Source code on Github
------------------------------------------------------------------------
-- The Agda standard library
--
-- Lexicographic ordering of lists
------------------------------------------------------------------------

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

module Data.List.Relation.Binary.Lex.Core where

open import Data.Empty using (; ⊥-elim)
open import Data.Unit.Base using (; tt)
open import Data.Product using (_×_; _,_; proj₁; proj₂; uncurry)
open import Data.List.Base using (List; []; _∷_)
open import Function.Base using (_∘_; flip; id)
open import Level using (Level; _⊔_)
open import Relation.Nullary using (¬_)
open import Relation.Binary.Core using (Rel)
open import Data.List.Relation.Binary.Pointwise.Base
   using (Pointwise; []; _∷_; head; tail)

private
  variable
    a ℓ₁ ℓ₂ : Level

-- The lexicographic ordering itself can be either strict or non-strict,
-- depending on whether type P is inhabited.

data Lex {A : Set a} (P : Set)
         (_≈_ : Rel A ℓ₁) (_≺_ : Rel A ℓ₂) :
         Rel (List A) (a  ℓ₁  ℓ₂) where
  base : P                              Lex P _≈_ _≺_ []       []
  halt :  {y ys}                       Lex P _≈_ _≺_ []       (y  ys)
  this :  {x xs y ys} (x≺y : x  y)    Lex P _≈_ _≺_ (x  xs) (y  ys)
  next :  {x xs y ys} (x≈y : x  y)
         (xs<ys : Lex P _≈_ _≺_ xs ys)  Lex P _≈_ _≺_ (x  xs) (y  ys)

----------------------------------------------------------------------
-- Lexicographic orderings, using a strict ordering as the base

Lex-< : {A : Set a} (_≈_ : Rel A ℓ₁) (_≺_ : Rel A ℓ₂) 
        Rel (List A) (a  ℓ₁  ℓ₂)
Lex-< = Lex 

Lex-≤ : {A : Set a} (_≈_ : Rel A ℓ₁) (_≺_ : Rel A ℓ₂) 
        Rel (List A) (a  ℓ₁  ℓ₂)
Lex-≤ = Lex