From a154e2b98c0b1a74766eeee4a557751e6ad5de8d Mon Sep 17 00:00:00 2001
From: imiel <imiel@engineer.city>
Date: Sun, 19 Apr 2026 04:17:45 +0000
Subject: [PATCH] Initial

---
 .gitignore            |   4 ++
 BidirTT.lean          |   7 ++
 BidirTT/Check.lean    | 105 +++++++++++++++++++++++++++++
 BidirTT/Context.lean  |  30 +++++++++
 BidirTT/Eval.lean     | 150 ++++++++++++++++++++++++++++++++++++++++++
 BidirTT/Examples.lean |  72 ++++++++++++++++++++
 BidirTT/Pretty.lean   |  19 ++++++
 BidirTT/Syntax.lean   |  32 +++++++++
 BidirTT/Value.lean    |  27 ++++++++
 Main.lean             |  30 +++++++++
 README.md             |  17 +++++
 Tests.lean            | 106 +++++++++++++++++++++++++++++
 lakefile.lean         |  17 +++++
 lean-toolchain        |   1 +
 14 files changed, 617 insertions(+)
 create mode 100644 .gitignore
 create mode 100644 BidirTT.lean
 create mode 100644 BidirTT/Check.lean
 create mode 100644 BidirTT/Context.lean
 create mode 100644 BidirTT/Eval.lean
 create mode 100644 BidirTT/Examples.lean
 create mode 100644 BidirTT/Pretty.lean
 create mode 100644 BidirTT/Syntax.lean
 create mode 100644 BidirTT/Value.lean
 create mode 100644 Main.lean
 create mode 100644 README.md
 create mode 100644 Tests.lean
 create mode 100644 lakefile.lean
 create mode 100644 lean-toolchain

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..a76ef6a
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,4 @@
+.lake/
+build/
+lakefile.olean
+lake-manifest.json
diff --git a/BidirTT.lean b/BidirTT.lean
new file mode 100644
index 0000000..8d5226d
--- /dev/null
+++ b/BidirTT.lean
@@ -0,0 +1,7 @@
+import BidirTT.Syntax
+import BidirTT.Value
+import BidirTT.Pretty
+import BidirTT.Eval
+import BidirTT.Context
+import BidirTT.Check
+import BidirTT.Examples
diff --git a/BidirTT/Check.lean b/BidirTT/Check.lean
new file mode 100644
index 0000000..ce80f3b
--- /dev/null
+++ b/BidirTT/Check.lean
@@ -0,0 +1,105 @@
+import BidirTT.Context
+import BidirTT.Eval
+import BidirTT.Pretty
+
+namespace BidirTT
+
+abbrev TCM := Except String
+
+private def showTy (l : Lvl) (v : Val) : String :=
+  match quote l v with
+  | Except.ok tm     => prettyTm tm
+  | Except.error err => s!"<ill-scoped value: {err}>"
+
+mutual
+  partial def check (cxt : Cxt) : Raw → Val → TCM Tm
+    | .lam x t, .pi a c => do
+        let bodyTy ← cApp c (.var cxt.lvl)
+        let t' ← check (cxt.bind x a) t bodyTy
+        pure (.lam t')
+    | .pair t u, .sig a c => do
+        let t' ← check cxt t a
+        let vt ← eval cxt.env t'
+        let bodyTy ← cApp c vt
+        let u' ← check cxt u bodyTy
+        pure (.pair t' u')
+    | .letE x a t u, ty => do
+        let (a', _) ← inferUniverse cxt a
+        let va := eval cxt.env a'
+        let va ← va
+        let t' ← check cxt t va
+        let vt ← eval cxt.env t'
+        let u' ← check (cxt.define x vt va) u ty
+        pure (.letE a' t' u')
+    | r, ty => do
+        let (t', ty') ← infer cxt r
+        if (← conv cxt.lvl ty' ty) then
+          pure t'
+        else
