Chaddaï Fouché wrote:
You can indeed already do that, except it won't be a single instance since list have a bucketful of interesting properties. A good starting is looking at what list is an instance of and trying to identify the set of instance which interest us in this case, Foldable and Functor are probably a good start, embodying most of the interesting way to access a data structure as a list (head and tail don't really make sense for most of the alternatives, except other "sequence" library which currently provide this functionality in an ad-hoc way, see Sequence and ByteString for example of that). An alternative is Traversable. Thanks!
But before digging into this, maybe I should rephrase myself by giving a more specific (although useless) example of what I mean. When I write: data Foo = Foo Int Int deriving (Show,Eq) instance Num Foo where fromInteger x = Foo x' x' where x' = fromInteger x _ + _ = error "Not relevant for example" _ * _ = error "Not relevant for example" abs _ = error "Not relevant for example" signum _ = error "Not relevant for example" x = 42::Foo I don't have to apply the Foo data constructor to lift the number 42 because I guess the compiler has builtin support for calling fromInteger (or fromRational). I even don't have to add the type annotation most of the time when the compiler can infer the type needed, which is very cool, sometimes a bit annoying. However, now I try the same for lists data Bar = Bar [Int] [Int] -- A List type class does not exist, so this cannot work instance List Bar where fromList x = Bar x x -- This does not work y = [1..10]::Bar So the only way to do this, is to create a constructor function like bar x = Bar x x which means the usage of lists in Haskell is not as general as numbers, in the sense one cannot take advantage of the builtin syntactic sugar of lists like [x,y,z] := x : (y : (z : [])) So if I would like to use this compact notation for e.g. creating a Set datatype, I would have to create special constructor functions (fromList or mkSet or whatever) Is this correct? If so, I'm sure a good reason must exist for this :) Thanks, Peter