module Filter where

data Bool : Set where
  false : Bool
  true  : Bool

infixr 40 _::_

data List (A : Set) : Set where
  []   : List A
  _::_ : A -> List A -> List A

filter : {A : Set} -> (A -> Bool) -> List A -> List A
filter p [] = []
filter p (x :: xs) with p x
...                | true  = x :: filter p xs
...                | false = filter p xs

infix 20 _⊆_

data _⊆_ {A : Set} : List A -> List A -> Set where
  stop : [] ⊆ []
  drop : forall {xs y ys} -> xs ⊆ ys -> xs ⊆ y :: ys
  keep : forall {x xs ys} -> xs ⊆ ys -> x :: xs ⊆ x :: ys

subset : {A : Set}(p : A -> Bool)(xs : List A) -> filter p xs ⊆ xs
subset p [] = stop
subset p (x :: xs) with p x
...                | true  = keep (subset p xs)
...                | false = drop (subset p xs)