[m-rev.] diff: add checked conversion of int64s to ints

[Julien Fischer]

Add checked conversion of int64s to ints.

library/int64.m:
     As above.

Julien.

diff --git a/library/int64.m b/library/int64.m
index d1e7763..caaf4c1 100644
--- a/library/int64.m
+++ b/library/int64.m
@@ -51,6 +51,20 @@
  % Conversion to int.
  %

+    % to_int(I64, I):
+    %
+    % Convert an int64 into an int.
+    % Fails if I64 is not in [int.min_int, int.max_int].
+    %
+:- pred to_int(int64::in, int::out) is semidet.
+
+    % det_to_int(I64) = I:
+    %
+    % Convert an int64 into an int.
+    % Throws an exception if I64 is not in [int.min_int, int.max_int].
+    %
+:- func det_to_int(int64) = int.
+
      % cast_to_int(I64) = I:
      %
      % Convert an int64 to an int.
@@ -398,6 +412,20 @@ cast_from_int(_) = _ :-

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

+to_int(I64, I) :-
+    I64 =< cast_from_int(int.max_int),
+    I64 >= cast_from_int(int.min_int),
+    I = cast_to_int(I64).
+
+det_to_int(I64) = I :-
+    ( if to_int(I64, IPrime) then
+        I = IPrime
+    else
+        error("int64.det_to_int: cannot convert int64 to int")
+    ).
+
+%---------------------------------------------------------------------------%
+
  :- pragma no_determinism_warning(cast_to_int/1).

  :- pragma foreign_proc("C",