From 281bccb3738012584ae0bc84a4c958f4b95d7a05 Mon Sep 17 00:00:00 2001
From: Christopher Fujino <christopherfujino@gmail.com>
Date: Thu, 30 Jul 2026 17:26:51 -0700
Subject: [PATCH 1/9] wip broken

---
 MILESTONES.md                |  1 +
 bin/native_test.ml           |  4 ++--
 lib/interpreter/dune         |  7 ++++++-
 lib/interpreter/interpret.ml | 27 +++++++++++++++------------
 lib/interpreter/native.ml    | 12 +++++++-----
 lib/interpreter/native.mli   |  2 +-
 native/native.ml             |  2 +-
 7 files changed, 33 insertions(+), 22 deletions(-)

diff --git a/MILESTONES.md b/MILESTONES.md
index 761a5934..e4de750a 100644
--- a/MILESTONES.md
+++ b/MILESTONES.md
@@ -119,3 +119,4 @@
 ## Bugs
 - [ ] bug: stacktrace printing is wrong in REPL
 - [ ] bug: Do static methods need to take self as first arg?
+- [ ] bug: $cwd is NOT always absolute
diff --git a/bin/native_test.ml b/bin/native_test.ml
index 257dc337..2615ce8b 100644
--- a/bin/native_test.ml
+++ b/bin/native_test.ml
@@ -1,6 +1,6 @@
 let map_some cb = function Some v -> cb v | None -> failwith "Nope!"
 let map_none cb = function None -> cb () | Some _ -> failwith "Nope!"
-let map_ok cb = function Ok () -> cb () | Error _ -> failwith "Nope!"
+let map_ok cb = function Ok v -> cb v | Error _ -> failwith "Nope!"
 let map_error cb = function Ok _ -> failwith "Nope!" | Error msg -> cb msg
 
 let () =
@@ -12,4 +12,4 @@ let () =
   |> map_error (fun msg ->
       Printf.printf "CD-ing to /nosuchdir returned \"%s\"\n" msg);
   Sn.Native.chdir "/"
-  |> map_ok (fun () -> print_endline "CD-ing to / succeeded.")
+  |> map_ok (fun msg -> Printf.printf "CD-ing to / succeeded with \"%s\"." msg)
diff --git a/lib/interpreter/dune b/lib/interpreter/dune
index 8128f5c8..6f236672 100644
--- a/lib/interpreter/dune
+++ b/lib/interpreter/dune
@@ -1,5 +1,10 @@
 (library
  (name interpreter)
- (libraries compiler core_unix sloth_common core_unix.sys_unix)
+ (libraries
+   compiler
+   core_unix
+   core_unix.sys_unix
+   core_unix.filename_unix
+   sloth_common)
  (preprocess
   (pps ppx_jane)))
diff --git a/lib/interpreter/interpret.ml b/lib/interpreter/interpret.ml
index e0b62147..c57f254c 100644
--- a/lib/interpreter/interpret.ml
+++ b/lib/interpreter/interpret.ml
@@ -740,11 +740,19 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
       let middleware name prev next =
         match name with
         | "$cwd" ->
+            let cd_wrapper p =
+              match M.chdir p with
+              | Ok new_p -> (
+                  match
+                    Context.reassign globals.context_ids "$cwd"
+                      (Runtime.String new_p)
+                  with
+                  | Some () -> ()
+                  | None -> internal_failure "Failed to re-assign $cwd")
+              | Error msg -> fail ~globals pos msg
+            in
             (match Runtime.string_of_val next with
-            | Some s -> (
-                match M.chdir s with
-                | Ok () -> ()
-                | Error msg -> fail ~globals pos msg)
+            | Some s -> cd_wrapper s
             | None ->
                 fail ~globals pos
                 @@ Printf.sprintf
@@ -753,14 +761,9 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
             post_block_hook :=
               Some
                 (fun () ->
-                  let unit_res =
-                    Runtime.string_of_val prev
-                    |> option_value ~message:__LOC__
-                    |> M.chdir
-                  in
-                  match unit_res with
-                  | Ok () -> ()
-                  | Error msg -> fail ~globals pos msg)
+                  Runtime.string_of_val prev
+                  |> option_value ~message:__LOC__
+                  |> cd_wrapper)
         | _ -> ()
       in
 
diff --git a/lib/interpreter/native.ml b/lib/interpreter/native.ml
index ff33089b..4f77382d 100644
--- a/lib/interpreter/native.ml
+++ b/lib/interpreter/native.ml
@@ -35,7 +35,7 @@ module type Sig = sig
     string array ->
     (Runtime.t, string) Result.t
 
-  val chdir : string -> (unit, string) Result.t
+  val chdir : string -> (string, string) Result.t
   val directory_exists : string -> bool
   val file_exists : string -> bool
   val mkdir : string -> unit
@@ -97,10 +97,12 @@ module Prod : Sig = struct
     else Ok ()
 
   let chdir path =
-    try Ok (Core_unix.chdir path)
-    with Core_unix.Unix_error (err, _, _) ->
-      let err_msg = Core_unix.Error.message err in
-      Error (Printf.sprintf "`chdir(%s)` failed with \"%s\"" path err_msg)
+    let open Result.Monad_infix in
+    (try Ok (Core_unix.chdir path)
+     with Core_unix.Unix_error (err, _, _) ->
+       let err_msg = Core_unix.Error.message err in
+       Error (Printf.sprintf "`chdir(%s)` failed with \"%s\"" path err_msg))
+    >>= fun () -> Core_unix.Filename_unix.realpath "."
 
