From f33be724318bfed8d9d7856d38667669926006b3 Mon Sep 17 00:00:00 2001
From: Ana Pantilie <ana.pantilie95@gmail.com>
Date: Tue, 26 May 2026 18:51:42 +0300
Subject: [PATCH] WIP: contextual semantics

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 .../src/Untyped/ContextualSemantics.lagda.md  | 135 ++++++++++++++++++
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 create mode 100644 plutus-metatheory/src/Untyped/ContextualSemantics.lagda.md

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+---
+title: Contextual Semantics
+layout: page
+---
+
+```
+module Untyped.ContextualSemantics where
+```
+
+## Imports
+
+```
+open import Untyped
+open import Data.Nat using (ℕ; suc; _+_; _<_; _≤_)
+open import Data.List using (List; _++_)
+import Data.List as List
+open import Data.List.Relation.Unary.All using (All)
+open import Builtin
+open import Builtin.Signature using (args♯; Sig)
+open import Untyped.RenamingSubstitution using (_[_])
+open import Data.Vec using (Vec)
+import Data.List.NonEmpty as NE
+import Data.Vec as Vec
+import Data.Fin as Fin
+open import Relation.Binary.PropositionalEquality using (_≡_)
+
+```
+
+## TODO
+
+```
+variable
+  n : ℕ
+
+data B : n ⊢ → Set 
+data Value : n ⊢ → Set
+data A : {X : n ⊢} → B X → Set
+
+data B where
+  builtinB
+    : (b : Builtin)
+    → B {n} (builtin b)
+  appB
+    : {t₁ t₂ : n ⊢}
+    → B t₁
+    → Value t₂
+    → B (t₁ · t₂)
+  forceB
+    : {t : n ⊢}
+    → B t
+    → B (force t)
+
+β : {X : n ⊢} → B X → Builtin 
+β (builtinB b) = b
+β (appB b v) = β b
+β (forceB b) = β b
+
+||_|| : {X : n ⊢} → B X → ℕ
+||_|| (builtinB b) = 0 
+||_|| (appB b v) = 1 + || b ||
+||_|| (forceB b) = 1 + || b ||
+
+data A where
+  builtinA
+    : {b : Builtin}
+    → A {n} (builtinB b)
+  appA
+    : {t₁ t₂ : n ⊢} (b : B (t₁ · t₂))
+    → || b || < args♯ (signature (β b))
+    -- TODO: quantification?
+    → A {n} b
+  forceA
+    : {t : n ⊢} (b : B (force t))
+    → || b || < 1 -- TODO: we don't formalize quantifications, in practice I've only seen them applied directly to 'builtin b', but this should be fixed separately
+    → A {n} b
+
+data Value where
+  conᵥ : (t : TmCon) → Value {n} (con t)
+  delayᵥ : (t : n ⊢) → Value (delay t)
+  ƛᵥ : (t : suc n ⊢) → Value (ƛ t)
+  constrᵥ : (i : ℕ) (ts : List (n ⊢)) → All Value ts → Value (constr i ts)
+  bAppᵥ : {t : n ⊢} {b : B t} → A b → Value t
+
+data Frame : n ⊢ → Set where
+  □
+    : {t : n ⊢}
+    → Frame t
+  _ᶠ·_
+    : {t₁ : n ⊢}
+    → Frame t₁
+    → (t₂ : n ⊢)
+    → Frame (t₁ · t₂) 
+  _·ᶠ_
+    : {t₁ t₂ : n ⊢}
+    → Value t₁
+    → Frame t₂
+    → Frame (t₁ · t₂)
+  forceᶠ
+    : {t : n ⊢}
+    → Frame t
+    → Frame (force t)
+  constrᶠ
+    : {f : n ⊢} {vs : List (n ⊢)}
+    → (i : ℕ)
+    → All Value vs
+    → Frame f
+    → (ts : List (n ⊢))
+    → Frame (constr i (vs ++ List.[ f ] ++ ts))
+  caseᶠ
+    : {f : n ⊢}
+    → Frame f
+    → (ts : List (n ⊢))
+    → Frame (case f ts)
+
+-- TODO: implement
+postulate
+  multiApp : {t : n ⊢} → Value t → List (n ⊢) → n ⊢
+
+-- TODO: rename constructors
+data _⟶_ : n ⊢ → n ⊢ → Set where
+  lamapp
+    : {t₁ : suc n ⊢} {t₂ : n ⊢}
+    → Value t₂
+    → (ƛ t₁ · t₂) ⟶ (t₁ [ t₂ ])
+  forcedelay
+    : {t : n ⊢}
+    → (force (delay t)) ⟶ t
+  caseconstr
+    : {i l : ℕ} {vs ts : List (n ⊢)} {f : n ⊢} 
+    → All Value vs
+    -- TODO: lookup i in ts, we need to deal with empty ts's and index out of bounds
+    → (case (constr i vs) ts) ⟶ (multiApp {!   !} vs)
+
+
+```
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