[Haskell-cafe] Issues(Bugs?) with GHC Type Families

[David Menendez]

On Thu, Mar 6, 2008 at 9:52 PM, Ryan Ingram <ryani.spam at gmail.com> wrote:
>  This is actually a general issue with the way typeclasses are defined
>  in Haskell; the accepted solution is what the "Show" typeclass does:
>
>
>  > class C a where
>  >    c :: a
>  >    cList :: [a]
>  >    cList = [c,c]
>
>
>  > instance C Char where
>  >    c = 'a'
>  >    cList = "a" -- replaces instance for String above
>
> > instance C a => C [a] where
>  >    c = cList
>  > cc = c :: String
>
>  I don't really like this solution; it feels like a hack and it relies
>  on knowing when you define the typeclass what sort of overlap you
>  expect.

The pattern above can be thought of as a specialized version of this:

class C a where
    c :: a

class ListOfC a where
    listOfC :: [a]

instance (ListOfC a) => C [a] where
    c = listOfC

This works for an arbitrary number of type constructors (good!), but
requires a ListOfC instance for everything you might want to put into
a list (cumbersome!).

You can make things slightly less verbose by putting a sensible
default in ListOfC,

class (C a) => ListOfC a where
    listOfC :: [a]
    listOfC = default_c

-- 
Dave Menendez <dave at zednenem.com>
<http://www.eyrie.org/~zednenem/>