does this have a name (recusive datatypes)

Brian Huffman
Wed, 10 Apr 2002 11:58:17 -0700

On Wednesday 10 April 2002 11:07 am, Hal Daume III wrote:
> Does this have a name:
> > data S s a = Nil | S a (s (S s a))
> it seems to capture the essense of many recursive data structures.  With:
> > newtype Id a = Id a
> > newtype Pair a = Pair (a,a)
> probably more...

It seems to me that this is very similar to the "Mu" datatype from Mark 
Jones' paper "Functional Programming with Overloading and Higher-Order 
Polymorphism" ( He has examples of 
isomorphisms with lists and rose trees, etc. Here is an example:

data Mu f = In (f (Mu f))

type IntList = Mu IntListF
data IntListF a = Nil | Cons Int a

nil = In Nil
cons x xs = In (Cons x xs)

> Is there any theory about what types of recursive data structures can be
> captured with "S" and what types cannot?  It seems that those

The paper also mentions some stuff about anamorphisms and catamorphisms, 
which are apparently like generalized folds and unfolds, which you might be 
interested in. (I don't pretend to be an expert as I have just found out 
about them myself.)

> Also, if we want to write a show instance for S s, this seems to be
> impossible.  Is it?  If so, is this a weakness in Haskell (cyclic instance
> declarations) or is it theoretically not possible?
>  - Hal

Here is my attempt at a Show instance for S s (It works, but I'm not sure how 
to get rid of all the escaped quote marks):

data S s a = Nil | S a (s (S s a))

newtype Id a = Id a
instance Functor Id   where fmap f (Id i) = Id (f i)
instance Show a => Show (Id a) where
  show (Id a) = show a

instance Functor s => Functor (S s) where
   fmap f Nil = Nil
   fmap f (S a ss) = S (f a) (fmap (fmap f) ss)

instance (Functor s, Show a, Show (s String)) => Show (S s a) where
  show Nil = "Nil"
  show (S a ss) = show a ++ show (fmap show ss)

infixr 5 `cons`
cons x xs = S x (Id xs)

test :: S Id Int
test = 1 `cons` 2 `cons` 3 `cons` Nil

main = print test
-- 1"2\"3\\\"Nil\\\"\""