diff --git a/src/exec.ml b/src/exec.ml
index 2d91b75..1b33609 100644
--- a/src/exec.ml
+++ b/src/exec.ml
@@ -37,12 +37,30 @@ let builtin_mem =
   fun x -> List.assoc x l
 let builtin_typctx = List.map (function(x,(t,_))->(x,(t,true))) builtins
 
+let rec exp_captures bound = function
+  | EInt _ | EBool _ | EFnB _ -> []
+  | EVar x -> if List.mem x bound then [] else [x]
+  | EFn (x, _, e) -> exp_captures (x::bound) e
+  | EApp (e1, e2) -> List.concat_map (exp_captures bound) [e1;e2]
+  | EIf (b, e1, e2) -> List.concat_map (exp_captures bound) [b;e1;e2]
+  | EScope s -> fst (stmt_captures bound s)
+and stmt_captures bound = function
+  | SExp e | SAsgn (_, e) -> (exp_captures bound e, bound)
+  | SDecl (x, _) -> ([], [x])
+  | SSeq (s1, s2) ->
+      let (c1, b1) = stmt_captures bound s1 in
+      let (c2, b2) = stmt_captures b1 s2 in
+      (c1@c2, b2)
+
 let rec eval (m: mem) (e: exp) : value =
   match e with
   | EInt i -> VInt i
   | EBool b -> VBool b
   | EVar x -> m x
-  | EFn (x, _, e) -> VFn (m, x, e)
+  | EFn (x, _, e) ->
+      let c = exp_captures [x] e in
+      let m x = List.assoc x (List.map (fun x -> (x, m x)) c) in
+      VFn (m, x, e)
   | EFnB (_, _, f) -> VFnB (m, f)
   | EApp (e1, e2) ->
       (match eval m e1, eval m e2 with