   let mkdir = Core_unix.mkdir ~perm:0o775
 
diff --git a/lib/interpreter/native.mli b/lib/interpreter/native.mli
index 5d2557bc..5a10993e 100644
--- a/lib/interpreter/native.mli
+++ b/lib/interpreter/native.mli
@@ -28,7 +28,7 @@ module type Sig = sig
     string array ->
     (Runtime.t, string) Result.t
 
-  val chdir : string -> (unit, string) Result.t
+  val chdir : string -> (string, string) Result.t
   val directory_exists : string -> bool
   val file_exists : string -> bool
   val mkdir : string -> unit
diff --git a/native/native.ml b/native/native.ml
index f531a3b7..37f716fa 100644
--- a/native/native.ml
+++ b/native/native.ml
@@ -2,7 +2,7 @@
    Check for correctness if switching to Core! *)
 
 external file_read_all : string -> string option = "file_read_all"
-external chdir : string -> (unit, string) Result.t = "_chdir"
+external chdir : string -> (string, string) Result.t = "_chdir"
 (*
   val fd_read_all : Core_unix.File_descr.t -> (Runtime.t, string) Result.t
   val fd_write_all : Core_unix.File_descr.t -> string -> (unit, string) Result.t

From 06802b7924b8b9b99ed19cc17f4fda4a8a3c47c6 Mon Sep 17 00:00:00 2001
From: Christopher Fujino <christopherfujino@gmail.com>
Date: Thu, 30 Jul 2026 17:41:54 -0700
Subject: [PATCH 2/9] wip

---
 lib/interpreter/dune         | 7 +------
 lib/interpreter/interpret.ml | 8 ++++----
 lib/interpreter/native.ml    | 9 +++------
 lib/interpreter/native.mli   | 5 +++++
 4 files changed, 13 insertions(+), 16 deletions(-)

diff --git a/lib/interpreter/dune b/lib/interpreter/dune
index 6f236672..d5f1866f 100644
--- a/lib/interpreter/dune
+++ b/lib/interpreter/dune
@@ -1,10 +1,5 @@
 (library
  (name interpreter)
- (libraries
-   compiler
-   core_unix
-   core_unix.sys_unix
-   core_unix.filename_unix
-   sloth_common)
+ (libraries compiler core_unix sloth_common core_unix.sys_unix core_unix.filename_unix)
  (preprocess
   (pps ppx_jane)))
diff --git a/lib/interpreter/interpret.ml b/lib/interpreter/interpret.ml
index c57f254c..13602bc7 100644
--- a/lib/interpreter/interpret.ml
+++ b/lib/interpreter/interpret.ml
@@ -747,7 +747,7 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
                     Context.reassign globals.context_ids "$cwd"
                       (Runtime.String new_p)
                   with
-                  | Some () -> ()
+                  | Some () -> Printf.printf "[DEBUG] %s\n" new_p; ()
                   | None -> internal_failure "Failed to re-assign $cwd")
               | Error msg -> fail ~globals pos msg
             in
@@ -771,7 +771,7 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
         List.fold assignments ~init:(inner_globals, First ())
           ~f:(fun prev (name, expr) ->
             prev >>= fun globals () ->
-            interpret_expr globals expr >>= fun globals next_path ->
+            interpret_expr globals expr >>= fun globals next_value ->
             let prev =
               match Context.get globals.context_ids name with
               | Some prev -> prev
@@ -780,8 +780,8 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
                     name
                   |> fail ~globals pos
             in
-            middleware name prev next_path;
-            (match Context.reassign globals.context_ids name next_path with
+            middleware name prev next_value;
+            (match Context.reassign globals.context_ids name next_value with
             | Some () -> ()
             | None -> internal_failure __LOC__);
             (globals, First ()))
diff --git a/lib/interpreter/native.ml b/lib/interpreter/native.ml
index 4f77382d..4414b1af 100644
--- a/lib/interpreter/native.ml
+++ b/lib/interpreter/native.ml
@@ -8,13 +8,10 @@ type processMode =
   | BlockBuffer (* proc&! -> ProcessResult *)
 
 module type Sig = sig
-  (* TODO we should never be returning Runtime.t, but more specific types *)
-
   val file_read_all : string -> string
   val fd_read_all : Core_unix.File_descr.t -> (Runtime.t, string) Result.t
   val fd_write_all : Core_unix.File_descr.t -> string -> (unit, string) Result.t
 
-  (* TODO does this type of `data` need to be more generic to support binary? *)
   val write : Core_unix.File_descr.t -> data:string -> (unit, string) Result.t
   val read : Core_unix.File_descr.t -> (Runtime.t, string) Result.t
   val wait : Runtime.process_handle -> (Runtime.t, string) Result.t
@@ -102,7 +99,7 @@ module Prod : Sig = struct
      with Core_unix.Unix_error (err, _, _) ->
        let err_msg = Core_unix.Error.message err in
        Error (Printf.sprintf "`chdir(%s)` failed with \"%s\"" path err_msg))
-    >>= fun () -> Core_unix.Filename_unix.realpath "."
+    >>= fun () -> Ok (Filename_unix.realpath path)
 
   let mkdir = Core_unix.mkdir ~perm:0o775
 
@@ -436,8 +433,8 @@ module Make_test () : TestSig = struct
     this_pid
 
   let proc_expectations : Mock_process.spec option ref = ref None
-  let chdir _ = Ok ()
-  (* This function only exists to cause OS side-effects, no-op in tests *)
+  let chdir path = Ok path
+  (* Note: this will not normalize paths as the real one does *)
 
   let mkdir path =
     match Hashtbl.add path_to_entity ~key:path ~data:Directory with
diff --git a/lib/interpreter/native.mli b/lib/interpreter/native.mli
index 5a10993e..08c497cb 100644
--- a/lib/interpreter/native.mli
+++ b/lib/interpreter/native.mli
@@ -7,13 +7,18 @@ type processMode =
   | BlockBuffer (* proc&! -> ProcessResult *)
 
