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