+          throw s!"type mismatch: expected {showTy cxt.lvl ty}, got {showTy cxt.lvl ty'}"
+
+  partial def infer (cxt : Cxt) : Raw → TCM (Tm × Val)
+    | .var x =>
+        match cxt.lookup x with
+        | some (i, a) => pure (.var i, a)
+        | none        => throw s!"unknown variable {x}"
+    | .univ i => pure (.univ i, .univ (i + 1))
+    | .app t u => do
+        let (t', tty) ← infer cxt t
+        match tty with
+        | .pi a c => do
+            let u' ← check cxt u a
+            let vu ← eval cxt.env u'
+            let bodyTy ← cApp c vu
+            pure (.app t' u', bodyTy)
+        | _ => throw s!"expected Pi type in application, got {showTy cxt.lvl tty}"
+    | .fst t => do
+        let (t', tty) ← infer cxt t
+        match tty with
+        | .sig a _ => pure (.fst t', a)
+        | _ => throw s!"expected Sigma type in .1, got {showTy cxt.lvl tty}"
+    | .snd t => do
+        let (t', tty) ← infer cxt t
+        match tty with
+        | .sig _ c => do
+            let vt ← eval cxt.env t'
+            let fstv ← vFst vt
+            let bodyTy ← cApp c fstv
+            pure (.snd t', bodyTy)
+        | _ => throw s!"expected Sigma type in .2, got {showTy cxt.lvl tty}"
+    | .pi 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 (.pi a' b', .univ (Nat.max i 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))
+    | .ann t a => do
+        let (a', _) ← inferUniverse cxt a
+        let va ← eval cxt.env a'
+        let t' ← check cxt t va
+        pure (t', va)
+    | .letE x a t u => do
+        let (a', _) ← inferUniverse cxt a
+        let va ← eval cxt.env a'
+        let t' ← check cxt t va
+        let vt ← eval cxt.env t'
+        let (u', uty) ← infer (cxt.define x vt va) u
+        pure (.letE a' t' u', uty)
+    | .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
+    let (t, ty) ← infer cxt r
+    match ty with
+    | .univ level => pure (t, level)
+    | _ => throw s!"expected a universe, got {showTy cxt.lvl ty}"
+end
+
+def checkTop (r : Raw) : TCM (Tm × Val) :=
+  infer Cxt.empty r
+
+end BidirTT
diff --git a/BidirTT/Context.lean b/BidirTT/Context.lean
new file mode 100644
index 0000000..abf70dc
--- /dev/null
+++ b/BidirTT/Context.lean
@@ -0,0 +1,30 @@
+import BidirTT.Value
+
+namespace BidirTT
+
+structure Cxt where
+  env   : Env
+  types : List (Name × Val)
+  lvl   : Lvl
+  deriving Inhabited
+
+def Cxt.empty : Cxt := ⟨[], [], 0⟩
+
+def Cxt.bind (cxt : Cxt) (x : Name) (a : Val) : Cxt :=
+  { env   := .var cxt.lvl :: cxt.env
+  , types := (x, a) :: cxt.types
+  , lvl   := cxt.lvl + 1 }
+
+def Cxt.define (cxt : Cxt) (x : Name) (v a : Val) : Cxt :=
+  { env   := v :: cxt.env
+  , types := (x, a) :: cxt.types
+  , lvl   := cxt.lvl + 1 }
+
+private def lookupGo : Name → List (Name × Val) → Nat → Option (Nat × Val)
+  | _, [],              _ => none
+  | x, (y, a) :: rest,  i => if x == y then some (i, a) else lookupGo x rest (i+1)
+
+def Cxt.lookup (cxt : Cxt) (x : Name) : Option (Nat × Val) :=
+  lookupGo x cxt.types 0
+
+end BidirTT
