[m-rev.] for review: Make string.left and string.right not create unused substrings.

[Peter Wang]

---
 library/string.m | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/library/string.m b/library/string.m
index 97b005841..4363c0c5f 100644
--- a/library/string.m
+++ b/library/string.m
@@ -4171,7 +4171,7 @@ left(S1, N) = S2 :-
     left(S1, N, S2).
 
 left(String, Count, LeftString) :-
-    split(String, Count, LeftString, _RightString).
+    between(String, 0, Count, LeftString).
 
 left_by_codepoint(String, Count) = LeftString :-
     left_by_codepoint(String, Count, LeftString).
@@ -4184,8 +4184,8 @@ right(S1, N) = S2 :-
 
 right(String, RightCount, RightString) :-
     length(String, Length),
-    LeftCount = Length - RightCount,
-    split(String, LeftCount, _LeftString, RightString).
+    Start = Length - RightCount,
+    between(String, Start, Length, RightString).
 
 right_by_codepoint(String, RightCount) = RightString :-
     right_by_codepoint(String, RightCount, RightString).
-- 
2.23.0