prelude.poet

  ;;;; pairs
  
  (define-type Pair a b)
  (define-type Empty)
  
  (define Pair:foldl
    f x (Pair a b) -> (foldl f (f x a) b))

  (define Pair:first
    (Pair a _) -> (Some a))

  (define Pair:rest
    (Pair _ (Empty)) -> Empty
    (Pair _ a)       -> a)
  
;;;; snip

 (define-interface Foldable
    foldl XS (F X XS)
    foldr XS (F X XS))
  
  (extend-interface Foldable
    array
    (foldl array:foldl)
    (foldr array:foldr)
    array-like
    (foldl array:foldl)
    (foldr array:foldr)
    Pair
    (foldl Pair:foldl)
    Empty
    (foldl (fn (_ X _) X)))