[m-rev.] for review: add builtin.unsafe_cast_any_to_ground/1

[Julien Fischer]

For review by anyone.

Estimated hours taken: 0.1
Branches: main

Move G12's any_util.unsafe_cast_ground/1 function into the Mercury standard
library.  (The name has changed.)  Most non-trivial uses of solver types 
require similar functionality so we may as well provide it in the standard
library.

library/builtin.m:
 	Add unsafe_cast_any_to_ground/1 as a builtin.

NEWS:
 	Announce the new function.

Julien.

Index: NEWS
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/NEWS,v
retrieving revision 1.457
diff -u -r1.457 NEWS
--- NEWS	1 May 2007 01:13:09 -0000	1.457
+++ NEWS	9 May 2007 07:15:13 -0000
@@ -17,6 +17,9 @@

  Changes to the Mercury standard library:

+* We have added a new builtin predicate, unsafe_cast_any_to_ground/1, that
+  can be useful when manipulating polymorphic values that have inst any.
+
  * Predicates and functions which create strings from lists of characters
    now fail, throw an exception or return an error value if a null character
    is found.  Unexpected null characters in strings are a potential source of
Index: library/builtin.m
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/library/builtin.m,v
retrieving revision 1.126
diff -u -r1.126 builtin.m
--- library/builtin.m	15 Jan 2007 02:23:59 -0000	1.126
+++ library/builtin.m	9 May 2007 07:15:13 -0000
@@ -148,9 +148,8 @@
      % not `unique', then the behaviour is undefined.  (If you lie to the
      % compiler, the compiler will get its revenge!)
      %
-:- pred unsafe_promise_unique(T::in, T::uo) is det.
-
  :- func unsafe_promise_unique(T::in) = (T::uo) is det.
+:- pred unsafe_promise_unique(T::in, T::uo) is det.

      % A synonym for fail/0; this name is more in keeping with Mercury's
      % declarative style rather than its Prolog heritage.
@@ -159,6 +158,17 @@

  %-----------------------------------------------------------------------------%

+    % This function is useful for converting polymorphic non-solver type
+    % values with inst any to inst ground (the compiler recognises that inst
+    % any is equivalent to ground for non-polymorphic non-solver type values.)
+    %
+    % Do not call this on solver type values unless you are absolutely sure
+    % they are semantically ground.
+    %
+:- func unsafe_cast_any_to_ground(T::ia) = (T::out) is det.
+
+%-----------------------------------------------------------------------------%
+
      % A call to the function `promise_only_solution(Pred)' constitutes a
      % promise on the part of the caller that `Pred' has at most one
      % solution, i.e. that `not some [X1, X2] (Pred(X1), Pred(X2), X1 \=
@@ -1109,6 +1119,29 @@

  semipure_true :-
      semipure private_builtin.semip.
+
+%-----------------------------------------------------------------------------%
+
+:- pragma foreign_proc("C",
+    unsafe_cast_any_to_ground(X::ia) = (Y::out),
+    [promise_pure, will_not_call_mercury, thread_safe, will_not_modify_trail],
+"
+    Y = X;
+").
+
+:- pragma foreign_proc("C#",
+    unsafe_cast_any_to_ground(X::ia) = (Y::out),
+    [promise_pure, will_not_call_mercury, thread_safe, will_not_modify_trail],
+"
+    Y = X;
+").
+
+:- pragma foreign_proc("Java",
+    unsafe_cast_any_to_ground(X::ia) = (Y::out),
+    [promise_pure, will_not_call_mercury, thread_safe, will_not_modify_trail],
+"
+    Y = X;
+").

  %-----------------------------------------------------------------------------%
  :- end_module builtin.

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