[Haskell-cafe] Re: Classes: functional dependency (type -> value)

[Heinrich Apfelmus]

Belka wrote:
> Hello, communion people!
> 
> I seek for your advice in a matter, where it's really hard for me to
> determine a good programming style.
> Here's the problem. I'm generalizing multiple authorization procedures to
> one, using class definition. (if of any interest, the code is in the end.) 
> The problem essense is folowing:
> ----------------
> data SomeRole = Role1 | Role2 | Role3
> 
> class SomeClass a b c | a -> b, c where
>       f1 :: ...
>       f2 :: ...
>       ...
>       fn :: ...
>       role :: SomeRole -- <-- here is the problem
> 
> I want to have a fuctional dependency from a type "a" on a value of *role*,
> so that I could easily "inspect" the *role* from within any other class
> members.
> Is it possible? Or do I rougly violate some style traditions?

The problem is that when you write

   role

there is no way to choose the right instance? That is, where does the
compiler get  a, b, c  from when looking just at an invocation of  role
? Therefore, the type of  role  has to involve  a  , for example as in

   class SomeClass a b c ... where
        ...
        role :: a -> SomeRole

and used as

   role (undefined :: Foo)



That being said, I think that type classes are not what you want here. I
suggest to simply use a regular data type

   data SomeThing a b c = SomeThing {
         f1   :: ...
       , f2   :: ...
         ...
       , fn   :: ...
       , role :: SomeRole
       }

Remember that  f1, f2, ...  can be functions, this is a functional
language, after all! "Instances" are then simply a concrete value, like
for example

   thething :: SomeThing Foo Bar Baz
   thething = SomeThing {
        f1 = id
     ,  f2 = filter (>3) . map length
     ,  ...
     ,  role = Role1
     }


Regards,
apfelmus

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