[Haskell-cafe] Redefining superclass default methods in a subclass
brianh at metamilk.com
Sat Jan 6 06:51:35 EST 2007
Bulat Ziganshin wrote:
> Hello Brian,
> Thursday, January 4, 2007, 10:00:05 PM, you wrote:
>> 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.
> i've runned into this problem with Streams library. finally i've
> decided to wrote bodies of such methods outside of class:
Thanks for the workaround, which solves the need to copy and paste method
bodies though not the problem of having to write out instance decls for a
potentially large chain of classes leading to the subclass of interest. Part
of the motivation for proposing that a superclass method default could be
redefined in a subclass (or a particular instance) is that it would allow
some refactorings of the class hierarchy without affecting code that just
uses the subclass - in particular it would allow existing code using Monad
to compile unchanged even when Monad moved down to Functor => Applicative =>
Monad because return and >>= are enough to get completely defined instances
for Functor and Applicative.
Best regards, Brian.
More information about the Haskell-Cafe