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
1 Superclass defaults
- A class declaration can contain method defaults for methods in that class and in its (indirect) superclasses.
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, andClass1in the list:Class2
- is a (indirect) superclass ofClass1, orClass2
- is a (indirect) superclass ofClass2, orClass1
- andClass1have a common subclassClass2in the list.Class3
In other words, the superclass relation gives a connected acyclic graph with a single source, the most specific class in the heirarchy.
If no implementation of a method is given in such an instance declaration, a default is used. Multiple classes can give a default form. If bothmandClass1have a default implementation, andClass2is a (indirect) superclass ofClass1, then the default fromClass2is ignored.Class1
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.
A class alias is declared with the syntax
The body can contain default implementations of methods from
class alias Head => Alias a = (Class1 a, Class2 a, ..) where ...,Class1and their superclasses.Class2
In a context, is treated the same asAlias a.(Head, Class1 a, Class2 a, ..)
In an instance head, is treated the same asAlias a,(_Alias a, Class1 a, Class2 a, ..)
whereis considered a subclass of_AliasandClass1that contains the default methods from the class alias body.Class2The name
class (Head, Class1 a, Class2 a, ..) => _Alias a where ...is a fresh name. I.e. it can not be refered to, it is used only for the purpose of this specification._Alias