 module type Sig = sig
+  (* TODO we should never be returning Runtime.t, but more specific types *)
+
   val file_read_all : string -> string
   val fd_read_all : Core_unix.File_descr.t -> (Runtime.t, string) Result.t
   val fd_write_all : Core_unix.File_descr.t -> string -> (unit, string) Result.t
+
+  (* TODO does this type of `data` need to be more generic to support binary? *)
   val write : Core_unix.File_descr.t -> data:string -> (unit, string) Result.t
   val read : Core_unix.File_descr.t -> (Runtime.t, string) Result.t
   val wait : Runtime.process_handle -> (Runtime.t, string) Result.t
   val pipe : unit -> Core_unix.File_descr.t * Core_unix.File_descr.t
+  (** unit -> read * write *)
 
   val open_file :
     mode:Core_unix.open_flag list ->

From a05331077459c9c6f1b7b1549ca1ba846aa624b6 Mon Sep 17 00:00:00 2001
From: Christopher Fujino <christopherfujino@gmail.com>
Date: Thu, 30 Jul 2026 21:44:33 -0700
Subject: [PATCH 3/9] wip broken

---
 lib/interpreter/context.ml      |  6 +++++
 lib/interpreter/context.mli     |  1 +
 lib/interpreter/dune            |  1 +
 lib/interpreter/globals.ml      |  4 +--
 lib/interpreter/interpret.ml    | 18 +++++++++++---
 lib/interpreter/native.ml       | 40 ++----------------------------
 lib/interpreter/native.mli      | 43 ++-------------------------------
 lib/interpreter/native_sig.mli  | 43 +++++++++++++++++++++++++++++++++
 lib/interpreter/stdlib_impl.ml  | 12 ++++-----
 lib/interpreter/stdlib_impl.mli |  4 +--
 10 files changed, 80 insertions(+), 92 deletions(-)
 create mode 100644 lib/interpreter/native_sig.mli

diff --git a/lib/interpreter/context.ml b/lib/interpreter/context.ml
index ebaf216b..abb3d418 100644
--- a/lib/interpreter/context.ml
+++ b/lib/interpreter/context.ml
@@ -33,3 +33,9 @@ let reassign t' name v =
       let tbl = List.hd_exn t' in
       Hashtbl.change tbl name ~f:(fun _ -> Some v);
       Some ()
+
+let debug t' to_s =
+  List.iter t' ~f:(fun tbl ->
+      Hashtbl.iter_keys tbl ~f:(fun key ->
+          let value = Hashtbl.find tbl key |> Option.value_exn in
+          Printf.printf "%s => %s\n" key (to_s value)))
diff --git a/lib/interpreter/context.mli b/lib/interpreter/context.mli
index afeb1b15..96679fd1 100644
--- a/lib/interpreter/context.mli
+++ b/lib/interpreter/context.mli
@@ -5,3 +5,4 @@ val push_empty : 'a t -> 'a t
 val get : 'a t -> string -> 'a option
 val bind : 'a t -> string -> 'a -> unit option
 val reassign : 'a t -> string -> 'a -> unit option
+val debug : 'a t -> ('a -> string) -> unit
diff --git a/lib/interpreter/dune b/lib/interpreter/dune
index d5f1866f..6e849f90 100644
--- a/lib/interpreter/dune
+++ b/lib/interpreter/dune
@@ -1,5 +1,6 @@
 (library
  (name interpreter)
  (libraries compiler core_unix sloth_common core_unix.sys_unix core_unix.filename_unix)
+ (modules_without_implementation native_sig)
  (preprocess
   (pps ppx_jane)))
diff --git a/lib/interpreter/globals.ml b/lib/interpreter/globals.ml
index 63433693..dc238e4f 100644
--- a/lib/interpreter/globals.ml
+++ b/lib/interpreter/globals.ml
@@ -1,7 +1,7 @@
 open Core
 
 type t = {
-  l : (module Native.Sig);
+  l : (module Native_sig.Sig);
   identifiers : Runtime.t Identifiers.t;
   context_ids : Runtime.t Context.t;
   (* We need to store these at the top level so that we can find these for
@@ -26,7 +26,7 @@ let push_frame t next_func pos =
 
 (* TODO memoize *)
 let make_globals m src script_path ~argv ~env ~version =
-  let module M = (val m : Native.Sig) in
+  let module M = (val m : Native_sig.Sig) in
   let classes = Hashtbl.create (module String) in
   let identifiers : Runtime.t Identifiers.t = Identifiers.create () in
   let context_ids = Context.create () in
diff --git a/lib/interpreter/interpret.ml b/lib/interpreter/interpret.ml
index 13602bc7..d17a1f13 100644
--- a/lib/interpreter/interpret.ml
+++ b/lib/interpreter/interpret.ml
@@ -728,16 +728,21 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
           | Exit _ -> (globals, either)
           | Return _ -> (globals, either)))
   | WithExpr (assignments, block, pos) ->
+      (* TODO *)
       let module M = (val globals.l) in
       let post_block_hook = ref None in
       let inner_globals =
         { globals with context_ids = Context.push_empty globals.context_ids }
       in
+      let debug tag =
+        let cwd = Context.get inner_globals.context_ids "$cwd" |> Option.value_exn |> Runtime.to_s in
+        Printf.printf "[DEBUG] %s\t$CWD = %s\n" tag cwd
+      in
       (*
       Process side effects for certain context variables
       These can't be recovered from, we must fail immediately
       *)
-      let middleware name prev next =
+      let middleware name prev next = (* TODO: this is wrong, we shouldn't re-assign $cwd here, we do it later; we should have a seperate syscall for realpath, and do it before here *)
         match name with
         | "$cwd" ->
             let cd_wrapper p =
@@ -747,7 +752,9 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
                     Context.reassign globals.context_ids "$cwd"
                       (Runtime.String new_p)
                   with
-                  | Some () -> Printf.printf "[DEBUG] %s\n" new_p; ()
+                  | Some () ->
+                      debug "B";
+                      ()
                   | None -> internal_failure "Failed to re-assign $cwd")
               | Error msg -> fail ~globals pos msg
             in
@@ -767,6 +774,7 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
         | _ -> ()
       in
 
