From 28c9f2f9f85b4adbb867abccb1efc176c7ca7361 Mon Sep 17 00:00:00 2001
From: imiel <imiel@engineer.city>
Date: Sun, 19 Apr 2026 15:50:59 +0000
Subject: [PATCH] Replace hardcoded universe levels with a proper level
 language and constraint solving

---
 BidirTT/Check.lean    |  8 +++----
 BidirTT/Eval.lean     |  8 +++----
 BidirTT/Examples.lean |  2 ++
 BidirTT/Level.lean    | 52 +++++++++++++++++++++++++++++++++++++++++++
 BidirTT/Pretty.lean   |  2 +-
 BidirTT/Syntax.lean   |  6 +++--
 BidirTT/Value.lean    |  2 +-
 Tests.lean            | 23 ++++++++++++++++++-
 8 files changed, 90 insertions(+), 13 deletions(-)
 create mode 100644 BidirTT/Level.lean

diff --git a/BidirTT/Check.lean b/BidirTT/Check.lean
index cf7d820..82a5880 100644
--- a/BidirTT/Check.lean
+++ b/BidirTT/Check.lean
@@ -122,7 +122,7 @@ mutual
             let _ := level
             pure (.idElim motive' r' target' eq', resultTy)
         | _ => throw s!"expected Id type in idElim, got {showTy cxt.lvl eqTy}"
-    | .univ i => pure (.univ i, .univ (i + 1))
+    | .univ i => pure (.univ i.normalise, .univ i.succ')
     | .app t u => do
         let (t', tty) ← infer cxt t
         match tty with
@@ -150,12 +150,12 @@ mutual
         let (a', i) ← inferUniverse cxt a
         let va ← eval cxt.env a'
         let (b', j) ← inferUniverse (cxt.bind x va) b
-        pure (.pi a' b', .univ (Nat.max i j))
+        pure (.pi a' b', .univ (i.max' j))
     | .sig x a b => do
         let (a', i) ← inferUniverse cxt a
         let va ← eval cxt.env a'
         let (b', j) ← inferUniverse (cxt.bind x va) b
-        pure (.sig a' b', .univ (Nat.max i j))
+        pure (.sig a' b', .univ (i.max' j))
     | .ann t a => do
         let (a', _) ← inferUniverse cxt a
         let va ← eval cxt.env a'
@@ -171,7 +171,7 @@ mutual
     | .lam _ _  => throw "cannot infer type of lambda, use an annotation"
     | .pair _ _ => throw "cannot infer type of pair, use an annotation"
 
-  partial def inferUniverse (cxt : Cxt) (r : Raw) : TCM (Tm × Nat) := do
+  partial def inferUniverse (cxt : Cxt) (r : Raw) : TCM (Tm × Level) := do
     let (t, ty) ← infer cxt r
     match ty with
     | .univ level => pure (t, level)
diff --git a/BidirTT/Eval.lean b/BidirTT/Eval.lean
index e9cd58b..e15516b 100644
--- a/BidirTT/Eval.lean
+++ b/BidirTT/Eval.lean
@@ -67,7 +67,7 @@ mutual
         let vy ← eval env y
         let vp ← eval env p
         vIdElim vm vr vy vp
-    | _,   .univ i     => pure (.univ i)
+    | _,   .univ i     => pure (.univ i.normalise)
     | env, .letE _ t u => do
         let vt ← eval env t
         eval (vt :: env) u
@@ -187,7 +187,7 @@ mutual
         let qu ← quote l u
         pure (.id qa qt qu)
     | _, .refl     => pure .refl
-    | _, .univ i   => pure (.univ i)
+    | _, .univ i   => pure (.univ i.normalise)
 end
 
 private def andThen (lhs : EvalM Bool) (rhs : Unit → EvalM Bool) : EvalM Bool := do
@@ -239,7 +239,7 @@ mutual
     | _, _, _ => pure false
 
