[Haskell-beginners] Fwd: Functional Dependencies

[dan portin]

Hi,

*unflt* was a poor choice of name. *flt2*, which flattens a datatype
containing a flattenable datatype, would have been a better name.

>
> If I was making a flatten class, I wouldn't want Monad as a constraint
> on the result container. Monad doesn't really say anything about
> "containers" - there are some containers that are Monads, but there
> are monads that aren't containers and there are containers that aren't
> monads.
>

You're right. My initial class was

class Monoid m => t -> m where
   flt :: t -> m

instance (Tree a) [a] where flt = fltTree
instance (Maybe a) [a] where flt = maybeToList

for some tree type like `data Tree a = Nil | Node a (Tree a) (Tree a)` (for
simplicity). The problem is that I couldn't figure out how to type the
"flatten twice" function, which required a declaration like

flt2 :: (F t' m) => t' t -> m

which is clearly incorrect (since it gives rise to a kind mismatch). "Monad"
and "Functor" were introduced just to get the definition of *unflt* to work,
and was unsatisfactory.

I need to be able to assert that *flt2* is a nested flattenable datatype,
and that the return type of the nested and containing datatype is the same
monoid. I can't figure out how to type this, however.
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