module test

:: DList a = Nil | Cons a (Link (DList a))
:: Link a = {prev :: a, next :: a}

fromList :: ([a] -> DList a)
fromList = go Nil
where
	go :: (DList a) [a] -> DList a
	go prev [] = Nil
	go prev [a:as] =
		let head = Cons a {prev=prev,next=tail}
		    tail = go head as
		in head

toList :: (DList a) -> [a]
toList Nil = []
toList (Cons el {next}) = [el:toList next]

nil :: DList a
nil = Nil

(:!) infixr 0 :: a (DList a) -> DList a
(:!) elh Nil = Cons elh {prev=nil,next=nil}
(:!) elh (Cons elt link) =
	let tail = Cons elt {link & prev=head}
	    head = Cons elh {prev=nil,next=tail}
	in head

Start =
	(toList (1 :! 2 :! 3 :! nil)
	,toList (fromList [1,2,3])
	)