--- a/Makefile	Wed Nov 05 09:39:00 2008 -0500
+++ b/Makefile	Sun Nov 09 13:44:31 2008 -0500
@@ -1,7 +1,7 @@
 MODULES_NODOC := Axioms Tactics MoreSpecif DepList
 MODULES_PROSE := Intro
 MODULES_CODE  := StackMachine InductiveTypes Predicates Coinductive Subset \
-	MoreDep DataStruct Equality Match Reflection Firstorder Hoas
+	MoreDep DataStruct Equality Match Reflection Firstorder Hoas Interps
 MODULES_DOC   := $(MODULES_PROSE) $(MODULES_CODE)
 MODULES       := $(MODULES_NODOC) $(MODULES_DOC)
 VS            := $(MODULES:%=src/%.v)

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/Interps.v	Sun Nov 09 13:44:31 2008 -0500
@@ -0,0 +1,124 @@
+(* Copyright (c) 2008, Adam Chlipala
+ * 
+ * This work is licensed under a
+ * Creative Commons Attribution-Noncommercial-No Derivative Works 3.0
+ * Unported License.
+ * The license text is available at:
+ *   http://creativecommons.org/licenses/by-nc-nd/3.0/
+ *)
+
+(* begin hide *)
+Require Import String List.
+
+Require Import Axioms Tactics.
+
+Set Implicit Arguments.
+(* end hide *)
+
+
+(** %\chapter{Type-Theoretic Interpreters}% *)
+
+(** TODO: Prose for this chapter *)
+
+
+(** * Simply-Typed Lambda Calculus *)
+
+Module STLC.
+  Inductive type : Type :=
+  | Nat : type
+  | Arrow : type -> type -> type.
+
+  Infix "-->" := Arrow (right associativity, at level 60).
+
+  Section vars.
+    Variable var : type -> Type.
+
+    Inductive exp : type -> Type :=
+    | Var : forall t,
+      var t
+      -> exp t
+
+    | Const : nat -> exp Nat
+    | Plus : exp Nat -> exp Nat -> exp Nat
+
+    | App : forall t1 t2,
+      exp (t1 --> t2)
+      -> exp t1
+      -> exp t2
+    | Abs : forall t1 t2,
+      (var t1 -> exp t2)
+      -> exp (t1 --> t2).
+  End vars.
+
+  Definition Exp t := forall var, exp var t.
+
+  Implicit Arguments Var [var t].
+  Implicit Arguments Const [var].
+  Implicit Arguments Plus [var].
+  Implicit Arguments App [var t1 t2].
+  Implicit Arguments Abs [var t1 t2].
+
+  Notation "# v" := (Var v) (at level 70).
+
+  Notation "^ n" := (Const n) (at level 70).
+  Infix "++" := Plus.
+
+  Infix "@" := App (left associativity, at level 77).
+  Notation "\ x , e" := (Abs (fun x => e)) (at level 78).
+  Notation "\ ! , e" := (Abs (fun _ => e)) (at level 78).
+
+  Fixpoint typeDenote (t : type) : Set :=
+    match t with
+      | Nat => nat
+      | t1 --> t2 => typeDenote t1 -> typeDenote t2
+    end.
+
+  Fixpoint expDenote t (e : exp typeDenote t) {struct e} : typeDenote t :=
+    match e in (exp _ t) return (typeDenote t) with
+      | Var _ v => v
+        
+      | Const n => n
+      | Plus e1 e2 => expDenote e1 + expDenote e2
+        
+      | App _ _ e1 e2 => (expDenote e1) (expDenote e2)
+      | Abs _ _ e' => fun x => expDenote (e' x)
+    end.
+
+  Definition ExpDenote t (e : Exp t) := expDenote (e _).
+
+  Section cfold.
+    Variable var : type -> Type.
+
+    Fixpoint cfold t (e : exp var t) {struct e} : exp var t :=
+      match e in exp _ t return exp _ t with
+        | Var _ v => #v
+
+        | Const n => ^n
+        | Plus e1 e2 =>
+          let e1' := cfold e1 in
+          let e2' := cfold e2 in
+          match e1', e2' with
+            | Const n1, Const n2 => ^(n1 + n2)
+            | _, _ => e1' ++ e2'
+          end
+
+        | App _ _ e1 e2 => cfold e1 @ cfold e2
+        | Abs _ _ e' => Abs (fun x => cfold (e' x))
+      end.
+  End cfold.
+
+  Definition Cfold t (E : Exp t) : Exp t := fun _ => cfold (E _).
+
+  Lemma cfold_correct : forall t (e : exp _ t),
+    expDenote (cfold e) = expDenote e.
+    induction e; crush; try (ext_eq; crush);
+      repeat (match goal with
+                | [ |- context[cfold ?E] ] => dep_destruct (cfold E)
+              end; crush).
+  Qed.
+
+  Theorem Cfold_correct : forall t (E : Exp t),
+    ExpDenote (Cfold E) = ExpDenote E.
+    unfold ExpDenote, Cfold; intros; apply cfold_correct.
+  Qed.
+End STLC.

--- a/src/Intro.v	Wed Nov 05 09:39:00 2008 -0500
+++ b/src/Intro.v	Sun Nov 09 13:44:31 2008 -0500
@@ -207,6 +207,8 @@
 \hline
 Higher-Order Abstract Syntax & \texttt{Hoas.v} \\
 \hline
+Type-Theoretic Interpreters & \texttt{Interps.v} \\
+\hline
 \end{tabular} \end{center}
 
 % *)

--- a/src/toc.html	Wed Nov 05 09:39:00 2008 -0500
+++ b/src/toc.html	Sun Nov 09 13:44:31 2008 -0500
@@ -17,5 +17,6 @@
 <li><a href="Reflection.html">Proof by Reflection</a>
 <li><a href="Firstorder.html">First-Order Abstract Syntax</a>
 <li><a href="Hoas.html">Higher-Order Abstract Syntax</a>
+<li><a href="Interps.html">Type-Theoretic Interpreters</a>
 
 </body></html>