[Haskell-cafe] Re: class default method proposal
apfelmus at quantentunnel.de
Tue Dec 11 12:22:16 EST 2007
Jules Bean wrote:
> David Menendez wrote:
>> Duncan Coutts wrote:
>>> So my suggestion is that we let classes declare default
>>> implementations of methods from super-classes.
>> It creates ambiguity if two classes declare defaults for a common
>> My standard example involves Functor, Monad, and Comonad. Both Monad
>> and Comonad could provide a default implementation for fmap. But let's
>> say I have a type which is both a Monad and a Comonad: which default
>> implementation gets used?
>> I'm disappointed to see this objection isn't listed on the wiki.
> Doesn't sound like a very big problem. That would just be a compile time
> error ("More than one default for fmap possible for Foo, please reslve
And how would you resolve that ambiguity?
module Control.Functor.UsefulStuff (hylo) where
hylo :: Functor f => (a -> f a) -> (f b -> b) -> a -> b
hylo f g = g . fmap (hylo f g) . f
module BANG where
import Foo (Foo)
import Control.Functor.UsefulStuff (hylo)
bar :: Bar -> Foo Bar
baz :: Foo Baz -> Baz
bang = hylo bar baz
The problem is that the ambiguity may arise by just importing different
modules while not having access to the offending call to fmap .
Also note that as much as I'd like explicit import/export of type class
instances, the current implicit and global export is no accident, it's
crucial for well-definedness. See also the second half of
In other words, the main problem of all those superclass/explicit
import/export proposals is that there are no proofs of the fact that
they only allow well-defined programs. The homework isn't done yet,
discussing adoption is too early.
More information about the Haskell-Cafe