mirror of
https://github.com/idris-lang/Idris2.git
synced 2024-12-18 08:42:11 +03:00
77 lines
2.5 KiB
Idris
77 lines
2.5 KiB
Idris
module Data.List.Elem
|
|
|
|
import Decidable.Equality
|
|
|
|
--------------------------------------------------------------------------------
|
|
-- List membership proof
|
|
--------------------------------------------------------------------------------
|
|
|
|
||| A proof that some element is found in a list.
|
|
public export
|
|
data Elem : a -> List a -> Type where
|
|
||| A proof that the element is at the head of the list
|
|
Here : Elem x (x :: xs)
|
|
||| A proof that the element is in the tail of the list
|
|
There : Elem x xs -> Elem x (y :: xs)
|
|
|
|
export
|
|
Uninhabited (Here = There e) where
|
|
uninhabited Refl impossible
|
|
|
|
export
|
|
Uninhabited (There e = Here) where
|
|
uninhabited Refl impossible
|
|
|
|
export
|
|
thereInjective : {0 e1, e2 : Elem x xs} -> There e1 = There e2 -> e1 = e2
|
|
thereInjective Refl = Refl
|
|
|
|
export
|
|
DecEq (Elem x xs) where
|
|
decEq Here Here = Yes Refl
|
|
decEq Here (There later) = No absurd
|
|
decEq (There later) Here = No absurd
|
|
decEq (There this) (There that) with (decEq this that)
|
|
decEq (There this) (There this) | Yes Refl = Yes Refl
|
|
decEq (There this) (There that) | No contra = No (contra . thereInjective)
|
|
|
|
export
|
|
Uninhabited (Elem {a} x []) where
|
|
uninhabited Here impossible
|
|
uninhabited (There p) impossible
|
|
|
|
||| An item not in the head and not in the tail is not in the list at all.
|
|
export
|
|
neitherHereNorThere : Not (x = y) -> Not (Elem x xs) -> Not (Elem x (y :: xs))
|
|
neitherHereNorThere xny xnxs Here = xny Refl
|
|
neitherHereNorThere xny xnxs (There xxs) = xnxs xxs
|
|
|
|
||| Check whether the given element is a member of the given list.
|
|
export
|
|
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 xny with (isElem x xs)
|
|
isElem x (y :: xs) | No xny | Yes xxs = Yes (There xxs)
|
|
isElem x (y :: xs) | No xny | No xnxs = No (neitherHereNorThere xny xnxs)
|
|
|
|
||| Remove the element at the given position.
|
|
public export
|
|
dropElem : (xs : List a) -> (p : Elem x xs) -> List a
|
|
dropElem (x :: ys) Here = ys
|
|
dropElem (x :: ys) (There p) = x :: dropElem ys p
|
|
|
|
||| Erase the indices, returning the numeric position of the element
|
|
public export
|
|
elemToNat : Elem x xs -> Nat
|
|
elemToNat Here = Z
|
|
elemToNat (There p) = S (elemToNat p)
|
|
|
|
||| Find the element with a proof at a given position, if it is valid
|
|
public export
|
|
indexElem : Nat -> (xs : List a) -> Maybe (x ** Elem x xs)
|
|
indexElem _ [] = Nothing
|
|
indexElem Z (y :: ys) = Just (y ** Here)
|
|
indexElem (S n) (y :: ys) = map (\(x ** p) => (x ** There p)) (indexElem n ys)
|