I think
instance Bar (Ret c) => Foo c where
...
will do what you are asking.
Alex
On Tue, Sep 29, 2009 at 10:25 PM, DNM wrote:
>
> Dan, thanks again for the response.
>
> I changed my code to use type families to let each Cls instance (actually a
> more complicated instance in my code) deter
You can require the associated type to have a particular instance, like
this:
class (Bar (Ret c)) => Cls c where
type Ret c
foo :: c -> Ret c
Another option is to use existential types:
data HasBar = forall a. Bar a => HasBar a
class Cls c where
foo :: c -> HasBar
You then have to wr
class Cls c where
type Ret c :: (Bar *) => * -- or a better name
foo :: c -> Ret c
which isn't legal Haskell.
OK, that's exactly the same thing I've met when developing
compose-trans. I needed guarantees that something is a Monad.
My way of doing that was to make "Bar" ("Monad" in my c
Dan, thanks again for the response.
I changed my code to use type families to let each Cls instance (actually a
more complicated instance in my code) determine which Bar instance type it
will return, but this didn't seem to work. The problem is that the client
of the typeclass instance methds ('
