ocaml: phase 6 Filename module + Char.compare/equal/escaped (+7 tests, 569 total)

Filename module (forward-slash only, no Windows-separator detection):

  basename '/foo/bar/baz.ml'        = 'baz.ml'
  dirname  '/foo/bar/baz.ml'        = '/foo/bar'
  extension 'baz.tar.gz'            = '.gz'
  chop_extension 'hello.ml'         = 'hello'
  concat 'a' 'b'                    = 'a/b'
  is_relative 'a/b'                 = true
  current_dir_name = '.', parent_dir_name = '..', dir_sep = '/'

Char additions:

  equal a b                         = (a = b)
  compare a b                       = code(a) - code(b)
  escaped '\n'                      = '\\n'    (likewise t, r, \\, ")

lib/ocaml/runtime.sx

@@ -474,6 +474,69 @@
      let is_alnum c = is_alpha c || is_digit c
      let is_whitespace c =
        c = \" \" || c = \"\\t\" || c = \"\\n\" || c = \"\\r\"
+     let equal a b = a = b
+     let compare a b = _char_code a - _char_code b
+     let escaped c =
+       if c = \"\\n\" then \"\\\\n\"
+       else if c = \"\\t\" then \"\\\\t\"
+       else if c = \"\\r\" then \"\\\\r\"
+       else if c = \"\\\\\" then \"\\\\\\\\\"
+       else if c = \"\\\"\" then \"\\\\\\\"\"
+       else c
+   end ;;
+
+   module Filename = struct
+     (* Minimal Filename: basename / dirname / extension / concat /
+        chop_suffix.  Forward-slash only — doesn't try to detect
+        Windows-style separators. *)
+     let _last_slash s =
+       let n = _string_length s in
+       let rec aux i =
+         if i < 0 then -1
+         else if _string_get s i = \"/\" then i
+         else aux (i - 1)
+       in
+       aux (n - 1)
+
+     let basename s =
+       let i = _last_slash s in
+       if i < 0 then s
+       else _string_sub s (i + 1) (_string_length s - i - 1)
+
+     let dirname s =
+       let i = _last_slash s in
+       if i < 0 then \".\"
+       else if i = 0 then \"/\"
+       else _string_sub s 0 i
+
+     let extension s =
+       let b = basename s in
+       let n = _string_length b in
+       let rec aux i =
+         if i < 0 then \"\"
+         else if _string_get b i = \".\" then
+           _string_sub b i (n - i)
+         else aux (i - 1)
+       in
+       aux (n - 1)
+
+     let chop_extension s =
+       let ext = extension s in
+       let nx = _string_length ext in
+       if nx = 0 then s
+       else _string_sub s 0 (_string_length s - nx)
+
+     let concat a b =
+       if _string_length a = 0 then b
+       else if _string_get a (_string_length a - 1) = \"/\" then a ^ b
+       else a ^ \"/\" ^ b
+
+     let is_relative s =
+       _string_length s = 0 || _string_get s 0 <> \"/\"
+
+     let current_dir_name = \".\"
+     let parent_dir_name = \"..\"
+     let dir_sep = \"/\"
    end ;;
 
    module Int = struct

lib/ocaml/test.sh

@@ -1404,6 +1404,24 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 5152)
 (eval "(ocaml-run \"let f ?x ~y = x + y in f ?x:1 ~y:2\")")
 
+;; ── Filename module ──────────────────────────────────────────
+(epoch 5160)
+(eval "(ocaml-run \"Filename.basename \\\"/foo/bar/baz.ml\\\"\")")
+(epoch 5161)
+(eval "(ocaml-run \"Filename.dirname \\\"/foo/bar/baz.ml\\\"\")")
+(epoch 5162)
+(eval "(ocaml-run \"Filename.extension \\\"baz.tar.gz\\\"\")")
+(epoch 5163)
+(eval "(ocaml-run \"Filename.concat \\\"a\\\" \\\"b\\\"\")")
+(epoch 5164)
+(eval "(ocaml-run \"Filename.chop_extension \\\"hello.ml\\\"\")")
+
+;; ── Char.compare / equal / escaped ─────────────────────────
+(epoch 5170)
+(eval "(ocaml-run \"Char.compare \\\"b\\\" \\\"a\\\"\")")
+(epoch 5171)
+(eval "(ocaml-run \"Char.equal \\\"a\\\" \\\"a\\\"\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -2232,6 +2250,17 @@ check 5150 "f ~x:3 ~y:7 sum"              '10'
 check 5151 "f ~x ~y punning"              '20'
 check 5152 "f ?x:1 ~y:2 (no Some wrap)"   '3'
 
+# ── Filename module ─────────────────────────────────────────────
+check 5160 "basename /foo/bar/baz.ml"     '"baz.ml"'
+check 5161 "dirname /foo/bar/baz.ml"      '"/foo/bar"'
+check 5162 "extension baz.tar.gz"         '".gz"'
+check 5163 "concat a b"                   '"a/b"'
+check 5164 "chop_extension hello.ml"      '"hello"'
+
+# ── Char.compare / equal ────────────────────────────────────────
+check 5170 "Char.compare b a"             '1'
+check 5171 "Char.equal a a"               'true'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"

plans/ocaml-on-sx.md

@@ -407,6 +407,12 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 6 — Filename module + Char.compare/equal/escaped
+  (+7 tests, 569 total). Filename: basename, dirname, extension,
+  chop_extension, concat, is_relative + dir_sep / current_dir_name /
+  parent_dir_name constants. Forward-slash only, doesn't try to
+  detect Windows separators. Char additions: equal, compare (via
+  code subtraction), escaped (handles `\n`/`\t`/`\r`/`\\`/`\"`).
 - 2026-05-09 Phase 4 — basic labeled / optional argument syntax
   (label dropped, positional semantics) (+3 tests, 562 total). Three
   parser changes: