From: Markus Triska <triska@metalevel.at>
Date: Fri, 11 Aug 2023 20:46:46 +0000 (+0200)
Subject: ADDED: succ/2, part of the Prologue for Prolog
X-Git-Tag: v0.9.2~10^2~1
X-Git-Url: https://git.sagredo.dev/?a=commitdiff_plain;h=26c0b4fc75d1f2b80cf13de62a49a7851ef04a57;p=scryer-prolog.git

ADDED: succ/2, part of the Prologue for Prolog

Specification:

   https://www.complang.tuwien.ac.at/ulrich/iso-prolog/prologue#succ
---

diff --git a/src/lib/iso_ext.pl b/src/lib/iso_ext.pl
index afc28969..deccb345 100644
--- a/src/lib/iso_ext.pl
+++ b/src/lib/iso_ext.pl
@@ -14,6 +14,7 @@ but they're not part of the ISO Prolog standard at the moment.
                     partial_string/3,
                     partial_string_tail/2,
                     setup_call_cleanup/3,
+                    succ/2,
                     call_nth/2,
                     countall/2,
                     copy_term_nat/2,
@@ -112,6 +113,23 @@ bb_get(Key, Value) :-
     ).
 
 
+%% succ(?I, ?S).
+%
+% True iff S is the successor of the non-negative integer I.
+% At least one of the arguments must be instantiated.
+
+succ(I, S) :-
+    can_be(not_less_than_zero, I),
+    can_be(not_less_than_zero, S),
+    (   integer(I) ->
+        S is I+1
+    ;   integer(S) ->
+        S > 0,
+        I is S-1
+    ;   instantiation_error(succ/2)
+    ).
+
+
 % setup_call_cleanup.
 
 :- meta_predicate(call_cleanup(0, 0)).