[m-rev.] for review: Move some code from std_util.m to maybe.m

[Paul Bone]

For review by anyone.

---

Move some code from std_util.m to maybe.m

Deprecate the maybe_pred and maybe_func code in std_lib.m and add a
predicate version of pred_to_maybe and a new function func_to_maybe to
maybe.m.

library/std_util.m:
library/maybe.m:
    As above.

NEWS:
    Update news file.
---
 NEWS               |  7 +++++++
 library/maybe.m    | 15 ++++++++++++++-
 library/std_util.m |  5 +++++
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/NEWS b/NEWS
index ec77eea..4f5ac05 100644
--- a/NEWS
+++ b/NEWS
@@ -279,6 +279,8 @@ Changes to the Mercury standard library:
    - fold2_maybe/6
    - maybe_default/2
    - pred_to_maybe/1
+   - pred_to_maybe/2
+   - func_to_maybe/1
 
 * The following predicates and functions have been added to the calendar
   module:
@@ -393,6 +395,11 @@ Changes to the Mercury standard library:
     to group the prettyprinter parameters together in a value of the type
     that was designed for this purpose.
 
+* We have deprecated the following functions from the std_util module:
+
+  - maybe_pred/3
+  - maybe_func/2
+
 Changes to the Mercury compiler:
 
 * We have added a new option --warn-dead-preds. While the existing option
diff --git a/library/maybe.m b/library/maybe.m
index 89a5da1..2351d8e 100644
--- a/library/maybe.m
+++ b/library/maybe.m
@@ -2,6 +2,7 @@
 % vim: ft=mercury ts=4 sw=4 et wm=0 tw=0
 %---------------------------------------------------------------------------%
 % Copyright (C) 1994-2006, 2010-2011 The University of Melbourne.
+% Copyright (C) 2016 The Mercury Team.
 % This file may only be copied under the terms of the GNU Library General
 % Public License - see the file COPYING.LIB in the Mercury distribution.
 %---------------------------------------------------------------------------%
@@ -173,9 +174,15 @@
     %
     % Make a maybe value from a semidet predicate.
     %
-:- func pred_to_maybe(pred(X)) = maybe(X).
+:- func pred_to_maybe(pred(T)) = maybe(T).
 :- mode pred_to_maybe(pred(out) is semidet) = out is det.
 
+:- pred pred_to_maybe(pred(T), maybe(T)).
+:- mode pred_to_maybe(pred(out) is semidet, out) is det.
+
+:- func func_to_maybe((func) = T) = maybe(T).
+:- mode func_to_maybe((func) = out is semidet) = out is det.
+
     % Return the value from within the maybe or a default value if there is
     % none.
     %
@@ -218,12 +225,18 @@ map_fold3_maybe(P, yes(T0), yes(T), !A, !B, !C) :-
 maybe_is_yes(yes(X), X).
 
 pred_to_maybe(Pred) = Result :-
+    pred_to_maybe(Pred, Result).
+
+pred_to_maybe(Pred, Result) :-
     ( if Pred(X) then
         Result = yes(X)
     else
         Result = no
     ).
 
+func_to_maybe(PF) =
+    ( if Y = apply(PF) then yes(Y) else no ).
+
 maybe_default(_, yes(X)) = X.
 maybe_default(D, no) = D.
 
diff --git a/library/std_util.m b/library/std_util.m
index 39b3854..9b55de3 100644
--- a/library/std_util.m
+++ b/library/std_util.m
@@ -2,6 +2,7 @@
 % vim: ft=mercury ts=4 sw=4 et wm=0 tw=0
 %---------------------------------------------------------------------------%
 % Copyright (C) 1994-2006, 2008 The University of Melbourne.
+% Copyright (C) 2016 The Mercury Team.
 % This file may only be copied under the terms of the GNU Library General
 % Public License - see the file COPYING.LIB in the Mercury distribution.
 %---------------------------------------------------------------------------%
@@ -53,9 +54,13 @@
     % X succeeds, Y is bound to `yes(Z)' where Z is the output of the
     % call, or to `no' if the call fails.
     %
+    % use maybe.pred_to_maybe instead
+:- pragma obsolete(maybe_pred/3).
 :- pred maybe_pred(pred(T1, T2), T1, maybe(T2)).
 :- mode maybe_pred(pred(in, out) is semidet, in, out) is det.
 
+    % use maybe.func_to_maybe instead
+:- pragma obsolete(maybe_func/2).
 :- func maybe_func(func(T1) = T2, T1) = maybe(T2).
 :- mode maybe_func(func(in) = out is semidet, in) = out is det.
 
-- 
2.7.4