+      debug "C";
       let _, either =
         List.fold assignments ~init:(inner_globals, First ())
           ~f:(fun prev (name, expr) ->
@@ -780,15 +788,19 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
                     name
                   |> fail ~globals pos
             in
+            debug "D";
             middleware name prev next_value;
+            debug "E";
             (match Context.reassign globals.context_ids name next_value with
             | Some () -> ()
             | None -> internal_failure __LOC__);
             (globals, First ()))
         >>= fun globals () -> (globals, interpret_block globals block)
       in
+      debug "F";
       (* This needs to run regardless of either's state *)
       let _ = Option.map !post_block_hook ~f:(fun hook -> hook ()) in
+      debug "G";
       (globals, either)
   | CatchExpr { subject; capture; catch; pos = _ } -> (
       let globals, subject_either = interpret_expr globals subject in
@@ -1155,7 +1167,7 @@ and cast_to_file_descriptor ~globals ~pos ~mode ~m t :
   match t with
   | FileDescriptor fd -> First fd
   | File { path } -> (
-      let module M = (val m : Native.Sig) in
+      let module M = (val m : Native_sig.Sig) in
       match M.open_file ~mode path with
       | Ok fd -> First fd
       | Error msg -> failure_obj ~globals ~pos msg)
diff --git a/lib/interpreter/native.ml b/lib/interpreter/native.ml
index 4414b1af..c176f404 100644
--- a/lib/interpreter/native.ml
+++ b/lib/interpreter/native.ml
@@ -1,44 +1,8 @@
 open Core
 open Sloth_common.Common
+open Native_sig
 
-type processMode =
-  | BlockInherit (* proc! -> null *)
-  | ForkInherit (* ? -> ProcessHandle *)
-  | ForkBuffer (* proc& -> ProcessHandle *)
-  | BlockBuffer (* proc&! -> ProcessResult *)
-
-module type Sig = sig
-  val file_read_all : string -> string
-  val fd_read_all : Core_unix.File_descr.t -> (Runtime.t, string) Result.t
-  val fd_write_all : Core_unix.File_descr.t -> string -> (unit, string) Result.t
-
-  val write : Core_unix.File_descr.t -> data:string -> (unit, string) Result.t
-  val read : Core_unix.File_descr.t -> (Runtime.t, string) Result.t
-  val wait : Runtime.process_handle -> (Runtime.t, string) Result.t
-
-  val pipe : unit -> Core_unix.File_descr.t * Core_unix.File_descr.t
-  (** unit -> read * write *)
-
-  val open_file :
-    mode:Core_unix.open_flag list ->
-    string ->
-    (Core_unix.File_descr.t, string) Result.t
-
-  val close : Core_unix.File_descr.t -> (unit, string) Result.t
-
-  val proc_exec :
-    mode:processMode ->
-    Runtime.process ->
-    string array ->
-    (Runtime.t, string) Result.t
-
-  val chdir : string -> (string, string) Result.t
-  val directory_exists : string -> bool
-  val file_exists : string -> bool
-  val mkdir : string -> unit
-end
-
-module Prod : Sig = struct
+module Prod : Native_sig.Sig = struct
   let pipe = Core_unix.pipe ~close_on_exec:true
   let file_read_all = In_channel.read_all
 
diff --git a/lib/interpreter/native.mli b/lib/interpreter/native.mli
index 08c497cb..c357fcbe 100644
--- a/lib/interpreter/native.mli
+++ b/lib/interpreter/native.mli
@@ -1,48 +1,9 @@
 open Core
 
-type processMode =
-  | BlockInherit (* proc! -> null *)
-  | ForkInherit (* ? -> ProcessHandle *)
-  | ForkBuffer (* proc& -> ProcessHandle *)
-  | BlockBuffer (* proc&! -> ProcessResult *)
-
-module type Sig = sig
-  (* TODO we should never be returning Runtime.t, but more specific types *)
-
-  val file_read_all : string -> string
-  val fd_read_all : Core_unix.File_descr.t -> (Runtime.t, string) Result.t
-  val fd_write_all : Core_unix.File_descr.t -> string -> (unit, string) Result.t
-
-  (* TODO does this type of `data` need to be more generic to support binary? *)
-  val write : Core_unix.File_descr.t -> data:string -> (unit, string) Result.t
-  val read : Core_unix.File_descr.t -> (Runtime.t, string) Result.t
-  val wait : Runtime.process_handle -> (Runtime.t, string) Result.t
-  val pipe : unit -> Core_unix.File_descr.t * Core_unix.File_descr.t
-  (** unit -> read * write *)
-
-  val open_file :
-    mode:Core_unix.open_flag list ->
-    string ->
-    (Core_unix.File_descr.t, string) Result.t
-
-  val close : Core_unix.File_descr.t -> (unit, string) Result.t
-
-  val proc_exec :
-    mode:processMode ->
-    Runtime.process ->
-    string array ->
-    (Runtime.t, string) Result.t
-
-  val chdir : string -> (string, string) Result.t
-  val directory_exists : string -> bool
-  val file_exists : string -> bool
-  val mkdir : string -> unit
-end
-
-module Prod : Sig
+module Prod : Native_sig.Sig
 
