Source code on Github
------------------------------------------------------------------------
-- The Agda standard library
--
-- List membership and some related definitions
------------------------------------------------------------------------

{-# OPTIONS --cubical-compatible --safe #-}

open import Relation.Binary

module Data.List.Membership.Setoid {c } (S : Setoid c ) where

open import Function.Base using (_∘_; id; flip)
open import Data.Fin.Base using (Fin; zero; suc)
open import Data.List.Base as List using (List; []; _∷_; length; lookup)
open import Data.List.Relation.Unary.Any
  using (Any; index; map; here; there)
open import Data.Product as Prod using (; _×_; _,_)
open import Relation.Unary using (Pred)
open import Relation.Nullary using (¬_)

open Setoid S renaming (Carrier to A)

------------------------------------------------------------------------
-- Definitions

infix 4 _∈_ _∉_

_∈_ : A  List A  Set _
x  xs = Any (x ≈_) xs

_∉_ : A  List A  Set _
x  xs = ¬ x  xs

------------------------------------------------------------------------
-- Operations

open Data.List.Relation.Unary.Any using (_∷=_; _─_) public

mapWith∈ :  {b} {B : Set b}
           (xs : List A)  (∀ {x}  x  xs  B)  List B
mapWith∈ []       f = []
mapWith∈ (x  xs) f = f (here refl)  mapWith∈ xs (f  there)

------------------------------------------------------------------------
-- Finding and losing witnesses

module _ {p} {P : Pred A p} where

  find :  {xs}  Any P xs   λ x  x  xs × P x
  find (here px)   = (_ , here refl , px)
  find (there pxs) = Prod.map id (Prod.map there id) (find pxs)

  lose : P Respects _≈_    {x xs}  x  xs  P x  Any P xs
  lose resp x∈xs px = map (flip resp px) x∈xs

------------------------------------------------------------------------
-- DEPRECATED
------------------------------------------------------------------------
-- Please use the new names as continuing support for the old names is
-- not guaranteed.

-- Version 0.16

map-with-∈ = mapWith∈
{-# WARNING_ON_USAGE map-with-∈
"Warning: map-with-∈ was deprecated in v0.16.
Please use mapWith∈ instead."
#-}