   partial def conv : Lvl → Val → Val → EvalM Bool
-    | _, .univ i,     .univ j     => pure (i == j)
+    | _, .univ i,     .univ j     => Level.eqv i j
     | l, .pi a c,     .pi a' c'   =>
         andThen (conv l a a') fun _ => do
           let b  ← cApp c (.neu (.var l))
@@ -299,7 +299,7 @@ mutual
 end
 
 partial def sub : Lvl → Val → Val → EvalM Bool
-  | _, .univ i,     .univ j     => pure (i <= j)
+  | _, .univ i,     .univ j     => Level.leq i j
   | l, .pi a c,     .pi a' c'   =>
       andThen (sub l a' a) fun _ => do
         let b  ← cApp c (.neu (.var l))
diff --git a/BidirTT/Examples.lean b/BidirTT/Examples.lean
index e78d998..78c09a1 100644
--- a/BidirTT/Examples.lean
+++ b/BidirTT/Examples.lean
@@ -46,6 +46,8 @@ def fstDepPair : Raw := .fst depPairAnn
 
 def sndDepPair : Raw := .snd depPairAnn
 
+def univMax : Raw := .univ (.max 0 1)
+
 def natTwo : Raw :=
   .succ (.succ .zero)
 
diff --git a/BidirTT/Level.lean b/BidirTT/Level.lean
new file mode 100644
index 0000000..793ac2e
--- /dev/null
+++ b/BidirTT/Level.lean
@@ -0,0 +1,52 @@
+namespace BidirTT
+
+inductive Level where
+  | zero : Level
+  | succ : Level → Level
+  | max  : Level → Level → Level
+  deriving Repr, Inhabited, BEq, DecidableEq
+
+def Level.ofNat : Nat → Level
+  | 0 => .zero
+  | n + 1 => .succ (Level.ofNat n)
+
+instance (n : Nat) : OfNat Level n where
+  ofNat := Level.ofNat n
+
+partial def Level.eval : Level → Nat
+  | .zero => 0
+  | .succ l => l.eval + 1
+  | .max l r => Nat.max l.eval r.eval
+
+def Level.normalise (l : Level) : Level :=
+  .ofNat l.eval
+
+def Level.succ' (l : Level) : Level :=
+  .normalise (.succ l)
+
+def Level.max' (l r : Level) : Level :=
+  .normalise (.max l r)
+
+def Level.pretty (l : Level) : String :=
+  s!"{l.eval}"
+
+abbrev LevelConstraint := Level × Level
+
+def solveLevelConstraints (constraints : List LevelConstraint) : Except String Unit := do
+  match constraints.find? fun (lhs, rhs) => lhs.eval > rhs.eval with
+  | some (lhs, rhs) =>
+      throw s!"unsatisfiable level constraint {lhs.pretty} <= {rhs.pretty}"
+  | none =>
+      pure ()
+
+def Level.leq (lhs rhs : Level) : Except String Bool := do
+  match solveLevelConstraints [(lhs, rhs)] with
+  | Except.ok _ => pure true
+  | Except.error _ => pure false
+
+def Level.eqv (lhs rhs : Level) : Except String Bool := do
+  match solveLevelConstraints [(lhs, rhs), (rhs, lhs)] with
+  | Except.ok _ => pure true
+  | Except.error _ => pure false
+
+end BidirTT
diff --git a/BidirTT/Pretty.lean b/BidirTT/Pretty.lean
index 74596ab..a33cd07 100644
--- a/BidirTT/Pretty.lean
+++ b/BidirTT/Pretty.lean
@@ -71,7 +71,7 @@ mutual
         let (py, nextFresh) := prettyTmWith names nextFresh y
         let (pp, nextFresh) := prettyTmWith names nextFresh p
         (s!"(idElim {pm} {pr} {py} {pp})", nextFresh)
-    | .univ i     => (s!"U{i}", nextFresh)
+    | .univ i     => (s!"U{i.pretty}", nextFresh)
     | .letE a t u =>
         let x := s!"x{nextFresh}"
         let (pa, nextFresh) := prettyTmWith names (nextFresh + 1) a
diff --git a/BidirTT/Syntax.lean b/BidirTT/Syntax.lean
index f0b140a..9776abc 100644
--- a/BidirTT/Syntax.lean
+++ b/BidirTT/Syntax.lean
@@ -1,3 +1,5 @@
+import BidirTT.Level
+
 namespace BidirTT
 