 module type TestSig = sig
-  include Sig
+  include Native_sig.Sig
 
   type fs_entity = File of string ref * Core_unix.File_descr.t | Directory
 
diff --git a/lib/interpreter/native_sig.mli b/lib/interpreter/native_sig.mli
new file mode 100644
index 00000000..86c48f0a
--- /dev/null
+++ b/lib/interpreter/native_sig.mli
@@ -0,0 +1,43 @@
+open Core
+
+(* This is its own, interface-only module to work around:
+  https://discuss.ocaml.org/t/why-do-i-need-to-repeat-type-declarations-between-interfaces-and-implementations-or-how-do-i-get-around-this/3350 *)
+
+type processMode =
+  | BlockInherit (* proc! -> null *)
+  | ForkInherit (* ? -> ProcessHandle *)
+  | ForkBuffer (* proc& -> ProcessHandle *)
+  | BlockBuffer (* proc&! -> ProcessResult *)
+
+module type Sig = sig
+  (* TODO we should never be returning Runtime.t, but more specific types *)
+
+  val file_read_all : string -> string
+  val fd_read_all : Core_unix.File_descr.t -> (Runtime.t, string) Result.t
+  val fd_write_all : Core_unix.File_descr.t -> string -> (unit, string) Result.t
+
+  (* TODO does this type of `data` need to be more generic to support binary? *)
+  val write : Core_unix.File_descr.t -> data:string -> (unit, string) Result.t
+  val read : Core_unix.File_descr.t -> (Runtime.t, string) Result.t
+  val wait : Runtime.process_handle -> (Runtime.t, string) Result.t
+  val pipe : unit -> Core_unix.File_descr.t * Core_unix.File_descr.t
+  (** unit -> read * write *)
+
+  val open_file :
+    mode:Core_unix.open_flag list ->
+    string ->
+    (Core_unix.File_descr.t, string) Result.t
+
+  val close : Core_unix.File_descr.t -> (unit, string) Result.t
+
+  val proc_exec :
+    mode:processMode ->
+    Runtime.process ->
+    string array ->
+    (Runtime.t, string) Result.t
+
+  val chdir : string -> (string, string) Result.t
+  val directory_exists : string -> bool
+  val file_exists : string -> bool
+  val mkdir : string -> unit
+end
diff --git a/lib/interpreter/stdlib_impl.ml b/lib/interpreter/stdlib_impl.ml
index 64224fbe..9099d57f 100644
--- a/lib/interpreter/stdlib_impl.ml
+++ b/lib/interpreter/stdlib_impl.ml
@@ -37,7 +37,7 @@ let make_method ~arity ~name cb =
  fun self -> Ok (make_native_func ~arity ~name (cb self))
 
 let make_ids m =
-  let module M = (val m : Native.Sig) in
+  let module M = (val m : Native_sig.Sig) in
   [
     ( "print",
       make_native_func ~arity:(Some 1) ~name:"print" (fun ctx _ args ->
@@ -114,7 +114,7 @@ let make_ids m =
   ]
 
 let make_protos m =
-  let module M = (val m : Native.Sig) in
+  let module M = (val m : Native_sig.Sig) in
   [
     {
       name = "List";
@@ -365,7 +365,7 @@ let make_protos m =
                              `HashMap[String]String`, but got %s"
                          @@ Runtime.to_s env)
                 in
-                match M.proc_exec ~mode:Native.ForkBuffer self env with
+                match M.proc_exec ~mode:Native_sig.ForkBuffer self env with
                 | Ok t' -> First t'
                 | Error err -> Second (Runtime.Error (None, String err))) );
           ( "forkInherit",
@@ -388,7 +388,7 @@ let make_protos m =
                              `HashMap[String]String`, but got %s"
                          @@ Runtime.to_s env)
                 in
-                match M.proc_exec ~mode:Native.ForkInherit self env with
+                match M.proc_exec ~mode:Native_sig.ForkInherit self env with
                 | Ok t' -> First t'
                 | Error err -> Second (Runtime.Error (None, String err))) );
           ( "blockBuffer",
@@ -411,7 +411,7 @@ let make_protos m =
                              `HashMap[String]String`, but got %s"
                          @@ Runtime.to_s env)
                 in
-                match M.proc_exec ~mode:Native.BlockBuffer self env with
+                match M.proc_exec ~mode:Native_sig.BlockBuffer self env with
                 | Ok t' -> First t'
                 | Error err -> Second (Runtime.Error (None, String err))) );
           ( "blockInherit",
@@ -434,7 +434,7 @@ let make_protos m =
                              `HashMap[String]String`, but got %s"
                          @@ Runtime.to_s env)
                 in
-                match M.proc_exec ~mode:Native.BlockInherit self env with
+                match M.proc_exec ~mode:Native_sig.BlockInherit self env with
                 | Ok t' -> First t'
                 | Error err -> Second (Runtime.Error (None, String err))) );
           ( "stdout",
diff --git a/lib/interpreter/stdlib_impl.mli b/lib/interpreter/stdlib_impl.mli
index bd8316ad..c5bc9b05 100644
--- a/lib/interpreter/stdlib_impl.mli
+++ b/lib/interpreter/stdlib_impl.mli
@@ -5,8 +5,8 @@ type proto_t = {
   static_getters : (string * (Runtime.t -> (Runtime.t, string) Result.t)) list;
 }
 
