[Haskell-cafe] Why superclass' instances are bad idea?

Wvv vitea3v at rambler.ru
Thu Sep 26 22:02:57 CEST 2013

Thanks a lot!
This makes clear. I haven't noticed before that OverlappingInstances don't
look at constraint!

John Lato-2 wrote
> This line
>     instance Monad m => Applicative m where
> tells the compiler "Every type (of the appropriate kind) is an instance of
> Applicative.  And it needs to have a Monad instance as well."
> That's what Edward means when he said that it means "every Applicative is
> a
> Monad".  Theoretically the statement makes no sense, but that's what this
> instance head means.  Everything is Applicative, and it also needs a Monad
> instance to use that Applicative.
> Consider what happens for something that isn't a Monad, e.g. ZipList.
> Since it's not a Monad, it would need its own instance
>     instance Applicative ZipList where
>     ...
> But now you'd need to enable OverlappingInstances, because ZipList matches
> both this instance and the general one you've defined above (GHC doesn't
> consider constraints when matching instance heads).  OverlappingInstances
> is much more problematic than the other extensions because it could (and
> almost certainly would in this case) give rise to incoherence (see the
> warning under
> http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#instance-overlap
> ).
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