X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=code%2Funtyped_evaluator.ml;h=92efa6ea1d9522c0ad1605825995643fd3bbbc02;hp=7af3eb43424db407a1bf24720841dbe67378efc1;hb=1666d4c9858609dd1c32d7ab7aff976debe73cf7;hpb=45bfd0c44c531bd94e00b5a11285691c48062157

diff --git a/code/untyped_evaluator.ml b/code/untyped_evaluator.ml
index 7af3eb43..92efa6ea 100644
--- a/code/untyped_evaluator.ml
+++ b/code/untyped_evaluator.ml
@@ -2,7 +2,7 @@
    This is a simplified version of the code at ...
    You can use this code as follows:
 
-   1. First, use a text editor to fill in the (* COMPLETE THIS *) portions
+   1. First, use a text editor to fill in the (* COMPLETE THIS *) portions.
    2. Then see if OCaml will compile it, by typing `ocamlc -c untyped_evaluator.ml` in a Terminal.
    3. If it doesn't work, go back to step 1.
    4. If it does work, then you can start up the OCaml toplevel using `ocaml -I DIRECTORY`,
@@ -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 ^ "`"))