--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/Firstorder.v	Sun Nov 02 12:16:55 2008 -0500
@@ -0,0 +1,299 @@
+(* 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 List String.
+
+Require Import Tactics.
+
+Set Implicit Arguments.
+(* end hide *)
+
+
+(** %\section{Formalizing Programming Languages and Compilers}
+
+   \chapter{First-Order Variable Representations}% *)
+
+(** TODO: Prose for this chapter *)
+
+
+(** * Concrete Binding *)
+
+Module Concrete.
+
+  Definition var := string.
+  Definition var_eq := string_dec.
+
+  Inductive exp : Set :=
+  | Const : bool -> exp
+  | Var : var -> exp
+  | App : exp -> exp -> exp
+  | Abs : var -> exp -> exp.
+
+  Inductive type : Set :=
+  | Bool : type
+  | Arrow : type -> type -> type.
+
+  Infix "-->" := Arrow (right associativity, at level 60).
+
+  Definition ctx := list (var * type).
+
+  Reserved Notation "G |-v x : t" (no associativity, at level 90, x at next level).
+
+  Inductive lookup : ctx -> var -> type -> Prop :=
+  | First : forall x t G,
+    (x, t) :: G |-v x : t
+  | Next : forall x t x' t' G,
+    x <> x'
+    -> G |-v x : t
+    -> (x', t') :: G |-v x : t
+
+    where "G |-v x : t" := (lookup G x t).
+
+  Hint Constructors lookup.
+
+  Reserved Notation "G |-e e : t" (no associativity, at level 90, e at next level).
+
+  Inductive hasType : ctx -> exp -> type -> Prop :=
+  | TConst : forall G b,
+    G |-e Const b : Bool
+  | TVar : forall G v t,
+    G |-v v : t
+    -> G |-e Var v : t
+  | TApp : forall G e1 e2 dom ran,
+    G |-e e1 : dom --> ran
+    -> G |-e e2 : dom
+    -> G |-e App e1 e2 : ran
+  | TAbs : forall G x e' dom ran,
+    (x, dom) :: G |-e e' : ran
+    -> G |-e Abs x e' : dom --> ran
+
+    where "G |-e e : t" := (hasType G e t).
+
+  Hint Constructors hasType.
+
+  Notation "x ## G" := (forall t' : type, In (x, t') G -> False) (no associativity, at level 90).
+
+  Notation "G' # G" := (forall (x : var) (t : type), In (x, t) G -> x ## G') (no associativity, at level 90).
+
+  Lemma lookup_In : forall G x t,
+    G |-v x : t
+    -> In (x, t) G.
+    induction 1; crush.
+  Qed.
+
+  Hint Resolve lookup_In.
+
+  Lemma disjoint_invert1 : forall G x t G' x' t',
+    G |-v x : t
+    -> (x', t') :: G' # G
+    -> x <> x'.
+    crush; eauto.
+  Qed.
+
+  Lemma disjoint_invert2 : forall G' G p,
+    p :: G' # G
+    -> G' # G.
+    firstorder.
+  Qed.
+
+  Hint Resolve disjoint_invert1 disjoint_invert2.
+  Hint Extern 1 (_ <> _) => (intro; subst).
+
+  Lemma weaken_lookup' : forall G x t,
+    G |-v x : t
+    -> forall G', G' # G
+      -> G' ++ G |-v x : t.
+    induction G' as [ | [x' t'] tl ]; crush; eauto 9.
+  Qed.
+
+  Lemma weaken_lookup : forall G2 x t G',
+    G' # G2
+    -> forall G1, G1 ++ G2 |-v x : t
+      -> G1 ++ G' ++ G2 |-v x : t.
+    Hint Resolve weaken_lookup'.
+
+    induction G1 as [ | [x' t'] tl ]; crush;
+      match goal with
+        | [ H : _ |-v _ : _ |- _ ] => inversion H; crush
+      end.
+  Qed.
+
+  Hint Resolve weaken_lookup.
+
+  Lemma hasType_push : forall x t G1 G2 e t',
+    ((x, t) :: G1) ++ G2 |-e e : t'
+    -> (x, t) :: G1 ++ G2 |-e e : t'.
+    trivial.
+  Qed.
+
+  Hint Resolve hasType_push.
+
+  Theorem weaken_hasType' : forall G' G e t,
+    G |-e e : t
+      -> forall G1 G2, G = G1 ++ G2
+        -> G' # G2
+        -> G1 ++ G' ++ G2 |-e e : t.
+    induction 1; crush; eauto.
+  Qed.
+
+  Theorem weaken_hasType : forall G e t,
+    G |-e e : t
+    -> forall G', G' # G
+      -> G' ++ G |-e e : t.
+    intros; change (G' ++ G) with (nil ++ G' ++ G);
+      eapply weaken_hasType'; eauto.
+  Qed.
+
+  Theorem weaken_hasType_closed : forall e t,
+    nil |-e e : t
+    -> forall G, G |-e e : t.
+    intros; rewrite (app_nil_end G); apply weaken_hasType; auto.
+  Qed.
