On Oct 12, 2005, at 8:00 PM, John Meacham wrote:
[longish proposal for class aliases]
Very nicely done, by the way.
== Notes ==
* class aliases are also able to introduce new superclass
constraints, such as
in the Num example we also want to enforce a (Eq a, Show a)
superclass
constraint. the interpretation is straightforward, Num in type
signatures
expands as if those were part of the alias and when declaring
instances the
existence of instances for the superclasses are checked, but not
filled in
automatically. I didn't show an example so as to not confuse the
basic idea
and because I have not come up with a syntax I am happy with.
(suggestions
welcome)
It sounds like there may be a simpler initial extension lurking under
this, see below.
...
* I had an earlier supertyping proposal you might know about, I
feel this is
a much better proposal even though it doesn't fully subsume my
supertyping
proposal, I feel it solves the problems it was meant to solve in
a cleaner
and easier to implement way.
Having read the previous proposal, I'm inclined to agree. I feel
like I can explain this one in a couple of minutes, and the listener
will be able to figure out most of the subtleties without additional
help.
* You may wonder why for the num example I put Additive a in the
class alias
even though it was already a superclass of AdditiveNegation. that is
because class aliases do not change the meaning of superclasses,
you need
to explicitly list a class if you want instance declarations to
propagate
methods to it. superclasses are checked just like normal in class
aliases.
This is the one possible exception to that. Again, see below.
* incidental but not earth-shattering benefits include being able to
declare an instance for a class and all its superclasses at once,
smarter defaults when you are combining related classes, and much
nicer type signatures by being able to create your own aliases for
common combinations of classes.
It seems to me this is a simpler extension here which might serve at
least as a conceptual stepping-stone to full class aliases---the
ability to declare an instance for a class and all its superclasses
at once. Given that ability, class aliases actually look like a
relatively simple extension.
One final thing which would be nice is the ability to define
instances of superclass methods in a subclass declaration. But this
takes things in a different direction entirely.
-Jan-Willem Maessen
--
John Meacham - ⑆repetae.net⑆john⑈
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