[m-rev.] diff: fundeps, polymorphic instances

[Mark Brown]

On 30-Oct-2006, Julien Fischer <juliensf at csse.unimelb.edu.au> wrote:
> On Sun, 29 Oct 2006, Mark Brown wrote:
> 
> >Estimated hours taken: 16
> >Branches: main
> >
> >Support polymorphic instances of typeclasses with functional dependencies.
> >We do this by allowing type variables in the range arguments, which must be
> >distinct according to existing typeclass restrictions, to be determined
> >from type variables in the domain arguments by the functional dependencies
> >on the instance constraints.
> 
> That looks fine; it's probably worth a brief mention in the
> NEWS file.

Estimated hours taken: 0.25
Branches: main

NEWS:
	Mention the recent change to functional dependencies support.

Index: NEWS
===================================================================
RCS file: /home/mercury1/repository/mercury/NEWS,v
retrieving revision 1.427
diff -u -r1.427 NEWS
--- NEWS	12 Oct 2006 06:30:16 -0000	1.427
+++ NEWS	30 Oct 2006 00:56:05 -0000
@@ -3,6 +3,8 @@
 
 Changes to the Mercury language:
 
+* Some of the restrictions on typeclass instances have been relaxed, allowing
+  support for polymorphic instances with functional dependencies.
 * We now support trace goals, which can be used to print progress messages or
   log messages in the middle of arbitrary computations.
 * Mutables can now be marked as constant, which is useful when working with
@@ -44,6 +46,23 @@
 
 Changes to the Mercury language:
 
+* The restriction on typeclass instances that all type variables appearing in
+  the range of a functional dependency must be monomorphic has been relaxed.
+  We now support cases where the type variables in the range are determined
+  by the type variables in the domain, using the functional dependency
+  information from any instance constraints.
+
+  For example, given the typeclass
+
+	:- typeclass foo(A, B) <= (A -> B).
+
+  the following instance is now valid:
+
+	:- instance foo(list(S), list(T)) <= foo(S, T).
+
+  since the variable T, in the range, is determined from the variable S by the
+  functional dependencies on the foo(S, T) constraint.
+
 * A new language construct allows programmers to include debugging and/or
   logging code in the middle of arbitrary computations. Trace goals
   may have both compile time and run time conditions placed on their execution.
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