-val make_ids : (module Native.Sig) -> (string * Runtime.t) list
-val make_protos : (module Native.Sig) -> proto_t list
+val make_ids : (module Native_sig.Sig) -> (string * Runtime.t) list
+val make_protos : (module Native_sig.Sig) -> proto_t list
 
 val context_ids :
   cwd:string ->

From ad39d3bfdbc2aec1801d8ca6ca2a916010a6ad72 Mon Sep 17 00:00:00 2001
From: Christopher Fujino <christopherfujino@gmail.com>
Date: Fri, 31 Jul 2026 12:32:18 -0700
Subject: [PATCH 4/9] add failing integration test

---
 lib/interpreter/dune                  |  7 ++++++-
 lib/interpreter/interpret.ml          |  8 ++++++--
 lib/interpreter/native.mli            |  1 -
 lib/interpreter/native_sig.mli        |  1 +
 test/integration_tests/cwd/test.sloth | 16 +++++++++++++++-
 tool/ci.ml                            |  4 ++--
 6 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/lib/interpreter/dune b/lib/interpreter/dune
index 6e849f90..6b2cf1ce 100644
--- a/lib/interpreter/dune
+++ b/lib/interpreter/dune
@@ -1,6 +1,11 @@
 (library
  (name interpreter)
- (libraries compiler core_unix sloth_common core_unix.sys_unix core_unix.filename_unix)
+ (libraries
+  compiler
+  core_unix
+  sloth_common
+  core_unix.sys_unix
+  core_unix.filename_unix)
  (modules_without_implementation native_sig)
  (preprocess
   (pps ppx_jane)))
diff --git a/lib/interpreter/interpret.ml b/lib/interpreter/interpret.ml
index d17a1f13..22be904e 100644
--- a/lib/interpreter/interpret.ml
+++ b/lib/interpreter/interpret.ml
@@ -735,14 +735,18 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
         { globals with context_ids = Context.push_empty globals.context_ids }
       in
       let debug tag =
-        let cwd = Context.get inner_globals.context_ids "$cwd" |> Option.value_exn |> Runtime.to_s in
+        let cwd =
+          Context.get inner_globals.context_ids "$cwd"
+          |> Option.value_exn |> Runtime.to_s
+        in
         Printf.printf "[DEBUG] %s\t$CWD = %s\n" tag cwd
       in
       (*
       Process side effects for certain context variables
       These can't be recovered from, we must fail immediately
       *)
-      let middleware name prev next = (* TODO: this is wrong, we shouldn't re-assign $cwd here, we do it later; we should have a seperate syscall for realpath, and do it before here *)
+      let middleware name prev next =
+        (* TODO: this is wrong, we shouldn't re-assign $cwd here, we do it later; we should have a seperate syscall for realpath, and do it before here *)
         match name with
         | "$cwd" ->
             let cd_wrapper p =
diff --git a/lib/interpreter/native.mli b/lib/interpreter/native.mli
index c357fcbe..df28ad3a 100644
--- a/lib/interpreter/native.mli
+++ b/lib/interpreter/native.mli
@@ -1,5 +1,4 @@
 open Core
-
 module Prod : Native_sig.Sig
 
 module type TestSig = sig
diff --git a/lib/interpreter/native_sig.mli b/lib/interpreter/native_sig.mli
index 86c48f0a..a9a09060 100644
--- a/lib/interpreter/native_sig.mli
+++ b/lib/interpreter/native_sig.mli
@@ -20,6 +20,7 @@ module type Sig = sig
   val write : Core_unix.File_descr.t -> data:string -> (unit, string) Result.t
   val read : Core_unix.File_descr.t -> (Runtime.t, string) Result.t
   val wait : Runtime.process_handle -> (Runtime.t, string) Result.t
+
   val pipe : unit -> Core_unix.File_descr.t * Core_unix.File_descr.t
   (** unit -> read * write *)
 
diff --git a/test/integration_tests/cwd/test.sloth b/test/integration_tests/cwd/test.sloth
index 64a71c9e..d8300e44 100755
--- a/test/integration_tests/cwd/test.sloth
+++ b/test/integration_tests/cwd/test.sloth
@@ -1,14 +1,28 @@
 #!/usr/bin/env sloth
 
+let sub = Directory("${$scriptDir}/sub")
+# Cleanup from previous failed runs
+if sub.exists() {
+  "rmdir ${ sub.path }"!
+}
+
 with ($cwd = $scriptDir) {
   let cwd = ("pwd"&!).stdout.trim()
   assert(cwd == $cwd)
 
-  let sub = Directory("${$cwd}/sub")
   sub.create()
   with ($cwd = sub.path) {
     cwd = ("pwd"&!).stdout.trim()
     assert(cwd == $cwd)
+
+    # Go back up to $scriptDir
+    with ($cwd = '..') {
+      cwd = ("pwd"&!).stdout.trim()
+      if cwd != $cwd {
+        throw "${ cwd } != ${ $cwd }"
+      }
+      assert(cwd == $cwd)
+    }
   }
 
   cwd = ("pwd"&!).stdout.trim()
diff --git a/tool/ci.ml b/tool/ci.ml
index 4d533f10..f32a1197 100644
--- a/tool/ci.ml
+++ b/tool/ci.ml
@@ -122,5 +122,5 @@ let () =
   match res with
   | Ok -> print_endline "CI suite successful!"
   | Error errs ->
-      print_endline "CI suite failed with the following errors:";
-      List.iter (fun msg -> Printf.printf "-> %s\n" msg) errs
+      print_endline "\n\nCI suite failed with the following errors:";
+      List.iter (fun msg -> Printf.printf " - %s\n" msg) errs

From 73fd596fda36ee443f042929ef5f4f207c242198 Mon Sep 17 00:00:00 2001
From: Christopher Fujino <christopherfujino@gmail.com>
Date: Fri, 31 Jul 2026 12:54:17 -0700
Subject: [PATCH 5/9] fix