 abbrev Name := String
@@ -23,7 +25,7 @@ inductive Raw where
   | id   : Raw → Raw → Raw → Raw
   | refl : Raw
   | idElim : Name → Name → Raw → Raw → Raw → Raw → Raw
-  | univ : Nat → Raw
+  | univ : Level → Raw
   | letE : Name → Raw → Raw → Raw → Raw
   | ann  : Raw → Raw → Raw
   deriving Repr, Inhabited, BEq, DecidableEq
@@ -49,7 +51,7 @@ inductive Tm where
   | id   : Tm → Tm → Tm → Tm
   | refl : Tm
   | idElim : Tm → Tm → Tm → Tm → Tm
-  | univ : Nat → Tm
+  | univ : Level → Tm
   | letE : Tm → Tm → Tm → Tm
   deriving Repr, Inhabited, BEq, DecidableEq
 
diff --git a/BidirTT/Value.lean b/BidirTT/Value.lean
index 220a01e..a3cfd43 100644
--- a/BidirTT/Value.lean
+++ b/BidirTT/Value.lean
@@ -27,7 +27,7 @@ mutual
     | empty : Val
     | id   : Val → Val → Val → Val
     | refl : Val
-    | univ : Nat → Val
+    | univ : Level → Val
 
   inductive Closure where
     | mk : List Val → Tm → Closure
diff --git a/Tests.lean b/Tests.lean
index f45672b..7ef79e1 100644
--- a/Tests.lean
+++ b/Tests.lean
@@ -25,8 +25,12 @@ structure TestCase where
 def cases : List TestCase := [
   ⟨"U0 is typed by U1",          Examples.univ0,
     .okTy (.univ 1)⟩,
+  ⟨"U(max 0 1) is typed by U2",  Examples.univMax,
+    .okTy (.univ 2)⟩,
   ⟨"U0 subsumes into U2",        .ann (.univ 0) (.univ 2),
     .okTy (.univ 2)⟩,
+  ⟨"U0 subsumes into U(max 0 2)", .ann (.univ 0) (.univ (.max 0 2)),
+    .okTy (.univ 2)⟩,
   ⟨"Nat is typed by U0",         .nat,
     .okTy (.univ 0)⟩,
   ⟨"succ zero infers Nat",       (.succ .zero),
@@ -164,6 +168,22 @@ def runNeutralRepresentationChecks : IO Bool := do
     IO.println "fail  stuck eliminators stay in the neutral fragment"
     pure false
 
+def runLevelSolverChecks : IO Bool := do
+  let satOk :=
+    match solveLevelConstraints [(.max 0 1, 1), (1, .succ 1)] with
+    | Except.ok _ => true
+    | Except.error _ => false
+  let unsatOk :=
+    match solveLevelConstraints [(.succ 1, 1)] with
+    | Except.error err => containsText err "unsatisfiable level constraint"
+    | Except.ok _ => false
+  if satOk && unsatOk then
+    IO.println "pass  level constraints are solved consistently"
+    pure true
+  else
+    IO.println "fail  level constraints are solved consistently"
+    pure false
+
 def runPrettyPrinterChecks : IO Bool := do
   match checkTop Examples.idAnn with
   | .ok (tm, ty) =>
@@ -192,8 +212,9 @@ def main : IO UInt32 := do
   let results ← cases.mapM runCase
   let safetyOk ← runInternalSafetyChecks
   let neutralOk ← runNeutralRepresentationChecks
+  let levelOk ← runLevelSolverChecks
   let prettyOk ← runPrettyPrinterChecks
-  let allResults := results ++ [safetyOk, neutralOk, prettyOk]
+  let allResults := results ++ [safetyOk, neutralOk, levelOk, prettyOk]
   let failed := allResults.countP (· == false)
   if failed == 0 then
     IO.println s!"\n{allResults.length} passed, 0 failed"