On Feb 6, 2008 9:00 AM, Henning Thielemann wrote:

On Wed, 6 Feb 2008, Matthew Pocock wrote:

...

class Buildable b where empty :: b a --makes an empty b with elements of type a insert :: a -> b a -> b a --inserts the new element into the buildable

How can this interface be used both for lists and maps? [...]

One solution is to change the class slightly: class Buildable t x where empty :: t insert :: x -> t -> t instance Buildable (Map k a) (k, a) where empty = Map.empty insert = uncurry Map.insert -- Denis

On Feb 6, 2008 9:40 AM, Henning Thielemann wrote:

On Wed, 6 Feb 2008, Denis Bueno wrote:

...

On Feb 6, 2008 9:00 AM, Henning Thielemann wrote:

...

On Wed, 6 Feb 2008, Matthew Pocock wrote:

...

class Buildable b where empty :: b a --makes an empty b with elements of type a insert :: a -> b a -> b a --inserts the new element into the buildable

How can this interface be used both for lists and maps? [...]

One solution is to change the class slightly:

class Buildable t x where empty :: t insert :: x -> t -> t

instance Buildable (Map k a) (k, a) where empty = Map.empty insert = uncurry Map.insert

ok, maybe with functional dependency t -> x

I'm not sure about that. It's often convenient to have two instances, one like the one I gave above, and others involving something that embeds a key-value pair: type SomethingWithKV k a = KV {getKV :: (k, a)} instance Buildable (Map k a) (SomethingWithKV k a) where empty = Map.empty insert s m = uncurry Map.insert (getKV s) m I have done this before -- it's very convenient, and I think makes the code that uses empty and insert more robust, and easier to read. -- Denis

On Feb 6, 2008 10:36 AM, Henning Thielemann wrote:

...

type SomethingWithKV k a = KV {getKV :: (k, a)} instance Buildable (Map k a) (SomethingWithKV k a) where empty = Map.empty insert s m = uncurry Map.insert (getKV s) m

I have done this before -- it's very convenient, and I think makes the code that uses empty and insert more robust, and easier to read.

But why do you want to have the special type SomethingWithKV, but not MapSomething ?

Do you mean a map that maps SomethingWithKV to whatever, instead of k to v? My point is that if you add the functional dependency, you can't write instances for types which wrap what you're really interested in. You have to manually unwrap those types everywhere they interact with a map. This is tedious and error-prone. Being able to write such instances is useful, I think, though not always clearer in all cases. -- Denis