diff --git a/BidirTT/Eval.lean b/BidirTT/Eval.lean
new file mode 100644
index 0000000..46e43aa
--- /dev/null
+++ b/BidirTT/Eval.lean
@@ -0,0 +1,150 @@
+import BidirTT.Value
+
+namespace BidirTT
+
+abbrev EvalM := Except String
+
+mutual
+  partial def eval : Env → Tm → EvalM Val
+    | env, .var i =>
+        match env[i]? with
+        | some v => pure v
+        | none =>
+            throw s!"bad de Bruijn index {i} in environment of size {env.length}"
+    | env, .lam t      => pure (.lam (.mk env t))
+    | env, .app t u    => do
+        let vt ← eval env t
+        let vu ← eval env u
+        vApp vt vu
+    | env, .pi a b     => do
+        let va ← eval env a
+        pure (.pi va (.mk env b))
+    | env, .sig a b    => do
+        let va ← eval env a
+        pure (.sig va (.mk env b))
+    | env, .pair t u   => do
+        let vt ← eval env t
+        let vu ← eval env u
+        pure (.pair vt vu)
+    | env, .fst t      => do
+        let vt ← eval env t
+        vFst vt
+    | env, .snd t      => do
+        let vt ← eval env t
+        vSnd vt
+    | _,   .univ i     => pure (.univ i)
+    | env, .letE _ t u => do
+        let vt ← eval env t
+        eval (vt :: env) u
+
+  partial def vApp : Val → Val → EvalM Val
+    | .lam c, u => cApp c u
+    | t,      u => pure (.app t u)
+
+  partial def vFst : Val → EvalM Val
+    | .pair a _ => pure a
+    | t         => pure (.fst t)
+
+  partial def vSnd : Val → EvalM Val
+    | .pair _ b => pure b
+    | t         => pure (.snd t)
+
+  partial def cApp : Closure → Val → EvalM Val
+    | .mk env body, v => eval (v :: env) body
+end
+
+partial def quote : Lvl → Val → EvalM Tm
+  | l, .var x =>
+      if x < l then
+        pure (.var (l - x - 1))
+      else
+        throw s!"bad level {x} while quoting at level {l}"
+  | l, .app t u  => do
+      let qt ← quote l t
+      let qu ← quote l u
+      pure (.app qt qu)
+  | l, .fst t    => do
+      let qt ← quote l t
+      pure (.fst qt)
+  | l, .snd t    => do
+      let qt ← quote l t
+      pure (.snd qt)
+  | l, .lam c    => do
+      let body ← cApp c (.var l)
+      let qb ← quote (l + 1) body
+      pure (.lam qb)
+  | l, .pi a c   => do
+      let qa ← quote l a
+      let body ← cApp c (.var l)
+      let qb ← quote (l + 1) body
+      pure (.pi qa qb)
+  | l, .sig a c  => do
+      let qa ← quote l a
+      let body ← cApp c (.var l)
+      let qb ← quote (l + 1) body
+      pure (.sig qa qb)
+  | l, .pair a b => do
+      let qa ← quote l a
+      let qb ← quote l b
+      pure (.pair qa qb)
+  | _, .univ i   => pure (.univ i)
+
+private def andThen (lhs : EvalM Bool) (rhs : Unit → EvalM Bool) : EvalM Bool := do
+  if (← lhs) then
+    rhs ()
+  else
+    pure false
+
+partial def conv : Lvl → Val → Val → EvalM Bool
+  | _, .univ i,     .univ j     => pure (i == j)
+  | l, .pi a c,     .pi a' c'   =>
+      andThen (conv l a a') fun _ => do
+        let b  ← cApp c (.var l)
+        let b' ← cApp c' (.var l)
+        conv (l + 1) b b'
+  | l, .sig a c,    .sig a' c'  =>
+      andThen (conv l a a') fun _ => do
