal field is nil.
+		The goal stack has the original query at the very bottom, protected by a catch frame where the ->goal field is nil.
 		This makes it so that we can continue until we hit the protective goal, at which point we have solved everything
 		and to get the result we can unify the original query with the one at the bottom of the stack, to get the bindings
 		applied.
 	*/
 		goals = malloc(sizeof(Goal));
 		goals->goal = copyterm(query, nil);
+		goals->catcher = nil;
 		goals->next = nil;
 		Goal *protector = malloc(sizeof(Goal));
 		protector->goal = nil;
+		protector->catcher = mkvariable(L"catch-var");
 		protector->next = goals;
 		goals = protector;
 
@@ -50,6 +51,11 @@ evalquery(Term *database, Term *query, Binding **resultbindings, Choicepoint **r
 Retry:
 		goal = goals->goal;
 
+		if(goals->catcher){
+			goals = goals->next;
+			continue;
+		}
+
 		if(debug)
 			print("Working goal: %S\n", prettyprint(goal));
 
@@ -81,7 +87,6 @@ Backtrack:
 					return 0;
 				if(debug)
 					print("Backtracking..\n");
-
 				Choicepoint *cp = choicestack;
 				choicestack = cp->next;
 				/* freegoals(goals) */
@@ -93,10 +98,10 @@ Backtrack:
 		
 		goals = goals->next;
 
-		/* Apply bindings to all goals on the stack. */
+		/* Apply bindings to all goals on the stack except catchframes */
 		Goal *g;
 		for(g = goals; g != nil; g = g->next){
-			if(g->goal != nil)
+			if(g->goal != nil && g->catcher == nil)
 				applybinding(g->goal, bindings);
 		}
 
@@ -122,6 +127,7 @@ addgoals(Goal *goals, Term *t)
 	}else{
 		Goal *g = malloc(sizeof(Goal));
 		g->goal = t;
+		g->catcher = nil;
 		g->next = goals;
 		goals = g;
 	}
@@ -266,6 +272,10 @@ copygoals(Goal *goals)
 			g->goal = copyterm(goals->goal, nil);
 		else
 			g->goal = nil;
+		if(goals->catcher)
+			g->catcher = copyterm(goals->catcher, nil);
+		else
+			g->catcher = nil;
 		g->next = copygoals(goals->next);
 		return g;
 	}else
-- 
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