Instances of multiple classes at once
Sat, 10 Feb 2001 16:48:30 +1100
On 08-Feb-2001, Dylan Thurston <email@example.com> wrote:
> On Thu, Feb 08, 2001 at 09:41:56PM +1100, Fergus Henderson wrote:
> > One point that needs to be resolved is the interaction with default methods.
> > Consider
> > class foo a where
> > f :: ...
> > f = ...
> > f2 :: ...
> > f2 = ...
> > class (foo a) => bar a where
> > b :: ...
> > instance bar T where
> > -- no definitions for f or f2
> > b = 42
> > Should this define an instance for `foo T'?
> > (I think not.)
> Whyever not?
Because too much Haskell code uses classes where the methods are
defined in terms of each other:
class Foo a where
-- you should define either f or f2
f :: ...
f = ... f2 ...
f2 :: ...
f2 = ... f ...
> Because there is no textual mention of class Foo in the
> instance for Bar?
Right, and because allowing the compiler to automatically generate
instances for class Foo without the programmer having considered
whether those instances are OK is too dangerous.
> Think about the case of a superclass with no methods;
> wouldn't you want to allow automatic instances in this case?
I think Marcin has a better idea:
| So maybe there should be a way to specify that default definitions
| are cyclic and some of them must be defined?
Fergus Henderson <firstname.lastname@example.org> | "I have always known that the pursuit
| of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh> | -- the last words of T. S. Garp.