On Sat, Jun 7, 2008 at 5:07 PM, Dmitri O.Kondratiev
{-- And what does the word "newbie" imply to you when answering my question? In what case using 'fundeps' and 'associated types' will make sence for this example? --}
Well, functional dependencies ("fundeps") and associated types are "advanced" extensions to the language (not Haskell98), and I personally would not consider it a good idea to approach them until you have a good handle on typeclasses. Actually, associated types are not that bad, save for the peculiar restrictions on what is possible to define (because of decidability issues). Fundeps are a bit stranger, IMO.. However, since you asked, I'll here try to expound a situation in which you might need associated types. Let's say you have a CellHash type that very efficiently stores key-value pairs, but only if the key is a string. So you have: data CellHash v = ... In the simple case, there is no way to make this an instance of Store, since Store requires a constructor with two parameters. And we can't really hack around it, because an instance of Store must support *any* key type by definition. Instead, we can use associated types, which associate *types* to instances in a class. These associated types will tell us about the acceptable keys and values of a particular instance: class Store s where type Key s :: * type Value s :: * put :: (Key s, Value s) -> s -> s get :: Key s -> s -> Value s The ":: *" annotates the kind of the result; for 'normal' value types it is *, for constructors with one parameter it is "* -> *", etc. It is straightforward to implement Store for your Cell and CellList. instance Store (Cell k v) where -- note that this is not "instance Store Cell" anymore type Key (Cell k v) = k type Value (Cell k v) = v -- as before But now we can make an instance for CellHash too: instance Store (CellHash v) where type Key (CellHash v) = String type Value (CellHash v) = v -- straightforward There's a situation where you would want to use associated types. But they are quite new, and can be hard to work with because of all the restrictions on their creation. For example, IIRC something like this would be illegal in the above instance: type Key (CellHash v) = HashKey String StringHashPolicy Because the right hand side is larger in terms of number of symbols than the left. This is untested, and I'm unsure of my knowledge here, so take the former with a grain of salt. Fundeps could be used to achieve the same thing, but are not without their caveats. Luke