---
 MILESTONES.md                  |  2 +-
 lib/common/common.ml           |  1 +
 lib/interpreter/interpret.ml   | 40 ++++++++++++++--------------------
 lib/interpreter/native.ml      | 20 ++++++++++-------
 lib/interpreter/native_sig.mli |  3 ++-
 5 files changed, 32 insertions(+), 34 deletions(-)

diff --git a/MILESTONES.md b/MILESTONES.md
index e4de750a..a0557572 100644
--- a/MILESTONES.md
+++ b/MILESTONES.md
@@ -41,6 +41,7 @@
 
 ## next
 - [x] Feature: Process.forkInherit()
+- [x] bug: $cwd is NOT always absolute
 
 ## v0.1 - initially tagged version
 - [x] Language versioning
@@ -119,4 +120,3 @@
 ## Bugs
 - [ ] bug: stacktrace printing is wrong in REPL
 - [ ] bug: Do static methods need to take self as first arg?
-- [ ] bug: $cwd is NOT always absolute
diff --git a/lib/common/common.ml b/lib/common/common.ml
index 6181e324..8ed94185 100644
--- a/lib/common/common.ml
+++ b/lib/common/common.ml
@@ -26,6 +26,7 @@ let wrap_error cb =
   | CompileError msg -> Error msg
   | RuntimeError msg -> Error msg
 
+(* TODO: why is this different from `internal_failure` above? *)
 let option_value opt ~message =
   match opt with Some v -> v | None -> raise @@ InternalFailure message
 
diff --git a/lib/interpreter/interpret.ml b/lib/interpreter/interpret.ml
index 22be904e..a7dca300 100644
--- a/lib/interpreter/interpret.ml
+++ b/lib/interpreter/interpret.ml
@@ -734,13 +734,6 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
       let inner_globals =
         { globals with context_ids = Context.push_empty globals.context_ids }
       in
-      let debug tag =
-        let cwd =
-          Context.get inner_globals.context_ids "$cwd"
-          |> Option.value_exn |> Runtime.to_s
-        in
-        Printf.printf "[DEBUG] %s\t$CWD = %s\n" tag cwd
-      in
       (*
       Process side effects for certain context variables
       These can't be recovered from, we must fail immediately
@@ -751,15 +744,7 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
         | "$cwd" ->
             let cd_wrapper p =
               match M.chdir p with
-              | Ok new_p -> (
-                  match
-                    Context.reassign globals.context_ids "$cwd"
-                      (Runtime.String new_p)
-                  with
-                  | Some () ->
-                      debug "B";
-                      ()
-                  | None -> internal_failure "Failed to re-assign $cwd")
+              | Ok _ -> ()
               | Error msg -> fail ~globals pos msg
             in
             (match Runtime.string_of_val next with
@@ -778,7 +763,6 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
         | _ -> ()
       in
 
-      debug "C";
       let _, either =
         List.fold assignments ~init:(inner_globals, First ())
           ~f:(fun prev (name, expr) ->
@@ -792,19 +776,27 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
                     name
                   |> fail ~globals pos
             in
-            debug "D";
+            let next_value =
+              match name with
+              | "$cwd" -> (
+                  match Runtime.string_of_val next_value with
+                  | Some path -> Runtime.String (M.realpath path)
+                  | None ->
+                      fail ~globals pos
+                      @@ Printf.sprintf
+                           "Cannot assign %s to $cwd because it must be a \
+                            `String`"
+                      @@ Runtime.to_s next_value)
+              | _ -> next_value
+            in
+            Context.reassign globals.context_ids name next_value
+            |> option_value ~message:__LOC__;
             middleware name prev next_value;
-            debug "E";
-            (match Context.reassign globals.context_ids name next_value with
-            | Some () -> ()
-            | None -> internal_failure __LOC__);
             (globals, First ()))
         >>= fun globals () -> (globals, interpret_block globals block)
       in
-      debug "F";
       (* This needs to run regardless of either's state *)
       let _ = Option.map !post_block_hook ~f:(fun hook -> hook ()) in
-      debug "G";
       (globals, either)
   | CatchExpr { subject; capture; catch; pos = _ } -> (
       let globals, subject_either = interpret_expr globals subject in
diff --git a/lib/interpreter/native.ml b/lib/interpreter/native.ml
index c176f404..d55519e1 100644
--- a/lib/interpreter/native.ml
+++ b/lib/interpreter/native.ml
@@ -58,12 +58,13 @@ module Prod : Native_sig.Sig = struct
     else Ok ()
 
   let chdir path =
-    let open Result.Monad_infix in
-    (try Ok (Core_unix.chdir path)
-     with Core_unix.Unix_error (err, _, _) ->
-       let err_msg = Core_unix.Error.message err in
-       Error (Printf.sprintf "`chdir(%s)` failed with \"%s\"" path err_msg))
-    >>= fun () -> Ok (Filename_unix.realpath path)
+    try Ok (Core_unix.chdir path)
+    with Core_unix.Unix_error (err, _, _) ->
+      let err_msg = Core_unix.Error.message err in
+      Error (Printf.sprintf "`chdir(%s)` failed with \"%s\"" path err_msg)
+
+  (** TODO make a Result.t *)
+  let realpath path = Filename_unix.realpath path
 
   let mkdir = Core_unix.mkdir ~perm:0o775
 
@@ -397,8 +398,11 @@ module Make_test () : TestSig = struct
     this_pid
 
   let proc_expectations : Mock_process.spec option ref = ref None
-  let chdir path = Ok path
-  (* Note: this will not normalize paths as the real one does *)
+  let chdir _ = Ok ()
+  (* Note: this function only exists to cause side effects *)
+
+  let realpath _ = "TODO normalize test realpath"
+  (* TODO: write test normalization algorithm? *)
 
   let mkdir path =
     match Hashtbl.add path_to_entity ~key:path ~data:Directory with
diff --git a/lib/interpreter/native_sig.mli b/lib/interpreter/native_sig.mli
index a9a09060..ef78572d 100644
--- a/lib/interpreter/native_sig.mli
+++ b/lib/interpreter/native_sig.mli
@@ -37,8 +37,9 @@ module type Sig = sig
     string array ->
     (Runtime.t, string) Result.t
 