+        let b  ← cApp c (.var l)
+        let b' ← cApp c' (.var l)
+        conv (l + 1) b b'
+  | l, .lam c,      .lam c'     =>
+      do
+        let body  ← cApp c (.var l)
+        let body' ← cApp c' (.var l)
+        conv (l + 1) body body'
+  | l, .lam c,      t           =>
+      do
+        let body  ← cApp c (.var l)
+        let apped ← vApp t (.var l)
+        conv (l + 1) body apped
+  | l, t,           .lam c      =>
+      do
+        let apped ← vApp t (.var l)
+        let body  ← cApp c (.var l)
+        conv (l + 1) apped body
+  | l, .pair a b,   .pair a' b' =>
+      andThen (conv l a a') fun _ => conv l b b'
+  | l, .pair a b,   p           =>
+      andThen
+        (do
+          let fstp ← vFst p
+          conv l a fstp)
+        fun _ => do
+          let sndp ← vSnd p
+          conv l b sndp
+  | l, p,           .pair a b   =>
+      andThen
+        (do
+          let fstp ← vFst p
+          conv l fstp a)
+        fun _ => do
+          let sndp ← vSnd p
+          conv l sndp b
+  | _, .var x,      .var y      => pure (x == y)
+  | l, .app t u,    .app t' u'  =>
+      andThen (conv l t t') fun _ => conv l u u'
+  | l, .fst t,      .fst t'     => conv l t t'
+  | l, .snd t,      .snd t'     => conv l t t'
+  | _, _,           _           => pure false
+
+end BidirTT
diff --git a/BidirTT/Examples.lean b/BidirTT/Examples.lean
new file mode 100644
index 0000000..6d7b52d
--- /dev/null
+++ b/BidirTT/Examples.lean
@@ -0,0 +1,72 @@
+import BidirTT.Syntax
+
+namespace BidirTT.Examples
+
+open BidirTT
+
+def idTy : Raw :=
+  .pi "A" (.univ 0) (.pi "_" (.var "A") (.var "A"))
+
+def idTm : Raw :=
+  .lam "A" (.lam "x" (.var "x"))
+
+def idAnn : Raw := .ann idTm idTy
+
+def constTy : Raw :=
+  .pi "A" (.univ 0) (.pi "B" (.univ 0)
+    (.pi "_" (.var "A") (.pi "_" (.var "B") (.var "A"))))
+
+def constTm : Raw :=
+  .lam "A" (.lam "B" (.lam "x" (.lam "_" (.var "x"))))
+
+def constAnn : Raw := .ann constTm constTy
+
+def swapTy : Raw :=
+  .pi "A" (.univ 0) (.pi "B" (.univ 0)
+    (.pi "_" (.sig "_" (.var "A") (.var "B"))
+             (.sig "_" (.var "B") (.var "A"))))
+
+def swapTm : Raw :=
+  .lam "A" (.lam "B" (.lam "p"
+    (.pair (.snd (.var "p")) (.fst (.var "p")))))
+
+def swapAnn : Raw := .ann swapTm swapTy
+
+def depPairTy : Raw :=
+  .sig "A" (.univ 2) (.var "A")
+
+def depPairTm : Raw :=
+  .pair
+    (.univ 1)
+    (.pi "_" (.univ 0) (.univ 0))
+
+def depPairAnn : Raw := .ann depPairTm depPairTy
+
+def fstDepPair : Raw := .fst depPairAnn
+
+def sndDepPair : Raw := .snd depPairAnn
+
+def omegaTy : Raw :=
+  .pi "A" (.univ 0) (.var "A")
+
+def omegaTm : Raw :=
+  .lam "x" (.app (.var "x") (.var "x"))
+
+def omegaAnn : Raw := .ann omegaTm omegaTy
+
+def unknownVar : Raw := .var "nope"
+
+def pairMismatch : Raw :=
+  .ann (.pair (.univ 1) (.univ 1))
+       (.sig "A" (.univ 2) (.var "A"))
+
+def badFst : Raw := .fst (.univ 0)
+
+def letUniverse : Raw :=
+  .ann
+    (.letE "A" (.univ 1) (.pi "_" (.univ 0) (.univ 0)) (.var "A"))
+    (.univ 1)
+
