[Haskell-cafe] Redefining superclass default methods in a subclass
zunino at di.unipi.it
Thu Jan 4 17:44:03 EST 2007
Brian Hulley wrote:
> Looking at some of the ideas in
> http://www.haskell.org/haskellwiki/The_Other_Prelude , it struck me that
> the class system at the moment suffers from the problem that as
> hierarchies get deeper, the programmer is burdened more and more by the
> need to cut-and-paste method definitions between instances because
> Haskell doesn't allow a superclass (or ancestor class) method default to
> be redefined in a subclass.
The class aliases proposal lists several similar shortcomings of the
current class system.
> Perhaps there is some reason this can't be done?
Some random thoughts:
How one would write instances? Using your Monad class, does
instance Monad F where
return = ...
(>>=) = ...
automatically define an instance for Applicative?
If it does: What if there already is such an instance? Which one gets
used for (>>)? The user-defined one or the Monad default? Is separate
compilation still possible? (If there is no instance right now, one
might pop out in another module...)
If it does not: How can one define it, without copy-and-pasting the default?
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