List.idr

module Data.List

%access public

using (xs : List a)
  ||| A proof that some element is found in a list
  data Elem : a -> List a -> Type where
       Here : Elem x (x :: xs)
       There : Elem x xs -> Elem x (y :: xs)

  instance Uninhabited (Elem {a} x []) where
       uninhabited Here impossible
       uninhabited (There p) impossible

  ||| Is the given element a member of the given list.
  |||
  ||| @x The element to be tested.
  ||| @xs The list to be checked against
  isElem : DecEq a => (x : a) -> (xs : List a) -> Dec (Elem x xs)
  isElem x [] = No absurd
  isElem x (y :: xs) with (decEq x y)
    isElem x (x :: xs) | (Yes refl) = Yes Here
    isElem x (y :: xs) | (No contra) with (isElem x xs)
      isElem x (y :: xs) | (No contra) | (Yes prf) = Yes (There prf)
      isElem x (y :: xs) | (No contra) | (No f) = No (mkNo contra f)
        where
          mkNo : {xs' : List a} ->
                 ((x' = y') -> _|_) -> (Elem x' xs' -> _|_) ->
                 Elem x' (y' :: xs') -> _|_
          mkNo f g Here = f refl
          mkNo f g (There x) = g x