diff --git a/impls/sml/env.sml b/impls/sml/env.sml
index 91814e15..60e0c480 100644
--- a/impls/sml/env.sml
+++ b/impls/sml/env.sml
@@ -9,5 +9,10 @@ fun get (d:mal_defs) s =
 fun set s v (d:mal_defs) =
     (s, v) :: (d |> List.filter (not o eq s o #1))
 
-fun lookup (INNER (d, out)) s = (case get d s of NONE => lookup out s | x => x)
+fun def s v (INNER (d, out)) = INNER (set s v d, out)
+  | def s v (ENV d)          = ENV (set s v d)
+
+fun let_in s v out = INNER (set s v [], out)
+
+fun lookup (INNER (d, out)) s = optOrElse (get d s) (fn _ => lookup out s)
   | lookup (ENV d)          s = get d s
diff --git a/impls/sml/step3_env.sml b/impls/sml/step3_env.sml
index e9b562f2..bba1f2cc 100644
--- a/impls/sml/step3_env.sml
+++ b/impls/sml/step3_env.sml
@@ -4,32 +4,31 @@ exception NotApplicable of string
 fun READ s =
     readStr s
 
-fun EVAL e ast =
-    case ast of
-        LIST [] => ast
-        | LIST l => eval_apply e ast
-        | _    => eval_ast e ast
+fun EVAL e (LIST (SYMBOL "def!"::args)) = eval_def e args
+  | EVAL e (ast as (LIST (_::_)))       = (e, eval_apply e ast)
+  | EVAL e ast                          = (e, eval_ast e ast)
 
-and eval_ast e ast =
-    case ast of
-        SYMBOL s => (case lookup e s of SOME v => v | NONE => raise NotDefined ("unable to resolve symbol '" ^ s ^ "'"))
-        | LIST l => LIST (List.map (EVAL e) l)
-        | _      => ast
+and eval_ast e (LIST l)   = LIST (List.map (#2 o (EVAL e)) l)
+  | eval_ast e (SYMBOL s) = valOrElse (lookup e s) (fn _ => raise NotDefined ("symbol '" ^ s ^ "' not found"))
+  | eval_ast e ast        = ast
 
 and eval_apply e ast =
     case eval_ast e ast of
         LIST ((FN f)::args) => f args
         | _ => raise NotApplicable "eval_apply needs a non-empty list"
 
+and eval_def e [SYMBOL s, ast] = let val (_, v) = EVAL e ast in (def s v e, v) end
+  | eval_def _ _ = raise NotApplicable "define needs a symbol and a value"
+
 fun PRINT f =
     prStr f
 
 fun rep e s =
-    s |> READ |> EVAL e |> PRINT
-    handle Nothing           => ""
-         | SyntaxError msg   => "SYNTAX ERROR: " ^ msg
-         | NotApplicable msg => "CANNOT APPLY: " ^ msg
-         | NotDefined msg    => "NOT DEFINED: " ^ msg
+    s |> READ |> EVAL e |> (fn (e, v) => (e, PRINT v))
+    handle Nothing           => (e, "")
+         | SyntaxError msg   => (e, "SYNTAX ERROR: " ^ msg)
+         | NotApplicable msg => (e, "CANNOT APPLY: " ^ msg)
+         | NotDefined msg    => (e, "NOT DEFINED: " ^ msg)
 
 fun malPlus  (INT a, INT b) = INT (a + b)
   | malPlus _ = raise NotApplicable "can only add integers"
@@ -55,14 +54,18 @@ val initEnv =
               | _     => raise NotApplicable "'/' requires arguments"))
      |> ENV
 
-fun repl () =
+fun repl () = repl' initEnv
+
+and repl' e =
     let open TextIO
     in (
         print("user> ");
         case inputLine(stdIn) of
-            SOME(line) => (
-                print((rep initEnv line) ^ "\n");
-                repl ()
-            )
+            SOME(line) =>
+                let val (e', s) = rep e line
+                    val _ = print(s ^ "\n")
+                in
+                    repl' e'
+                end
             | NONE => ()
     ) end
diff --git a/impls/sml/util.sml b/impls/sml/util.sml
index 2913b5fc..cc44333c 100644
--- a/impls/sml/util.sml
+++ b/impls/sml/util.sml
@@ -5,3 +5,9 @@ and takeWhile' f acc []      = rev acc
 infix 3 |> fun x |> f = f x
 
 fun eq a b = a = b
+
+fun optOrElse NONE b = b ()
+  | optOrElse a    _ = a
+
+fun valOrElse (SOME x) _ = x
+  | valOrElse a        b = b ()