From 3f316c5c9265618fe7095cc39c4cb10909cbe468 Mon Sep 17 00:00:00 2001
From: Peter Mikkelsen <peter@pmikkelsen.com>
Date: Fri, 16 Jul 2021 20:09:02 +0000
Subject: Implement a bit more of prolog flag predicates set_prolog_flag/2 and
 current_prolog_flag/2

---
 stdlib.pl | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

(limited to 'stdlib.pl')

diff --git a/stdlib.pl b/stdlib.pl
index 0986781..d093eed 100644
--- a/stdlib.pl
+++ b/stdlib.pl
@@ -549,3 +549,60 @@ nl :-
 nl(S) :-
 	put_char(S, '
 '). % This should really be \n
+
+% flags
+set_prolog_flag(Flag, Value) :-
+	is_nonvar(Flag),
+	is_nonvar(Value),
+	is_atom(Flag),
+	is_prolog_flag(Flag),
+	is_appropriate_flag_value(Flag, Value),
+	is_modifiable_flag(Flag),
+	'$set_prolog_flag'(Flag, Value).
+
+current_prolog_flag(Flag, Value) :-
+	is_atom_or_var(Flag),
+	( atom(Flag)
+	-> is_prolog_flag(Flag)
+	; true
+	),
+	current_prolog_flags(FlagsAndValues),
+	member(flag(Flag, Value), FlagsAndValues).
+
+is_prolog_flag(Flag) :-
+	member(Flag, 
+		[ bounded
+		, max_integer
+		, min_integer
+		, integer_rounding_function
+		, char_conversion
+		, debug
+		, max_arity
+		, unknown
+		, double_quotes]),
+	!
+	; domain_error(prolog_flag, Flag).
+
+is_modifiable_flag(Flag) :-
+	member(Flag, [char_conversion, debug, unknown, double_quotes]),
+	!
+	; permission_error(modify, flag, Flag).
+
+is_appropriate_flag_value(Flag, Value) :-
+	appropriate_flag_values(Flag, Values), 
+	member(Value, Values),
+	!
+	; domain_error(flag_value, Flag + Value).
+
+appropriate_flag_values(bounded, [true, false]).
+appropriate_flag_values(max_integer, [Val]) :-
+	current_prolog_flag(max_integer, Val).
+appropriate_flag_values(min_integer, [Val]) :-
+	current_prolog_flag(min_integer, Val).
+appropriate_flag_values(integer_rounding_function, [down, toward_zero]).
+appropriate_flag_values(char_conversion, [on, off]).
+appropriate_flag_values(debug, [on, off]).
+appropriate_flag_values(max_arity, [Val]) :-
+	current_prolog_flag(max_arity).
+appropriate_flag_values(unknown, [error, fail, warning]).
+appropriate_flag_values(double_quotes, [chars, codes, atom]).
\ No newline at end of file
-- 
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