[m-rev.] diff: optimise skip whitespace in parsing_utils

[Peter Wang]

Branches: main, 11.07

In parsing_utils, don't use a wrapper over the provided skip whitespace
predicate.  This incurred an additional higher-order call, and the use of the
`maybe' type incurred an extra memory allocation per successful call.

library/parsing_utils.m:
	As above.

diff --git a/library/parsing_utils.m b/library/parsing_utils.m
index 595da30..40dc9d5 100644
--- a/library/parsing_utils.m
+++ b/library/parsing_utils.m
@@ -336,7 +336,7 @@
     --->    src(
                 input_length        ::  int,
                 input_string        ::  string,
-                skip_ws_func        ::  func(src, ps) = maybe(ps),
+                skip_ws_pred        ::  skip_whitespace_pred,
 
                 furthest_offset     ::  mutvar(int),
                 % This mutable records the progress of the parser
@@ -399,17 +399,7 @@ new_src_and_ps(InputString, SkipWS, Src, PS) :-
     promise_pure (
         impure new_mutvar(fail_message_info(0, no), ErrorInfoMutVar),
         impure new_mutvar(0, FurthestOffsetMutvar),
-        % Convert the skip whitespace predicate to a function to
-        % avoid having to use a inst other than ground for src.
-        SkipWSFunc =
-            ( func(S, PS0) = MaybePS :-
-                ( SkipWS(S, _, PS0, PS1) ->
-                    MaybePS = yes(PS1)
-                ;
-                    MaybePS = no
-                )
-            ),
-        Src = src(string.length(InputString), InputString, SkipWSFunc,
+        Src = src(string.length(InputString), InputString, SkipWS,
             FurthestOffsetMutvar, ErrorInfoMutVar),
         PS = 0
     ).
@@ -419,8 +409,40 @@ new_src_and_ps(InputString, SkipWS, Src, PS) :-
 :- pred skip_whitespace(src::in, ps::in, ps::out) is semidet.
 
 skip_whitespace(Src, PS0, PS) :-
-    SkipWS = Src ^ skip_ws_func,
-    yes(PS) = SkipWS(Src, PS0).
+    SkipWS0 = Src ^ skip_ws_pred,
+    unsafe_skip_ws_pred_cast(SkipWS0, SkipWS),
+    SkipWS(Src, _, PS0, PS).
+
+:- pred unsafe_skip_ws_pred_cast(skip_whitespace_pred::in,
+    skip_whitespace_pred::out(parser)) is det.
+
+:- pragma foreign_proc("C",
+    unsafe_skip_ws_pred_cast(SkipWS0::in, SkipWS::out(parser)),
+    [will_not_call_mercury, promise_pure, thread_safe],
+"
+    SkipWS = SkipWS0;
+").
+
+:- pragma foreign_proc("C#",
+    unsafe_skip_ws_pred_cast(SkipWS0::in, SkipWS::out(parser)),
+    [will_not_call_mercury, promise_pure, thread_safe],
+"
+    SkipWS = SkipWS0;
+").
+
+:- pragma foreign_proc("Java",
+    unsafe_skip_ws_pred_cast(SkipWS0::in, SkipWS::out(parser)),
+    [will_not_call_mercury, promise_pure, thread_safe],
+"
+    SkipWS = SkipWS0;
+").
+
+:- pragma foreign_proc("Erlang",
+    unsafe_skip_ws_pred_cast(SkipWS0::in, SkipWS::out(parser)),
+    [will_not_call_mercury, promise_pure, thread_safe],
+"
+    SkipWS = SkipWS0
+").
 
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