+def univ0 : Raw := .univ 0
+
+end BidirTT.Examples
diff --git a/BidirTT/Pretty.lean b/BidirTT/Pretty.lean
new file mode 100644
index 0000000..e396206
--- /dev/null
+++ b/BidirTT/Pretty.lean
@@ -0,0 +1,19 @@
+import BidirTT.Syntax
+
+namespace BidirTT
+
+mutual
+  partial def prettyTm : Tm → String
+    | .var i      => s!"#{i}"
+    | .lam t      => s!"(fun => {prettyTm t})"
+    | .app t u    => s!"({prettyTm t} {prettyTm u})"
+    | .pi a b     => s!"(Pi {prettyTm a} -> {prettyTm b})"
+    | .sig a b    => s!"(Sigma {prettyTm a} * {prettyTm b})"
+    | .pair t u   => s!"({prettyTm t}, {prettyTm u})"
+    | .fst t      => s!"({prettyTm t}.1)"
+    | .snd t      => s!"({prettyTm t}.2)"
+    | .univ i     => s!"U{i}"
+    | .letE a t u => s!"(let : {prettyTm a} := {prettyTm t}; {prettyTm u})"
+end
+
+end BidirTT
diff --git a/BidirTT/Syntax.lean b/BidirTT/Syntax.lean
new file mode 100644
index 0000000..9bb2437
--- /dev/null
+++ b/BidirTT/Syntax.lean
@@ -0,0 +1,32 @@
+namespace BidirTT
+
+abbrev Name := String
+
+inductive Raw where
+  | var  : Name → Raw
+  | lam  : Name → Raw → Raw
+  | app  : Raw → Raw → Raw
+  | pi   : Name → Raw → Raw → Raw
+  | sig  : Name → Raw → Raw → Raw
+  | pair : Raw → Raw → Raw
+  | fst  : Raw → Raw
+  | snd  : Raw → Raw
+  | univ : Nat → Raw
+  | letE : Name → Raw → Raw → Raw → Raw
+  | ann  : Raw → Raw → Raw
+  deriving Repr, Inhabited, BEq, DecidableEq
+
+inductive Tm where
+  | var  : Nat → Tm
+  | lam  : Tm → Tm
+  | app  : Tm → Tm → Tm
+  | pi   : Tm → Tm → Tm
+  | sig  : Tm → Tm → Tm
+  | pair : Tm → Tm → Tm
+  | fst  : Tm → Tm
+  | snd  : Tm → Tm
+  | univ : Nat → Tm
+  | letE : Tm → Tm → Tm → Tm
+  deriving Repr, Inhabited, BEq, DecidableEq
+
+end BidirTT
diff --git a/BidirTT/Value.lean b/BidirTT/Value.lean
new file mode 100644
index 0000000..9267c64
--- /dev/null
+++ b/BidirTT/Value.lean
@@ -0,0 +1,27 @@
+import BidirTT.Syntax
+
+namespace BidirTT
+
+mutual
+  inductive Val where
+    | var  : Nat → Val
+    | app  : Val → Val → Val
+    | fst  : Val → Val
+    | snd  : Val → Val
+    | lam  : Closure → Val
+    | pi   : Val → Closure → Val
+    | sig  : Val → Closure → Val
+    | pair : Val → Val → Val
+    | univ : Nat → Val
+
+  inductive Closure where
+    | mk : List Val → Tm → Closure
+end
+
+abbrev Env := List Val
+abbrev Lvl := Nat
+
+instance : Inhabited Val := ⟨.univ 0⟩
+instance : Inhabited Closure := ⟨.mk [] (.univ 0)⟩
+
+end BidirTT
diff --git a/Main.lean b/Main.lean
new file mode 100644
index 0000000..2778915
--- /dev/null
+++ b/Main.lean
@@ -0,0 +1,30 @@
+import BidirTT
+
+open BidirTT
+
+def runOne (label : String) (r : Raw) : IO Unit := do
+  match checkTop r with
+  | .ok (t, ty) =>
+      IO.println s!"[ok]   {label}"
+      IO.println s!"  term : {BidirTT.prettyTm t}"
+      match quote 0 ty with
+      | Except.ok qt =>
+          IO.println s!"  type : {BidirTT.prettyTm qt}"
+      | Except.error err =>
+          IO.println s!"  type : <error: {err}>"
+  | .error e =>
+      IO.println s!"[err]  {label}: {e}"
+
+def main : IO Unit := do
