for review: mention quantification changes in NEWS

[Fergus Henderson]

Since the change to the quantification rule can break existing
code, I thought it worth mentioning in the NEWS file.
Do you think that is a good idea?

--------------------

Estimated hours taken: 0.5

NEWS:
	Mention that we've changed the rules for quantification of lambda
	expressions.

Index: NEWS
===================================================================
RCS file: /home/mercury1/repository/mercury/NEWS,v
retrieving revision 1.133
diff -u -r1.133 NEWS
--- NEWS	1999/03/07 14:02:28	1.133
+++ NEWS	1999/03/14 05:58:21
@@ -1,6 +1,31 @@
 NEWS since Mercury release 0.8:
 -------------------------------
 
+* We've made a small change to the rule for quantification of lambda
+  expressions.  
+
+  The new rule is that all variables in the arguments of lambda
+  expressions are treated as locally quantified to that lambda expression.
+  For function lambda expressions, variables in the result term
+  use the normal quantification rules.  See the "Data-terms" section
+  of the "Syntax" chapter of the Mercury language reference manual
+  for details.
+  
+  Previously, the exact quantification rule for lambda expressions was
+  not documented, but the compiler would locally quantify variables in
+  function return values, and it would only locally quantify variables
+  occuring at the top level of an argument term, not those occurring in
+  subterms.  Both of these were rather surprising for functional
+  programmers.
+
+  It is possible that this change may break some existing code using
+  predicate lambda expressions with compound terms as arguments, but we
+  expect this to be extremely rare.  If it does occur, the compiler
+  will issue a warning about variables having overlapping scopes, and
+  the work-around is simple: use a fresh variable for the lambda
+  predicate argument and unify it with the compound term in the body of
+  the lambda expression.
+
 * We've added a new predicate to the Mercury standard library:
 	bag__count_value/3.	
 

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  |  "I have always known that the pursuit
WWW: <http://www.cs.mu.oz.au/~fjh>  |  of excellence is a lethal habit"
PGP: finger fjh at 128.250.37.3        |     -- the last words of T. S. Garp.