Version 1 (modified by chak, 8 years ago) (diff) |
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## Class Families

Our translation of data families in combination with the desugaring of classes into data types motivates the concept of **indexed class families**, which turns out to be quite useful for generalising class APIs for commonly used data structures.

### An example

As a motivating example take the following problem from John Hughes' *Restricted Data Types*. Suppose we want to implement a set API as a type class. Then, we find that the signature

insert :: Set s => a -> s a -> s a

is too general. We need additional type constraints whose exact form *depends on the type constructor* we use to construct the sets; i.e., it varies on an instance by instance basis. For list, we just need `Eq`, but for sets as finite maps, we need `Ord`.

With indexed class families, we can define a set class as follows:

class Set s where class C s a empty :: s a insert :: C s a => a -> s a -> s a

Here, the **associated class** `C` of `Set` is indexed by the class parameter `s`.

In instances for sets as lists

instance Set [] where class Eq a => C [] a empty = [] insert x s | x `elem` s = s | otherwise = x:s instance C [] a

and sets as finite maps

newtype MapSet a = MapSet (Data.Map.Map a ()) instance Set MapSet where class Ord a => C MapSet a empty = Data.Map.empty insert x s = Data.Map.insert x () s instance C MapSet a

we instantiate `C` differently for different type indexes. The class family instances have no members in this case, but use existing classes as superclass to supply `insert` with the equality and ordering methods, respectively.