+  runOne "U0"            Examples.univ0
+  runOne "id"            Examples.idAnn
+  runOne "const"         Examples.constAnn
+  runOne "swap"          Examples.swapAnn
+  runOne "depPair"       Examples.depPairAnn
+  runOne "depPair.1"     Examples.fstDepPair
+  runOne "depPair.2"     Examples.sndDepPair
+  runOne "let universe"  Examples.letUniverse
+  runOne "omega (bad)"   Examples.omegaAnn
+  runOne "unknown var"   Examples.unknownVar
+  runOne "pair mismatch" Examples.pairMismatch
+  runOne "bad fst"       Examples.badFst
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..6019e69
--- /dev/null
+++ b/README.md
@@ -0,0 +1,17 @@
+# iris
+
+Small (toy) dependently typed core w. a bidirectional typechecker. It takes a named raw syntax, checks and elaborates it into a de Bruijn core and evaluates terms into semantic values, quotes them back for diagnostics and uses conversion checking to compare normal forms. The current kernel has explicit universe levels `U0`, `U1`, `U2`, dependent function and pair types, projections, annotations, and `let`
+
+One of the checked examples is:
+
+```lean
+def depPairTy : Raw :=
+  .sig "A" (.univ 2) (.var "A")
+
+def depPairTm : Raw :=
+  .pair
+    (.univ 1)
+    (.pi "_" (.univ 0) (.univ 0))
+```
+
+which elaborates to a pair whose first component is the type `U1` and whose second component inhabits it
diff --git a/Tests.lean b/Tests.lean
new file mode 100644
index 0000000..d5e087f
--- /dev/null
+++ b/Tests.lean
@@ -0,0 +1,106 @@
+import BidirTT
+
+open BidirTT
+
+inductive Expectation where
+  | okTy : Tm → Expectation
+  | errContains : String → Expectation
+
+private def renderType (ty : Val) : Except String Tm :=
+  BidirTT.quote 0 ty
+
+private def containsText (haystack needle : String) : Bool :=
+  needle.isEmpty || (haystack.splitOn needle).length > 1
+
+structure TestCase where
+  name   : String
+  input  : Raw
+  expect : Expectation
+
+def cases : List TestCase := [
+  ⟨"U0 is typed by U1",          Examples.univ0,
+    .okTy (.univ 1)⟩,
+  ⟨"id typechecks",             Examples.idAnn,
+    .okTy (.pi (.univ 0) (.pi (.var 0) (.var 1)))⟩,
+  ⟨"const typechecks",          Examples.constAnn,
+    .okTy (.pi (.univ 0) (.pi (.univ 0) (.pi (.var 1) (.pi (.var 1) (.var 3)))))⟩,
+  ⟨"swap typechecks",           Examples.swapAnn,
+    .okTy (.pi (.univ 0) (.pi (.univ 0) (.pi (.sig (.var 1) (.var 1)) (.sig (.var 1) (.var 3)))))⟩,
+  ⟨"dependent pair typechecks", Examples.depPairAnn,
+    .okTy (.sig (.univ 2) (.var 0))⟩,
+  ⟨"fst infers the first projection", Examples.fstDepPair,
+    .okTy (.univ 2)⟩,
+  ⟨"snd infers the dependent second projection", Examples.sndDepPair,
+    .okTy (.univ 1)⟩,
+  ⟨"let infers through definitions", Examples.letUniverse,
+    .okTy (.univ 1)⟩,
+  ⟨"self application rejected", Examples.omegaAnn,
+    .errContains "expected Pi type in application"⟩,
+  ⟨"unknown variable rejected", Examples.unknownVar,
+    .errContains "unknown variable nope"⟩,
+  ⟨"pair mismatch rejected at the Sigma body", Examples.pairMismatch,