-  val chdir : string -> (string, string) Result.t
+  val chdir : string -> (unit, string) Result.t
   val directory_exists : string -> bool
   val file_exists : string -> bool
   val mkdir : string -> unit
+  val realpath : string -> string
 end

From bcfda7c904104ca851f1666774c47a2638d11034 Mon Sep 17 00:00:00 2001
From: Christopher Fujino <christopherfujino@gmail.com>
Date: Fri, 31 Jul 2026 12:55:54 -0700
Subject: [PATCH 6/9] remove debug function

---
 lib/interpreter/context.ml  | 6 ------
 lib/interpreter/context.mli | 1 -
 2 files changed, 7 deletions(-)

diff --git a/lib/interpreter/context.ml b/lib/interpreter/context.ml
index abb3d418..ebaf216b 100644
--- a/lib/interpreter/context.ml
+++ b/lib/interpreter/context.ml
@@ -33,9 +33,3 @@ let reassign t' name v =
       let tbl = List.hd_exn t' in
       Hashtbl.change tbl name ~f:(fun _ -> Some v);
       Some ()
-
-let debug t' to_s =
-  List.iter t' ~f:(fun tbl ->
-      Hashtbl.iter_keys tbl ~f:(fun key ->
-          let value = Hashtbl.find tbl key |> Option.value_exn in
-          Printf.printf "%s => %s\n" key (to_s value)))
diff --git a/lib/interpreter/context.mli b/lib/interpreter/context.mli
index 96679fd1..afeb1b15 100644
--- a/lib/interpreter/context.mli
+++ b/lib/interpreter/context.mli
@@ -5,4 +5,3 @@ val push_empty : 'a t -> 'a t
 val get : 'a t -> string -> 'a option
 val bind : 'a t -> string -> 'a -> unit option
 val reassign : 'a t -> string -> 'a -> unit option
-val debug : 'a t -> ('a -> string) -> unit

From e14d7676c94112221c36a3c26edf37ab048f0558 Mon Sep 17 00:00:00 2001
From: Christopher Fujino <christopherfujino@gmail.com>
Date: Fri, 31 Jul 2026 12:58:32 -0700
Subject: [PATCH 7/9] remove todos

---
 lib/interpreter/interpret.ml | 2 --
 1 file changed, 2 deletions(-)

diff --git a/lib/interpreter/interpret.ml b/lib/interpreter/interpret.ml
index a7dca300..3d97f464 100644
--- a/lib/interpreter/interpret.ml
+++ b/lib/interpreter/interpret.ml
@@ -728,7 +728,6 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
           | Exit _ -> (globals, either)
           | Return _ -> (globals, either)))
   | WithExpr (assignments, block, pos) ->
-      (* TODO *)
       let module M = (val globals.l) in
       let post_block_hook = ref None in
       let inner_globals =
@@ -739,7 +738,6 @@ and interpret_expr globals (expr : Compiler.Optimizer.expr) =
       These can't be recovered from, we must fail immediately
       *)
       let middleware name prev next =
-        (* TODO: this is wrong, we shouldn't re-assign $cwd here, we do it later; we should have a seperate syscall for realpath, and do it before here *)
         match name with
         | "$cwd" ->
             let cd_wrapper p =

From 1580cef4c95071545b403d76ad3e3313b2b8dd09 Mon Sep 17 00:00:00 2001
From: Christopher Fujino <christopherfujino@gmail.com>
Date: Fri, 31 Jul 2026 13:01:17 -0700
Subject: [PATCH 8/9] fix revert

---
 native/native.ml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/native/native.ml b/native/native.ml
index 37f716fa..f531a3b7 100644
--- a/native/native.ml
+++ b/native/native.ml
@@ -2,7 +2,7 @@
    Check for correctness if switching to Core! *)
 
 external file_read_all : string -> string option = "file_read_all"
-external chdir : string -> (string, string) Result.t = "_chdir"
+external chdir : string -> (unit, string) Result.t = "_chdir"
 (*
   val fd_read_all : Core_unix.File_descr.t -> (Runtime.t, string) Result.t
   val fd_write_all : Core_unix.File_descr.t -> string -> (unit, string) Result.t

From 3f28c4e9b6d3c199c99714804e478dfb310d1e39 Mon Sep 17 00:00:00 2001
From: Christopher Fujino <christopherfujino@gmail.com>
Date: Fri, 31 Jul 2026 13:07:53 -0700
Subject: [PATCH 9/9] fix

---
 bin/native_test.ml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/bin/native_test.ml b/bin/native_test.ml
index 2615ce8b..727bc197 100644
--- a/bin/native_test.ml
+++ b/bin/native_test.ml
@@ -12,4 +12,4 @@ let () =
   |> map_error (fun msg ->
       Printf.printf "CD-ing to /nosuchdir returned \"%s\"\n" msg);
   Sn.Native.chdir "/"
-  |> map_ok (fun msg -> Printf.printf "CD-ing to / succeeded with \"%s\"." msg)
+  |> map_ok (fun () -> Printf.printf "CD-ing to / succeeded.\n")