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+---
+title: Contextual Semantics
+layout: page
+---
+
+```
+module Untyped.ContextualSemantics where
+```
+
+## Imports
+
+```
+open import Untyped
+open import Data.Nat using (ℕ; suc; _+_; _<_; _≤_; _≤ᵇ_)
+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; fv)
+open import Untyped.RenamingSubstitution using (_[_])
+open import Data.Vec using (Vec)
+open import Data.Maybe using (Maybe; just; nothing)
+open import Data.Bool using (if_then_else_)
+import Data.List.NonEmpty as List⁺
+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 ||
+
+||_||ₐ : {X : n ⊢} → B X → ℕ
+||_||ₐ (builtinB b) = 0 
+||_||ₐ (appB b v) = 1 + || b ||
+||_||ₐ (forceB b) = || b ||
+
+||_||ᶠ : {X : n ⊢} → B X → ℕ
+||_||ᶠ (builtinB b) = 0 
+||_||ᶠ (appB b v) = || b ||
+||_||ᶠ (forceB b) = 1 + || b ||
+
+data A where
+  builtinA
+    : {t : Builtin}
+    → (b : B {n} (builtin t))
+    → A {n} b
+  appA
+    : {t₁ t₂ : n ⊢}
+    → (b : B (t₁ · t₂))
+    -- we may apply app a number of times equal to the number of term arguments it has
+    → || b ||ₐ ≤ args♯ (signature (β b))
+    -- check that the builtin is not fully saturated
+    → || b || < args♯ (signature (β b)) + fv (signature (β b))
+    → A {n} b
+  forceA
+    : {t : n ⊢}
+    → (b : B (force t))
+    -- a builtin may be forced the number of times equal to the number of type arguments it has
+    → || b ||ᶠ ≤ fv (signature (β b))
+    -- check that the builtin is not fully saturated
+    → || b || < args♯ (signature (β b)) + fv (signature (β b))
+    → A {n} b
+
+βᴬ : {t : n ⊢} {b : B t} → A b → Builtin
+βᴬ (builtinA b) = β b
+βᴬ (appA b x x₁) = β b
+βᴬ (forceA b x x₁) = β b
+
+||_||ᴬ : {t : n ⊢} {b : B t} → A b → ℕ
+||_||ᴬ (builtinA b) = || b ||
+||_||ᴬ (appA b x x₁) = || b ||
+||_||ᴬ (forceA b x x₁) = || b ||
+
+||_||ᶠᴬ : {t : n ⊢} {b : B t} → A b → ℕ
+||_||ᶠᴬ (builtinA b) = || b ||ᶠ
+||_||ᶠᴬ (appA b x x₁) = || b ||ᶠ
+||_||ᶠᴬ (forceA b x x₁) = || b ||ᶠ
+
+-- this doesn't typecheck because i need to use the information
+-- that a is a partial application in order to lookup the next argument;
+-- that means that the x is always smaller than the total number of args
+-- and i should know that from the fact that a is a partial application
+nextᴬ
+  : {n : ℕ} {t : n ⊢} {b : B t}
+  → (a : A b)
+  → Maybe (Sig.fv⋆ (signature (βᴬ a)) / Sig.fv♯ (signature (βᴬ a)) ⊢⋆)
+nextᴬ a with || a ||ᴬ
+... | 0 =
+  if ( || a ||ᶠᴬ ≤ᵇ fv (signature (βᴬ a)) )
+    then nothing
+    else (just (List⁺.head (Sig.args (signature (βᴬ a)))))
+... | x = just (Vec.lookup (List⁺.toVec (Sig.args (signature (βᴬ a)))) {! Fin.fromℕ x  !})
+
+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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