+    .errContains "type mismatch: expected U1, got U2"⟩,
+  ⟨"bad fst rejected", Examples.badFst,
+    .errContains "expected Sigma type in .1, got U1"⟩
+]
+
+def runCase (tc : TestCase) : IO Bool := do
+  match tc.expect, checkTop tc.input with
+  | .okTy expectedTy, .ok (_, ty) =>
+      match renderType ty with
+      | Except.ok actualTy =>
+          if actualTy == expectedTy then
+            IO.println s!"PASS  {tc.name}"
+            pure true
+          else
+            IO.println s!"FAIL  {tc.name} (expected type {BidirTT.prettyTm expectedTy}, got {BidirTT.prettyTm actualTy})"
+            pure false
+      | Except.error err =>
+          IO.println s!"FAIL  {tc.name} (could not quote type: {err})"
+          pure false
+  | .okTy expectedTy, .error err =>
+      IO.println s!"FAIL  {tc.name} (expected type {BidirTT.prettyTm expectedTy}, got error {err})"
+      pure false
+  | .errContains needle, .error err =>
+      if containsText err needle then
+        IO.println s!"PASS  {tc.name}"
+        pure true
+      else
+        IO.println s!"FAIL  {tc.name} (expected error containing {needle}, got {err})"
+        pure false
+  | .errContains needle, .ok (_, ty) =>
+      match renderType ty with
+      | Except.ok actualTy =>
+          IO.println s!"FAIL  {tc.name} (expected error containing {needle}, got type {BidirTT.prettyTm actualTy})"
+          pure false
+      | Except.error err =>
+          IO.println s!"FAIL  {tc.name} (expected error containing {needle}, got quote failure {err})"
+          pure false
+
+def runInternalSafetyChecks : IO Bool := do
+  let malformedEvalOk :=
+    match eval [] (.var 0) with
+    | Except.error err => containsText err "bad de Bruijn index 0"
+    | Except.ok _ => false
+  let malformedQuoteOk :=
+    match quote 0 (.var 0) with
+    | Except.error err => containsText err "bad level 0"
+    | Except.ok _ => false
+  if malformedEvalOk && malformedQuoteOk then
+    IO.println "PASS  malformed core terms are rejected safely"
+    pure true
+  else
+    IO.println "FAIL  malformed core terms are rejected safely"
+    pure false
+
+def main : IO UInt32 := do
+  let results ← cases.mapM runCase
+  let safetyOk ← runInternalSafetyChecks
+  let allResults := results ++ [safetyOk]
+  let failed := allResults.countP (· == false)
+  if failed == 0 then
+    IO.println s!"\n{allResults.length} passed, 0 failed"
+    pure 0
+  else
+    IO.println s!"\n{allResults.length - failed} passed, {failed} failed"
+    pure 1
diff --git a/lakefile.lean b/lakefile.lean
new file mode 100644
index 0000000..59c9e53
--- /dev/null
+++ b/lakefile.lean
@@ -0,0 +1,17 @@
+import Lake
+open Lake DSL
+
+package bidirtt where
+  leanOptions := #[
+    ⟨`pp.unicode.fun, true⟩,
+    ⟨`autoImplicit, false⟩
+  ]
+
+@[default_target]
+lean_lib BidirTT where
+
+lean_exe bidirtt where
+  root := `Main
+
+lean_exe tests where
+  root := `Tests
diff --git a/lean-toolchain b/lean-toolchain
new file mode 100644
index 0000000..d0eb99f
--- /dev/null
+++ b/lean-toolchain
@@ -0,0 +1 @@
+leanprover/lean4:v4.15.0