Termination check on unionWith

Question

I\'m having a problem with termination checking, very similar to the one described in this question and also this Agda bug report/feature request.

The problem is convinc

Answer 1

Here's an alternative based on sized types, based on the answer to this later question. You can pick up the Data.Extended-key module from here, or you can tweak the code below so that it uses Data.AVL.Extended-key from the standard library instead.

Preamble:

{-# OPTIONS --sized-types #-}

open import Relation.Binary renaming (IsStrictTotalOrder to IsSTO)
open import Relation.Binary.PropositionalEquality as P using (_≡_)

-- A list of (key, value) pairs, sorted by key in strictly descending order.
module Temp
   {                                                                    

                                                        
            

Answer 2

It seems the first solution proposed for the earlier list-merge question does indeed work here, but only under Agda version 2.3.3+. Here's the full version, with a slightly nicer syntax for ∷.

data FiniteMap (l : Key) : Set (k ⊔ v ⊔ ℓ) where
   [] : FiniteMap l
   _∷[_]_ : (kv : KV) → let k = proj₁ kv in l < k → (m : FiniteMap k) → FiniteMap l

-- Split into two definitions to help the termination checker.
unionWith : ∀ {l} → Op₂ Value → Op₂ (FiniteMap l)
unionWith′ : ∀ {l} → Op₂ Value → (kv : KV) → let k = proj₁ kv in l < k → FiniteMap k → Op₁ (FiniteMap l)

unionWith _ [] [] = []
unionWith _ [] m = m
unionWith _ m [] = m
unionWith _⊕_ ((k , v) ∷[ k<l ] m) ((k′ , v′) ∷[ k′<l ] m′) with compare k k′
... | tri< k<k′ _ _ = (k , v) ∷[ k<l ] (unionWith _⊕_ m ((k′ , v′) ∷[ k<k′ ] m′))
... | tri≈ _ k≡k′ _ rewrite P.sym k≡k′ = (k , v ⊕ v′) ∷[ k<l ] (unionWith _⊕_ m m′)
... | tri> _ _ k′<k = (k′ , v′) ∷[ k′<l ] (unionWith′ _⊕_ (k , v) k′<k m m′)

unionWith′ _ (k , v) l<k m [] = (k , v) ∷[ l<k ] m
unionWith′ _⊕_ (k , v) l<k m ((k′ , v′) ∷[ k′<l ] m′) with compare k k′
... | tri< k<k′ _ _ = (k , v) ∷[ l<k ] (unionWith _⊕_ m ((k′ , v′) ∷[ k<k′ ] m′))
... | tri≈ _ k≡k′ _ rewrite P.sym k≡k′ = (k , v ⊕ v′) ∷[ l<k ] (unionWith _⊕_ m m′)
... | tri> _ _ k′<k = (k′ , v′) ∷[ k′<l ] (unionWith′ _⊕_ (k , v) k′<k m m′)