module Data.CoList
%access public export
%default total
||| Idris will know that it always can produce a new element in finite time
codata CoList : Type -> Type where
Nil : CoList a
(::) : a -> CoList a -> CoList a
||| Append two CoLists
(++) : CoList a -> CoList a -> CoList a
(++) [] right = right
(++) (x::xs) right = x :: (xs ++ right)
implementation Semigroup (CoList a) where
(<+>) = (++)
implementation Monoid (CoList a) where
neutral = []
implementation Functor CoList where
map f [] = []
map f (x::xs) = f x :: map f xs
implementation Show a => Show (CoList a) where
show xs = "[" ++ show' "" 20 xs ++ "]" where
show' : String -> (n : Nat) -> (xs : CoList a) -> String
show' acc Z _ = acc ++ "..."
show' acc (S n) [] = acc
show' acc (S n) [x] = acc ++ show x
show' acc (S n) (x :: xs) = show' (acc ++ (show x) ++ ", ") n xs
||| Take the first `n` elements of `xs`
|||
||| If there are not enough elements, return the whole coList.
||| @ n how many elements to take
||| @ xs the coList to take them from
takeCo : (n : Nat) -> (xs : CoList a) -> List a
takeCo Z _ = []
takeCo (S n) [] = []
takeCo (S n) (x::xs) = x :: takeCo n xs
||| The unfoldr builds a list from a seed value. In some cases, unfoldr can undo a foldr operation.
|||
||| ``` idris example
||| unfoldr (\b => if b == 0 then Nothing else Just (b, b-1)) 10
||| ```
|||
unfoldr : (a -> Maybe (a, a)) -> a -> CoList a
unfoldr f x =
case f x of
Just (y, new_x) => y :: (unfoldr f new_x)
_ => []