ocaml: phase 6 Printf width specifiers %5d/%-5d/%05d/%4s (+5 tests, 538 total)

The Printf walker now parses optional flags + width digits between
'%' and the spec letter:

  -  left-align (default is right-align)
  0  zero-pad (default is space-pad; only honoured when not left-aligned)
  Nd... decimal width digits (any number)

After formatting the argument into a base string with the existing
spec dispatch (%d/%i/%u/%s/%f/%c/%b/%x/%X/%o), the result is padded
to the requested width.

Workaround: width and spec_pos are returned packed as
  width * 1000000 + spec_pos
because the parser does not yet support tuple destructuring in let
('let (a, b) = expr in body' fails with 'expected ident'). TODO: lift
that limitation; for now the encoding round-trips losslessly for any
practical width.

  Printf.sprintf '%5d'  42      = '   42'
  Printf.sprintf '%-5d|' 42     = '42   |'
  Printf.sprintf '%05d' 42      = '00042'
  Printf.sprintf '%4s' 'hi'     = '  hi'
  Printf.sprintf 'hi=%-3d, hex=%04x' 9 15 = 'hi=9  , hex=000f'

lib/ocaml/runtime.sx

@@ -512,36 +512,89 @@
    end ;;
 
    module Printf = struct
-     (* sprintf walks fmt, accumulating prefix. When it sees a %X
-        spec, it returns a function of one arg that substitutes the
-        arg and recurses on the rest of fmt. With no specs, returns
-        the bare format string. Specs supported: %d %s %f %c %b
-        (and %% as a literal). Unknown specs are passed through. *)
+     (* sprintf walks fmt char-by-char. On '%' it parses optional
+        flags ('-' for left-justify, '0' for zero-pad), an optional
+        decimal width, and a final spec letter. Specs supported:
+        %d %i %u %s %f %c %b %x %X %o (and %% as a literal).
+        Width pads the formatted argument to at least N characters. *)
      let sprintf fmt =
        let n = _string_length fmt in
+       let is_spec c =
+         c = \"d\" || c = \"i\" || c = \"u\" || c = \"s\" || c = \"f\"
+         || c = \"c\" || c = \"b\" || c = \"x\" || c = \"X\" || c = \"o\"
+       in
+       let is_digit c =
+         let k = _char_code c in k >= 48 && k <= 57
+       in
+       let pad s width left zero =
+         let pad_len = width - _string_length s in
+         if pad_len <= 0 then s
+         else
+           let ch = if zero && (not left) then \"0\" else \" \" in
+           let rec mk k acc = if k = 0 then acc else mk (k - 1) (acc ^ ch) in
+           let padding = mk pad_len \"\" in
+           if left then s ^ padding else padding ^ s
+       in
+       (* Skip flag chars from p, returning new pos. Records flags in
+          shared refs (set above each call). *)
+       let parse_flags_loop p left_flag zero_flag =
+         let i = ref p in
+         let cont = ref true in
+         while !cont do
+           if !i < n then
+             let c = _string_get fmt !i in
+             if c = \"-\" then (left_flag := true; i := !i + 1)
+             else if c = \"0\" then (zero_flag := true; i := !i + 1)
+             else cont := false
+           else cont := false
+         done;
+         !i
+       in
+       let parse_width_loop p =
+         let i = ref p in
+         let w = ref 0 in
+         let cont = ref true in
+         while !cont do
+           if !i < n then
+             let c = _string_get fmt !i in
+             if is_digit c then
+               (w := !w * 10 + (_char_code c - 48); i := !i + 1)
+             else cont := false
+           else cont := false
+         done;
+         (!w) * 1000000 + (!i)
+       in
        let rec walk pos prefix =
          if pos >= n then prefix
          else if pos + 1 < n && _string_get fmt pos = \"%\" then
-           let spec = _string_get fmt (pos + 1) in
-           if spec = \"%\" then walk (pos + 2) (prefix ^ \"%\")
-           else if spec = \"d\" || spec = \"i\" || spec = \"s\"
-                || spec = \"f\" || spec = \"c\" || spec = \"b\"
-                || spec = \"x\" || spec = \"X\" || spec = \"o\"
-                || spec = \"u\" then
-             (fun arg ->
-                let s =
-                  if spec = \"d\" || spec = \"i\" || spec = \"u\"
-                    then _string_of_int arg
-                  else if spec = \"f\" then _string_of_float arg
-                  else if spec = \"x\" then _int_to_hex_lower arg
-                  else if spec = \"X\" then _int_to_hex_upper arg
-                  else if spec = \"o\" then _int_to_octal arg
-                  else if spec = \"b\" then
-                    (if arg then \"true\" else \"false\")
-                  else arg
-                in
-                walk (pos + 2) (prefix ^ s))
-           else walk (pos + 1) (prefix ^ _string_get fmt pos)
+           if _string_get fmt (pos + 1) = \"%\" then
+             walk (pos + 2) (prefix ^ \"%\")
+           else
+             let left_flag = ref false in
+             let zero_flag = ref false in
+             let after_flags = parse_flags_loop (pos + 1) left_flag zero_flag in
+             let packed = parse_width_loop after_flags in
+             let width = packed / 1000000 in
+             let spec_pos = packed - width * 1000000 in
+             if spec_pos < n && is_spec (_string_get fmt spec_pos) then
+               let spec = _string_get fmt spec_pos in
+               let left = !left_flag in
+               let zero = !zero_flag in
+               (fun arg ->
+                  let raw =
+                    if spec = \"d\" || spec = \"i\" || spec = \"u\"
+                      then _string_of_int arg
+                    else if spec = \"f\" then _string_of_float arg
+                    else if spec = \"x\" then _int_to_hex_lower arg
+                    else if spec = \"X\" then _int_to_hex_upper arg
+                    else if spec = \"o\" then _int_to_octal arg
+                    else if spec = \"b\" then
+                      (if arg then \"true\" else \"false\")
+                    else arg
+                  in
+                  let s = pad raw width left zero in
+                  walk (spec_pos + 1) (prefix ^ s))
+             else walk (pos + 1) (prefix ^ _string_get fmt pos)
          else walk (pos + 1) (prefix ^ _string_get fmt pos)
        in
        walk 0 \"\"

