[m-rev.] diff: avoid impure code in extras/trailed_update/samples/interpreter.m

[Fergus Henderson]

Estimated hours taken: 0.75
Branches: main

extras/trailed_update/samples/interpreter.m:
	Use `unsafe_promise_unique' rather than impure code.
	This is a cleaner solution.  (We don't want to be
	recommending the use of `impure' in sample code
	when better alternatives are available!)

Workspace: /home/ceres/fjh/mercury
Index: extras/trailed_update/samples/interpreter.m
===================================================================
RCS file: /home/mercury1/repository/mercury/extras/trailed_update/samples/interpreter.m,v
retrieving revision 1.5
diff -u -d -r1.5 interpreter.m
--- extras/trailed_update/samples/interpreter.m	13 Apr 2000 10:05:34 -0000	1.5
+++ extras/trailed_update/samples/interpreter.m	5 Nov 2002 06:27:22 -0000
@@ -74,34 +74,27 @@
 
 :- pred print_solutions(varset, map(var, my_var(S)), my_term(S),
 		store(S), database, io__state, io__state).
-:- mode print_solutions(in, in, in, mdi, in, di, uo) is det.
+:- mode print_solutions(in, in, in, mdi, in, di, uo) is cc_multi.
 
-/***
-% Alas, the following code gets a (spurious) unique mode error,
+% The call to unsafe_promise_unique here is needed because without it,
+% the following code gets a (spurious) unique mode error,
 % because the compiler thinks that `Store0' has inst `ground'
 % rather than `mostly_unique' when it is passed as a curried
 % argument of a higher-order term.  The compiler doesn't know
 % that unsorted_aggregate will only call its higher-order argument
 % once per forward execution.
 %
-% Instead, we use the impure code below.
+% It might be nicer to use do_while rather than unsorted_aggregate,
+% so that we can prompt the user after each solution to see if they
+% want to see the next solution.
 %
 print_solutions(VarSet, VarMap, MyGoal, Store0, Database) -->
 	unsorted_aggregate(
-		solve(Database, MyGoal, Store0),
+		(pred(Store::muo) is nondet :-
+			solve(Database, MyGoal, unsafe_promise_unique(Store0),
+				Store)),
 		write_solution(VarSet, VarMap, MyGoal)),
 	io__write_string("No (more) solutions.\n").
-***/
-
-:- pragma promise_pure(print_solutions/7).
-print_solutions(VarSet, VarMap, MyGoal, Store0, Database) -->
-	(
-		{ solve(Database, MyGoal, Store0, Store1) },
-		{ impure write_solution(VarSet, VarMap, MyGoal, Store1) },
-		{ fail }
-	;
-		io__write_string("No (more) solutions.\n")
-	).
 
 :- pred write_solution(varset, map(var, my_var(S)), my_term(S), store(S),
 			io__state, io__state).
@@ -114,18 +107,6 @@
 	{ my_term_to_term(MyGoal, Goal, VarSet0, VarSet, VarMap0, _VarMap,
 			Store0, _Store) },
 	term_io__write_term_nl(VarSet, Goal).
-
-:- impure pred write_solution(varset, map(var, my_var(S)), my_term(S),
-			store(S)).
-:- mode write_solution(in, in, in, mdi) is det.
-
-write_solution(VarSet0, VarToMyVarMap, MyGoal, Store0) :-
-	map__keys(VarToMyVarMap, Vars),
-	map__values(VarToMyVarMap, MyVars),
-	map__from_corresponding_lists(MyVars, Vars, VarMap0),
-	my_term_to_term(MyGoal, Goal, VarSet0, VarSet, VarMap0, _VarMap,
-			Store0, _Store),
-	impure unsafe_perform_io(term_io__write_term_nl(VarSet, Goal)).
 
 %-----------------------------------------------------------------------------%
 

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.
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