X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=code%2Funtyped_evaluator_complete.ml;h=d9b158c6af361b3c7a0bf15b18bb2ebc44ba659d;hp=908620583edd15f20936273613affe4600a7b29e;hb=42ea318d0e066534ffd016c299eb75e7f2efcca7;hpb=45bfd0c44c531bd94e00b5a11285691c48062157

diff --git a/code/untyped_evaluator_complete.ml b/code/untyped_evaluator_complete.ml
index 90862058..d9b158c6 100644
--- a/code/untyped_evaluator_complete.ml
+++ b/code/untyped_evaluator_complete.ml
@@ -15,9 +15,29 @@
       `reduce (App(Lambda("x",Var "x"),Lambda("y",Var "y")))`
       `evaluate (App(Lambda("x",Var "x"),Lambda("y",Var "y")))`
 
-   The environments play absolutely no role in this simplified V1 interpreter. In the
-   fuller code, they have a limited role in the V1 interpreter. In the V2 interpreter,
-   the environments are essential.
+   The two interpreters presented below are (V1) a substitute-and-replace
+   interpreter, and (V2) an environment-based interpreter. We discuss the
+   differences between these in the notes.
+
+   The implementeations we provide make both of these call-by-value. When given
+   a term `App(head, arg)`, the steps are: first, reduce or evaluate
+   `head`---it might involve further `App`s itself. Second, reduce or evaluate
+   `arg`.  Third, only _when_ `arg` reduces or evaluates to a result value, then
+   "perform" the application. What this last step amounts to is different in
+   the two interpreters. Call-by-name interpreters would "perform" the
+   application regardless, and without even trying to reduce or evaluate arg to
+   a result value beforehand.
+
+   Additionally, these implementations assume that free variables are "stuck"
+   terms rather than result values. That is a bit inconvenient with this
+   simplified program: it means that Lambdas (or Closures, in V2) are the only
+   result values. But in the fuller code from which this is simplified, it
+   makes more sense, because there we also have literal number and boolean
+   values as results, too.
+
+   The environments play absolutely no role in the simplified V1 interpreter
+   presented here. In the fuller code, they have a limited role in the V1
+   interpreter. In the V2 interpreter, the environments are essential.
 *)
 
 type identifier = string
@@ -36,16 +56,16 @@ and result = term
 
 (* This simplified code just provides a single implementation of environments;
    but the fuller code provides more. *)
-and env = identifier -> term option
+and env = (identifier * term) list
 
 (* Operations for environments *)
-let empty = fun _ -> None
-let push (ident : identifier) binding env =
-  fun (sought_ident : identifier) ->
-    if ident = sought_ident
-    then Some binding
-    else env sought_ident
-let lookup sought_ident env = env sought_ident
+let empty = []
+let shift (ident : identifier) binding env = (ident,binding) :: env
+let rec lookup (sought_ident : ident) (env : env) : term option =
+  match env with
+  | [] -> None
+  | (ident, binding) :: _ when ident = sought_ident -> Some binding
+  | _ :: env' -> lookup sought_ident env'
 
   
 (*
@@ -247,8 +267,9 @@ module V2 = struct
 
     | Var var ->
         (match lookup var env with
-        (* Free variables will never be pushed to the env, so we can be
-	   sure this is a result. *) CHECK
+	(* In this call-by-value design, only results get
+	   saved in the environment, so we can be sure this
+	   is a result. *)
         | Some res -> res
         | None -> failwith ("Unbound variable `" ^ var ^ "`"))
 
@@ -265,7 +286,7 @@ module V2 = struct
 	   bound to the result of evaluating arg under the
            current env *)
         let arg' = eval arg env in
-        let env' = push bound_var arg' env in
+        let env' = shift bound_var arg' env in
         eval body env'
 
     | App(head, arg) ->
@@ -276,7 +297,7 @@ module V2 = struct
 	    (* evaluate body under saved_env to govern its free
 	       variables, except that we add a binding of
                bound_var to arg' *)
-            let saved_env' = push bound_var arg' saved_env in
+            let saved_env' = shift bound_var arg' saved_env in
             eval body saved_env'
         | head' -> raise (Stuck(App(head',arg))))