lib/ocaml/test.sh

@@ -1330,6 +1330,18 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 5074)
 (eval "(ocaml-run \"Printf.sprintf \\\"%x %X %o\\\" 255 4096 8\")")
 
+;; ── Printf width specifiers ─────────────────────────────────
+(epoch 5080)
+(eval "(ocaml-run \"Printf.sprintf \\\"%5d\\\" 42\")")
+(epoch 5081)
+(eval "(ocaml-run \"Printf.sprintf \\\"%-5d|\\\" 42\")")
+(epoch 5082)
+(eval "(ocaml-run \"Printf.sprintf \\\"%05d\\\" 42\")")
+(epoch 5083)
+(eval "(ocaml-run \"Printf.sprintf \\\"%4s\\\" \\\"hi\\\"\")")
+(epoch 5084)
+(eval "(ocaml-run \"Printf.sprintf \\\"hi=%-3d, hex=%04x\\\" 9 15\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 360 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -2113,6 +2125,13 @@ check 5072 "%X 4096"                      '"1000"'
 check 5073 "%o 8"                         '"10"'
 check 5074 "%x %X %o multi"               '"ff 1000 10"'
 
+# ── Printf width specifiers ─────────────────────────────────────
+check 5080 "%5d 42 right-pad"             '"   42"'
+check 5081 "%-5d| 42 left-pad"            '"42   |"'
+check 5082 "%05d 42 zero-pad"             '"00042"'
+check 5083 "%4s hi"                       '"  hi"'
+check 5084 "%-3d %04x mixed"              '"hi=9  , hex=000f"'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL OCaml-on-SX tests passed"

plans/ocaml-on-sx.md

@@ -407,6 +407,17 @@ _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 — Printf width specifiers `%5d` / `%-5d` /
+  `%05d` / `%4s` etc. (+5 tests, 538 total). Walker now parses
+  optional `-` (left-align) and `0` (zero-pad) flags after `%`, then
+  optional decimal width digits, then the spec letter. After
+  formatting the arg into a base string, pads to the width using
+  spaces (or zeros if `0` flag and not `-`). Encoded width+spec_pos
+  return as `width * 1000000 + spec_pos` because the parser does not
+  yet support tuple destructuring in `let` (TODO: lift that
+  limitation; for now this round-trips losslessly for any practical
+  width). Examples: `%5d` 42 = "   42", `%-5d|` 42 = "42   |",
+  `%05d` 42 = "00042".
 - 2026-05-09 Phase 6 — Printf.sprintf adds %i, %u (aliases of %d),
   %x (lowercase hex), %X (uppercase hex), %o (octal) (+5 tests, 533
   total). New host primitives `_int_to_hex_lower`, `_int_to_hex_upper`,