X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=code%2Funtyped_evaluator.ml;h=92efa6ea1d9522c0ad1605825995643fd3bbbc02;hp=e5ce1c12f81fb6ad1b1d93f085d15f4e71a78462;hb=5f451af940e44f03c8cab0864398d2e644566787;hpb=7212dc7d99d0852cc195677af5ec51966a6d9065

diff --git a/code/untyped_evaluator.ml b/code/untyped_evaluator.ml
index e5ce1c12..92efa6ea 100644
--- a/code/untyped_evaluator.ml
+++ b/code/untyped_evaluator.ml
@@ -15,10 +15,6 @@
       `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 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.
-
    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.
@@ -38,6 +34,10 @@
    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
@@ -56,16 +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 shift (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'
 
   
 (*