ocaml: character type — Char of int, #\a parser, all char primitives

- Char of int variant in sx_types.ml (Unicode codepoint)
- type_of → "char", inspect → #\a / #\space / #\newline notation
- #\ char literal reader in sx_parser.ml (named + single-char)
- make-char char? char->integer integer->char char-upcase char-downcase
- char=? char<? char>? char<=? char>=? comparators
- char-ci=? char-ci<? char-ci>? char-ci<=? char-ci>=? case-insensitive
- char-alphabetic? char-numeric? char-whitespace? char-upper-case? char-lower-case?
- string->list (returns Char values) and list->string (accepts Char values)
- fix get_val in sx_runtime.ml: add Integer n case for list indexing
- fix raw_serialize in sx_server.ml: Integer and Char variants
- 4493/4493 tests — +43 passing, zero regressions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hosts/ocaml/bin/sx_server.ml

@@ -1377,6 +1377,7 @@ let rec dispatch env cmd =
         | Bool true -> "true"
         | Bool false -> "false"
         | Number n -> Sx_types.format_number n
+        | Integer n -> string_of_int n
         | String s -> "\"" ^ escape_sx_string s ^ "\""
         | Symbol s -> s
         | Keyword k -> ":" ^ k
@@ -1390,6 +1391,7 @@ let rec dispatch env cmd =
         | Island i -> "~" ^ i.i_name
         | SxExpr s -> s
         | RawHTML s -> "\"" ^ escape_sx_string s ^ "\""
+        | Char n -> Sx_types.inspect (Char n)
         | _ -> "nil"
       in
       send_ok_raw (raw_serialize result)

hosts/ocaml/lib/sx_parser.ml

@@ -120,6 +120,27 @@ let rec read_value s : value =
   | '"' -> String (read_string s)
   | '\'' -> advance s; List [Symbol "quote"; read_value s]
   | '`' -> advance s; List [Symbol "quasiquote"; read_value s]
+  | '#' when s.pos + 1 < s.len && s.src.[s.pos + 1] = '\\' ->
+    (* Character literal: #\a, #\space, #\newline, etc. *)
+    advance s; advance s;
+    if at_end s then raise (Parse_error "Unexpected end of input after #\\");
+    let char_start = s.pos in
+    (* Read a name if starts with ident char, else single char *)
+    if is_ident_start s.src.[s.pos] then begin
+      while s.pos < s.len && is_ident_char s.src.[s.pos] do advance s done;
+      let name = String.sub s.src char_start (s.pos - char_start) in
+      let cp = match name with
+        | "space" -> 32 | "newline" -> 10 | "tab" -> 9
+        | "return" -> 13 | "nul" -> 0 | "null" -> 0
+        | "escape" -> 27 | "delete" -> 127 | "backspace" -> 8
+        | "altmode" -> 27 | "rubout" -> 127
+        | _ -> Char.code name.[0]  (* single letter like #\a *)
+      in Char cp
+    end else begin
+      let c = s.src.[s.pos] in
+      advance s;
+      Char (Char.code c)
+    end
   | '#' when s.pos + 1 < s.len && s.src.[s.pos + 1] = ';' ->
     (* Datum comment: #; discards next expression *)
     advance s; advance s;

