On Mon, Jul 11, 2011 at 8:21 AM, Arlen Cuss
Hi Christopher,
As my first stab at it, it seemed like I should be able to create my own heterogeneous "list" data type -- i.e., a "list" data type that can contain elements of different types. (like, [3,'a',True], for example)
One problem I can see would be dealing with the contents of such a list. Imagine you have a list with many different types in it. Would every type appear in the type of the list? If so, that would have to be a consideration before you even get to the "=" sign in your data type definition. If not, whence do they come?
That approach can be found in the HList library where indeed the heterogeneous list at the data level is echoed by an heterogeneous list at the type level, the basic blocks are :
data HNil = HNil data HCons e l = HCons e l
but with just that you would have no guarantee that the "l" from HCons was HNil or HCons so you add a typeclass :
class HList l instance HList HNil instance (HList l) => HList (HCons e l)
and you don't export the data constructors but smart constructors instead :
hNil :: HNil hNil = HNil
hCons :: (HList l) => e -> l -> HCons e l hCons x xs = HCons x xs
And thus you have an heterogeneous list which you can manipulate usefully, though HList add plenty of stuff to allow cool tricks. -- Jedaï