[m-rev.] For review: the maybe_error type is now polymorphic in the type of error

[Paul Bone]

For review by anyone.

The maybe_error type now takes a second type argument allowing users to
customise the type of error that can be represented.

library/maybe.m:
    As above.

NEWS:
    Add a dot point for this change.
---
 NEWS            | 3 +++
 library/maybe.m | 6 ++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/NEWS b/NEWS
index 7d6114c..39b4139 100644
--- a/NEWS
+++ b/NEWS
@@ -48,6 +48,9 @@ Changes to the Mercury standard library:
   list.all_true_corresponding/3, list.all_false_corresponding/3 and
   list.same_length3/3.
 
+* The maybe_error type in the maybe module is now also polymorphic in type
+  of the error it can represent.
+
 Changes to the Mercury compiler:
 
 * The option `--warn-non-tail-recursion' no longer requires
diff --git a/library/maybe.m b/library/maybe.m
index 4796799..1643556 100644
--- a/library/maybe.m
+++ b/library/maybe.m
@@ -35,9 +35,11 @@
     --->    ok
     ;       error(string).
 
-:- type maybe_error(T)
+:- type maybe_error(T) == maybe_error(T, string).
+
+:- type maybe_error(T, E)
     --->    ok(T)
-    ;       error(string).
+    ;       error(E).
 
 :- inst maybe_error(I)
     --->    ok(I)
-- 
1.8.1.3