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Allow the repl to backtrack to give alternative results

--- a/TODO

+++ b/TODO

@@ -1,4 +1,3 @@

-* Add a repl

 * Stop comparing strings all the time

 * Stop copying the entire goal stack into every choicepoint

 * Stop creating choicepoints when it is not needed

--- a/eval.c

+++ b/eval.c

@@ -14,13 +14,12 @@

 static uvlong clausenr;

 int

-evalquery(Term *database, Term *query, Binding **resultbindings)

+evalquery(Term *database, Term *query, Binding **resultbindings, Choicepoint **resultchoicestack)

 {

 	Goal *goals;

-	Choicepoint *choicestack = nil;

+	Choicepoint *choicestack = *resultchoicestack;

-	clausenr = 2; /* Start at two since 0 is for the facts in the database, and 1 is for queries */

-

+	if(choicestack == nil){

 	/*

 		The goal stack has the original query at the very bottom, protected by a goal there 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

@@ -27,17 +26,21 @@

 		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->next = nil;

+		Goal *protector = malloc(sizeof(Goal));

+		protector->goal = nil;

+		protector->next = goals;

+		goals = protector;

-	goals = malloc(sizeof(Goal));

-	goals->goal = copyterm(query, nil);

-	goals->next = nil;

-	Goal *protector = malloc(sizeof(Goal));

-	protector->goal = nil;

-	protector->next = goals;

-	goals = protector;

+		/* Now add the actual goals */

+		goals = addgoals(goals, query);

-	/* Now add the actual goals */

-	goals = addgoals(goals, query);

+		clausenr = 2; /* Start at two since 0 is for the facts in the database, and 1 is for queries */

+	}else{

+		goto Backtrack;

+	}

 	while(goals->goal != nil){

 		Term *dbstart;

@@ -76,7 +79,9 @@

 Backtrack:

 				if(choicestack == nil)

 					return 0;

-				print("Backtracking..\n");

+				if(debug)

+					print("Backtracking..\n");

+

 				Choicepoint *cp = choicestack;

 				choicestack = cp->next;

 				/* freegoals(goals) */

@@ -104,6 +109,7 @@

 	}

 	goals = goals->next;

 	unify(query, goals->goal, resultbindings);

+	*resultchoicestack = choicestack;

 	return 1;

 }

--- a/fns.h

+++ b/fns.h

@@ -15,7 +15,7 @@

 Term *mkstring(Rune *);

 /* eval.c */

-int evalquery(Term *, Term *, Binding **);

+int evalquery(Term *, Term *, Binding **, Choicepoint **);

 int unify(Term *, Term *, Binding **);

 /* repl.c */

--- a/main.c

+++ b/main.c

@@ -43,7 +43,8 @@

 		Term *goal;

 		for(goal = initgoals; goal != nil; goal = goal->next){

 			Binding *bindings = nil;

-			evalquery(database, goal, &bindings);

+			Choicepoint *choicestack = nil;

+			evalquery(database, goal, &bindings, &choicestack);

 		}

 	}

--- a/parser.c

+++ b/parser.c

@@ -81,6 +81,7 @@

 Term *

 parse(int fd, int querymode)

 {

+	fd = dup(fd, -1);

 	parsein = Bfdopen(fd, OREAD);

 	if(parsein == nil){

 		print("Could not open file\n");

@@ -95,6 +96,7 @@

 		uvlong id = 1;

 		result = copyterm(result, &id);

 	}

+	Bterm(parsein);

 	return result;

 }

--- a/repl.c

+++ b/repl.c

@@ -1,9 +1,12 @@

 #include <u.h>

 #include <libc.h>

+#include <bio.h>

 #include "dat.h"

 #include "fns.h"

+Rune parsefindmore(int);

+

 void

 repl(Term *database)

 {

@@ -12,7 +15,10 @@

 		print("?- ");

 		Term *query = parse(fd, 1);

 		Binding *bindings = nil;

-		int success = evalquery(database, query, &bindings);

+		Choicepoint *choicestack = nil;

+		int success;

+FindMore:

+		success = evalquery(database, query, &bindings, &choicestack);

 		if(success == 0)

 			print("false.\n");

 		else{

@@ -20,10 +26,46 @@

 				print("true.\n");

 			else{

 				while(bindings){

-					print("%S = %S\n", bindings->name, prettyprint(bindings->value));

+					print("  %S = %S%s", 

+						bindings->name, 

+						prettyprint(bindings->value), 

+						bindings->next ? " ,\n" : "");

 					bindings = bindings->next;

 				}

 			}

+			if(choicestack != nil){

+				print(" ");

+				if(parsefindmore(fd) == L';'){

+					print(";\n");

+					goto FindMore;

+				}else

+					print(".\n");

+			}else{

+				print(".\n");

+			}

 		}

 	}

+}

+

+Rune

+parsefindmore(int fd)

+{

+	int consctl = open("/dev/consctl", OWRITE);

+	if(consctl > 0)

+		write(consctl, "rawon", 5);

+	else{

+		print("Could not open /dev/consctl\n");

+		exits("open");

+	}

+

+	fd = dup(fd, -1);

+	Biobuf *input = Bfdopen(fd, OREAD);

+	Rune peek = Bgetrune(input);

+	Bterm(input);

+

+	if(consctl > 0){

+		write(consctl, "rawoff", 6);

+		close(consctl);

+	}

+	return peek;

 }

\ No newline at end of file

--- a/stdlib.pl

+++ b/stdlib.pl

@@ -16,10 +16,13 @@

 If -> Then ; _ :- 

 	If, !, Then.

+

 _ -> _ ; Else :-

 	!, Else.

+

 If ; _ :-

 	If.

+

 _ ; Else :-

 	Else.