hosts/ocaml/lib/sx_primitives.ml

@@ -2225,4 +2225,80 @@ let () =
   register "symbol-interned?" (fun args ->
     match args with
     | [Symbol _] -> Bool true
-    | _ -> raise (Eval_error "symbol-interned?: expected 1 symbol"))
+    | _ -> raise (Eval_error "symbol-interned?: expected 1 symbol"));
+  (* Phase 13: character type *)
+  let char_downcase_cp n =
+    if n >= 65 && n <= 90 then n + 32 else n in
+  let char_upcase_cp n =
+    if n >= 97 && n <= 122 then n - 32 else n in
+  register "make-char" (fun args ->
+    match args with
+    | [Integer n] -> Char n
+    | _ -> raise (Eval_error "make-char: expected integer codepoint"));
+  register "char?" (fun args ->
+    match args with
+    | [Char _] -> Bool true | [_] -> Bool false
+    | _ -> raise (Eval_error "char?: expected 1 argument"));
+  register "char->integer" (fun args ->
+    match args with
+    | [Char n] -> Integer n
+    | _ -> raise (Eval_error "char->integer: expected char"));
+  register "integer->char" (fun args ->
+    match args with
+    | [Integer n] -> Char n
+    | _ -> raise (Eval_error "integer->char: expected integer"));
+  register "char-upcase" (fun args ->
+    match args with
+    | [Char n] -> Char (char_upcase_cp n)
+    | _ -> raise (Eval_error "char-upcase: expected char"));
+  register "char-downcase" (fun args ->
+    match args with
+    | [Char n] -> Char (char_downcase_cp n)
+    | _ -> raise (Eval_error "char-downcase: expected char"));
+  register "char=?"  (fun args -> match args with [Char a; Char b] -> Bool (a = b)  | _ -> raise (Eval_error "char=?: expected 2 chars"));
+  register "char<?"  (fun args -> match args with [Char a; Char b] -> Bool (a < b)  | _ -> raise (Eval_error "char<?: expected 2 chars"));
+  register "char>?"  (fun args -> match args with [Char a; Char b] -> Bool (a > b)  | _ -> raise (Eval_error "char>?: expected 2 chars"));
+  register "char<=?" (fun args -> match args with [Char a; Char b] -> Bool (a <= b) | _ -> raise (Eval_error "char<=?: expected 2 chars"));
+  register "char>=?" (fun args -> match args with [Char a; Char b] -> Bool (a >= b) | _ -> raise (Eval_error "char>=?: expected 2 chars"));
+  register "char-ci=?"  (fun args -> match args with [Char a; Char b] -> Bool (char_downcase_cp a = char_downcase_cp b) | _ -> raise (Eval_error "char-ci=?: expected 2 chars"));
+  register "char-ci<?"  (fun args -> match args with [Char a; Char b] -> Bool (char_downcase_cp a < char_downcase_cp b) | _ -> raise (Eval_error "char-ci<?: expected 2 chars"));
+  register "char-ci>?"  (fun args -> match args with [Char a; Char b] -> Bool (char_downcase_cp a > char_downcase_cp b) | _ -> raise (Eval_error "char-ci>?: expected 2 chars"));
+  register "char-ci<=?" (fun args -> match args with [Char a; Char b] -> Bool (char_downcase_cp a <= char_downcase_cp b) | _ -> raise (Eval_error "char-ci<=?: expected 2 chars"));
+  register "char-ci>=?" (fun args -> match args with [Char a; Char b] -> Bool (char_downcase_cp a >= char_downcase_cp b) | _ -> raise (Eval_error "char-ci>=?: expected 2 chars"));
+  register "char-alphabetic?" (fun args ->
+    match args with
+    | [Char n] -> Bool ((n >= 65 && n <= 90) || (n >= 97 && n <= 122))
+    | _ -> raise (Eval_error "char-alphabetic?: expected char"));
+  register "char-numeric?" (fun args ->
+    match args with
+    | [Char n] -> Bool (n >= 48 && n <= 57)
+    | _ -> raise (Eval_error "char-numeric?: expected char"));
+  register "char-whitespace?" (fun args ->
+    match args with
+    | [Char n] -> Bool (n = 32 || n = 9 || n = 10 || n = 13)
+    | _ -> raise (Eval_error "char-whitespace?: expected char"));
+  register "char-upper-case?" (fun args ->
+    match args with
+    | [Char n] -> Bool (n >= 65 && n <= 90)
+    | _ -> raise (Eval_error "char-upper-case?: expected char"));
+  register "char-lower-case?" (fun args ->
+    match args with
+    | [Char n] -> Bool (n >= 97 && n <= 122)
+    | _ -> raise (Eval_error "char-lower-case?: expected char"));
+  register "string->list" (fun args ->
+    match args with
+    | [String s] ->
+      let chars = ref [] in
+      String.iter (fun c -> chars := Char (Char.code c) :: !chars) s;
+      List (List.rev !chars)
+    | _ -> raise (Eval_error "string->list: expected string"));
+  register "list->string" (fun args ->
+    match args with
+    | [List chars] | [ListRef { contents = chars }] ->
+      let buf = Buffer.create (List.length chars) in
+      List.iter (function
+        | Char n -> Buffer.add_char buf (Char.chr (n land 0xFF))
+        | v -> raise (Eval_error ("list->string: expected char, got " ^ type_of v))
+      ) chars;
+      String (Buffer.contents buf)
+    | _ -> raise (Eval_error "list->string: expected list of chars"))

hosts/ocaml/lib/sx_runtime.ml

@@ -211,6 +211,8 @@ let get_val container key =
   | Dict d, Keyword k -> dict_get d k
   | (List l | ListRef { contents = l }), Number n ->
     (try List.nth l (int_of_float n) with _ -> Nil)
+  | (List l | ListRef { contents = l }), Integer n ->
+    (try List.nth l n with _ -> Nil)
   | Nil, _ -> Nil  (* nil.anything → nil *)
   | _, _ -> Nil    (* type mismatch → nil (matches JS/Python behavior) *)
 

hosts/ocaml/lib/sx_types.ml

@@ -75,6 +75,7 @@ and value =
   | Vector of value array  (** R7RS vector — mutable fixed-size array. *)
   | StringBuffer of Buffer.t  (** Mutable string buffer — O(1) amortized append. *)
   | HashTable of (value, value) Hashtbl.t  (** Mutable hash table with arbitrary keys. *)
+  | Char of int  (** Unicode codepoint — R7RS char type. *)
 
 (** CEK machine state — record instead of Dict for performance.
     5 fields × 55K steps/sec = 275K Hashtbl allocations/sec eliminated. *)
@@ -495,6 +496,7 @@ let type_of = function
   | Vector _       -> "vector"
   | StringBuffer _ -> "string-buffer"
   | HashTable _    -> "hash-table"
+  | Char _         -> "char"
 
 let is_nil = function Nil -> true | _ -> false
 let is_lambda = function Lambda _ -> true | _ -> false
@@ -842,3 +844,12 @@ let rec inspect = function
   | VmMachine m -> Printf.sprintf "<vm-machine:sp=%d frames=%d>" m.vm_sp (List.length m.vm_frames)
   | StringBuffer buf -> Printf.sprintf "<string-buffer:%d>" (Buffer.length buf)
   | HashTable ht -> Printf.sprintf "<hash-table:%d>" (Hashtbl.length ht)
+  | Char n ->
+    let name = match n with
+      | 32 -> "space" | 10 -> "newline" | 9 -> "tab"
+      | 13 -> "return" | 0 -> "nul" | 27 -> "escape"
+      | 127 -> "delete" | 8 -> "backspace"
+      | _ -> let buf = Buffer.create 1 in
+             Buffer.add_utf_8_uchar buf (Uchar.of_int n);
+             Buffer.contents buf
+    in "#\\" ^ name