Superclass defaults

From HaskellWiki

Revision as of 15:15, 29 October 2007 (edit) Twanvl (Talk | contribs) m (more typos) ← Previous diff		Current revision (18:07, 12 December 2007) (edit) (undo) Twanvl (Talk | contribs) m
Line 43:		Line 43:
	<li>		<li>
	A class alias is declared with the syntax		A class alias is declared with the syntax
-	<haskell>class alias Head => Alias a = (Class1 a, Class2 a, ..) where ...</haskell>	+	<haskell>class alias Ctx => Alias a = (Class1 a, Class2 a, ..) where ...</haskell>
	The body can contain default implementations of methods from <hask>Class1</hask>, <hask>Class2</hask> and their superclasses.		The body can contain default implementations of methods from <hask>Class1</hask>, <hask>Class2</hask> and their superclasses.
	</li><li>		</li><li>

---

Current revision

John Meacham's class alias proposal conflates two different issues:

• Class aliases: A single name for multiple classes.
• Method defaults in aliases: Allow defaults across classes.

I think it would be better to separate the two.

In particular, we would like to have a default implementation of 's . In John's propsal this is not possible, you need a new class alias which contains the default.

1 Superclass defaults

1. A class declaration can contain method defaults for methods in that class and in its (indirect) superclasses.
2. An instance declaration can specify multiple classes

   instance (Class1 a, Class2 a, ...) where ...

   Subject to the constraint that:

   • No class appears more than once in the list.
   • The arguments to each class are the same.
   • For each pair of classes,
     Class1
     and
     Class2
     in the list:
     • Class1
       is a (indirect) superclass of
       Class2
       , or
     • Class2
       is a (indirect) superclass of
       Class1
       , or
     • Class1
       and
       Class2
       have a common subclass
       Class3
       in the list.

     In other words, the superclass relation gives a connected acyclic graph with a single source, the most specific class in the heirarchy.

3. If no implementation of a method
   m
   is given in such an instance declaration, a default is used. Multiple classes can give a default for
   m
   . If both
   Class1
   and
   Class2
   have a default implementation, and
   Class1
   is a (indirect) superclass of
   Class2
   , then the default from
   Class1
   is ignored.

   It is an error if more than one default remains after this process.

2 Class aliases

The above can be extended with class aliasses, this part of the propsal is exactly the same as John's.

1. A class alias is declared with the syntax

   class alias Ctx => Alias a = (Class1 a, Class2 a, ..) where ...

   The body can contain default implementations of methods from
   Class1
   ,
   Class2
   and their superclasses.
2. In a context,
   Alias a
   is treated the same as
   (Head, Class1 a, Class2 a, ..)
   .
3. In an instance head,
   Alias a
   is treated the same as
   (_Alias a, Class1 a, Class2 a, ..)
   ,
   where
   _Alias
   is considered a subclass of
   Class1
   and
   Class2
   that contains the default methods from the class alias body.

   class (Head, Class1 a, Class2 a, ..) => _Alias a where ...

   The name
   _Alias
   is a fresh name. I.e. it can not be refered to, it is used only for the purpose of this specification.

Category: Proposals