+
+  Theorem weaken_hasType1 : forall G e t,
+    G |-e e : t
+    -> forall x t', x ## G
+      -> (x, t') :: G |-e e : t.
+    intros; change ((x, t') :: G) with (((x, t') :: nil) ++ G);
+      apply weaken_hasType; crush;
+        match goal with
+          | [ H : (_, _) = (_, _) |- _ ] => injection H
+        end; crush; eauto.
+  Qed.
+
+  Hint Resolve weaken_hasType_closed weaken_hasType1.
+
+  Section subst.
+    Variable x : var.
+    Variable e1 : exp.
+
+    Fixpoint subst (e2 : exp) : exp :=
+      match e2 with
+        | Const b => Const b
+        | Var x' =>
+          if var_eq x' x
+            then e1
+            else Var x'
+        | App e1 e2 => App (subst e1) (subst e2)
+        | Abs x' e' =>
+          Abs x' (if var_eq x' x
+            then e'
+            else subst e')
+      end.
+
+    Variable xt : type.
+    Hypothesis Ht' : nil |-e e1 : xt.
+
+    Lemma subst_lookup' : forall G2 x' t,
+      x' ## G2
+      -> (x, xt) :: G2 |-v x' : t
+      -> t = xt.
+      inversion 2; crush; elimtype False; eauto.
+    Qed.
+
+    Lemma subst_lookup : forall x' t G2,
+      x <> x'
+      -> forall G1, G1 ++ (x, xt) :: G2 |-v x' : t
+        -> G1 ++ G2 |-v x' : t.
+      induction G1 as [ | [x'' t'] tl ]; crush;
+        match goal with
+          | [ H : _ |-v _ : _ |- _ ] => inversion H
+        end; crush.
+    Qed.
+
+    Hint Resolve subst_lookup.
+
+    Lemma subst_lookup'' : forall G2 x' t,
+      x' ## G2
+      -> forall G1, x' ## G1
+        -> G1 ++ (x, xt) :: G2 |-v x' : t
+        -> t = xt.
+      Hint Resolve subst_lookup'.
+
+      induction G1 as [ | [x'' t'] tl ]; crush; eauto;
+        match goal with
+          | [ H : _ |-v _ : _ |- _ ] => inversion H
+        end; crush; elimtype False; eauto.
+    Qed.
+
+    Implicit Arguments subst_lookup'' [G2 x' t G1].
+
+    Lemma disjoint_cons : forall x x' t (G : ctx),
+      x ## G
+      -> x' <> x
+      -> x ## (x', t) :: G.
+      firstorder;
+        match goal with
+          | [ H : (_, _) = (_, _) |- _ ] => injection H
+        end; crush.
+    Qed.
+
+    Hint Resolve disjoint_cons.
+
+    Lemma shadow_lookup : forall G2 v t t' G1,
+      G1 |-v x : t'
+      -> G1 ++ (x, xt) :: G2 |-v v : t
+      -> G1 ++ G2 |-v v : t.
+      induction G1 as [ | [x'' t''] tl ]; crush;
+        match goal with
+          | [ H : nil |-v _ : _ |- _ ] => inversion H
+          | [ H1 : _ |-v _ : _, H2 : _ |-v _ : _ |- _ ] =>
+            inversion H1; crush; inversion H2; crush
+        end.
+    Qed.
+
+    Lemma shadow_hasType' : forall G2 G e t,
+      G |-e e : t
+      -> forall G1, G = G1 ++ (x, xt) :: G2
+        -> forall t'', G1 |-v x : t''
+          -> G1 ++ G2 |-e e : t.
+      Hint Resolve shadow_lookup.
+
+      induction 1; crush; eauto;
+        match goal with
+          | [ H : (?x0, _) :: _ ++ (x, _) :: _ |-e _ : _ |- _ ] =>
+            destruct (var_eq x0 x); subst; eauto
+        end.
+    Qed.      
+
+    Lemma shadow_hasType : forall G1 G2 e t t'',
+      G1 ++ (x, xt) :: G2 |-e e : t
+      -> G1 |-v x : t''
+      -> G1 ++ G2 |-e e : t.
+      intros; eapply shadow_hasType'; eauto.
+    Qed.
+
+    Hint Resolve shadow_hasType.
+
+    Theorem subst_hasType : forall G e2 t,
+      G |-e e2 : t
+        -> forall G1 G2, G = G1 ++ (x, xt) :: G2
+          -> x ## G1
+          -> x ## G2
+          -> G1 ++ G2 |-e subst e2 : t.
+      induction 1; crush;
+        try match goal with
+              | [ |- context[if ?E then _ else _] ] => destruct E
+            end; crush; eauto 6;
+        match goal with
+          | [ H1 : x ## _, H2 : x ## _, H3 : _ |-v x : _ |- _ ] =>
+            rewrite (subst_lookup'' H2 H1 H3)
+        end; crush.
+    Qed.
+
+    Theorem subst_hasType_closed : forall e2 t,
+      (x, xt) :: nil |-e e2 : t
+      -> nil |-e subst e2 : t.
+      intros; change (nil ++ nil |-e subst e2 : t);
+        eapply subst_hasType; eauto.
+    Qed.
+  End subst.
+
+End Concrete.