[Haskell-cafe] How to escape from typecheck error: Duplicate instance declarations ?

Alexander Solla alex.solla at gmail.com
Sat Jan 26 07:17:57 CET 2013

On Fri, Jan 25, 2013 at 12:39 PM, <s9gf4ult at gmail.com> wrote:

> **
> > This has the code smell of trying to use typeclasses for OOP.  That
> won't work.  (Yes, really.)
> I am not trying to use OOP, I am just writing some typecasting at all.
> > This would be correct.  Constraints on an instance are applied *after*
> the instance is selected, so when Haskell is looking for an instance, these
> two are identical.
> I didn't understand why these two instances are identical ? The
> constraints are different and OverlappingInstances should permit
> overlapping typeclasses in constraints and select more specific instance
> clause.

They are identical because constraints don't "count" for deciding that a
type is in a class.   For the purposes of deciding if a type is in a class,

instance Foo (Bar a)
instance Fizz a => Foo (Bar a)
instance Fuzz a => Foo (Bar a)

are exactly the same, and all three are therefore overlapping instances.
 None is more specific, because they all refer to the same type -- (Bar a).

Also, you can just use Typeable instead of that downcasting stuff.
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