Phase 7d: string interpolation text {expr} text

The readable way to build strings, as you noted `{a} {b}` beats `a + " " + b`.
A backtick string embeds any expression in `{…}` and desugars to the Phase-7c
`+` chain, wrapping each hole in `str(...)`:

  `hello {name}, n={count + 1}`
    ==  "hello " + name + ", n=" + str(count + 1)

- Lexer: a backtick captures its content raw as TK_INTERP.
- Parser: parse_interp splits literal runs from `{…}` holes (brace-depth aware,
  `{{`/`}}` escape to literal braces), re-lexes each hole as a full expression
  (save/restore toks/pi like an import), and folds it all into E_BIN(+) nodes —
  so no new AST or runtime beyond the existing concat.
- str(x): a string passes through; int/bool/fixed convert via a small emitted
  @fn_int_str prelude (digits from the end of a buffer, '-' for negatives),
  emitted once into any program that uses it.

examples/strings.ludic gains interpolation cases (now prints 1..7); the smoke
covers it. Grammar + ludic_syntax.h tokenize backtick strings (holes highlighted
as embedded code). LANGUAGE.md documents it as the preferred form.

Reseeded (22530 lines); C-free fixpoint holds; goldens identical; 18/18; vocab +
doc-fences clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/emit_core.ludic

@@ -19,6 +19,7 @@ var loc_ty:   []ptr
 var loc_mut:  []int          # 1 = mutable (var / param / loop-var), 0 = immutable (let)
 var nloc: int = 0
 var g_uses_str: bool = false  # a `str + str` / `str == str` was emitted -> emit the prelude
+var g_uses_intstr: bool = false  # `str(int)` was emitted -> emit the int->string prelude
 
 # loop targets for break/continue (innermost last)
 var brk_lbl: []ptr

selfhost/emit_decl.ludic

@@ -68,6 +68,7 @@ fn emit_program() -> void {
   head = buf_new()
   code = buf_new()
   g_uses_str = false
+  g_uses_intstr = false
   loc_name = new []ptr;  loc_reg = new []ptr;  loc_ty = new []ptr;  loc_mut = new []int
   brk_lbl = new []ptr;   cnt_lbl = new []ptr
   self_stk = new []ptr
@@ -84,6 +85,7 @@ fn emit_program() -> void {
     while i < len(prog) { if prog[i].kind == N_MAIN { emit_main(prog[i]) };  i = i + 1 }
   }
   if g_uses_str { emit_str_prelude() }          # @fn_str_eq / @fn_str_concat, after all uses are seen
+  if g_uses_intstr { emit_int_str() }           # @fn_int_str, for str(int) in interpolation
 }
 
 # Flush the emitted IR. With a null path it goes to stdout (the pipe the shell

selfhost/emit_expr.ludic

@@ -126,6 +126,12 @@ fn emit_call(e: Node) -> Val {
   if (name == "quit") { emit("  store i32 0, ptr @L_running\n");  return val("0", "void") }
   if (name == "len") { return emit_len(e) }
   if (name == "push") { return emit_push(e) }
+  if (name == "str") {                            # str(x): int/bool/fixed -> text, a string passes through
+    let a = emit_expr(e.kids[0])
+    if (llty(a.ty) == "ptr") { return a }
+    g_uses_intstr = true
+    return val(emit_bind(("call ptr @fn_int_str(i32 " + (a.code + ")"))), "str")
+  }
   if (name == "fx") { let a = emit_expr(e.kids[0]);  return val(emit_bind(("shl i32 " + (a.code + ", 16"))), "fixed") }
   if (name == "flr") { let a = emit_expr(e.kids[0]);  return val(emit_bind(("ashr i32 " + (a.code + ", 16"))), "int") }
   if is_intrinsic(name) { return emit_intrinsic(name, e) }

selfhost/emit_head.ludic

@@ -151,3 +151,26 @@ fn emit_str_prelude() -> void {
   emith("  %j1 = add i32 %j, 1\n  br label %c2\n")
   emith("fin:\n  %pe = getelementptr inbounds i8, ptr %out, i32 %sum\n  store i8 0, ptr %pe\n  ret ptr %out\n}\n")
 }
+
+# int -> decimal string, emitted (once) into any program that uses str(int)
+# (string interpolation of a number). Writes digits from the end of a 24-byte
+# buffer, prepends '-' for negatives, and returns a pointer into the buffer.
+fn emit_int_str() -> void {
+  emith("define ptr @fn_int_str(i32 %n0) {\n")
+  emith("entry:\n  %buf = call ptr @malloc(i64 24)\n")
+  emith("  %isneg = icmp slt i32 %n0, 0\n  %neg = sub i32 0, %n0\n")
+  emith("  %n = select i1 %isneg, i32 %neg, i32 %n0\n")
+  emith("  %term = getelementptr inbounds i8, ptr %buf, i32 23\n  store i8 0, ptr %term\n")
+  emith("  %iszero = icmp eq i32 %n0, 0\n  br i1 %iszero, label %zc, label %dl\n")
+  emith("zc:\n  %zp = getelementptr inbounds i8, ptr %buf, i32 22\n  store i8 48, ptr %zp\n  ret ptr %zp\n")
+  emith("dl:\n  br label %dloop\n")
+  emith("dloop:\n  %pos = phi i32 [ 22, %dl ], [ %pos2, %dbody ]\n  %cur = phi i32 [ %n, %dl ], [ %cur2, %dbody ]\n")
+  emith("  %done = icmp eq i32 %cur, 0\n  br i1 %done, label %sign, label %dbody\n")
+  emith("dbody:\n  %d = urem i32 %cur, 10\n  %ch = add i32 %d, 48\n  %ch8 = trunc i32 %ch to i8\n")
+  emith("  %pp = getelementptr inbounds i8, ptr %buf, i32 %pos\n  store i8 %ch8, ptr %pp\n")
+  emith("  %cur2 = udiv i32 %cur, 10\n  %pos2 = sub i32 %pos, 1\n  br label %dloop\n")
+  emith("sign:\n  br i1 %isneg, label %addneg, label %fin\n")
+  emith("addneg:\n  %sp = getelementptr inbounds i8, ptr %buf, i32 %pos\n  store i8 45, ptr %sp\n  %posn = sub i32 %pos, 1\n  br label %fin\n")
+  emith("fin:\n  %fpos = phi i32 [ %pos, %sign ], [ %posn, %addneg ]\n")
+  emith("  %rpos = add i32 %fpos, 1\n  %res = getelementptr inbounds i8, ptr %buf, i32 %rpos\n  ret ptr %res\n}\n")
+}

selfhost/lex.ludic

@@ -9,6 +9,7 @@ const TK_OP:  int = 3
 const TK_NL:  int = 4
 const TK_EOF: int = 5
 const TK_FLOAT: int = 6
+const TK_INTERP: int = 7      # `text {expr} text` — raw content, split by the parser
 
 property Tok { kind: int = 0, text: ptr = null, ival: int = 0, line: int = 0 }
 
@@ -69,6 +70,16 @@ fn lex(src: ptr) -> void {
       tok_push(TK_STR, out, 0, line)
       continue
     }
+    if c == 96 {                                   # `interpolated string` — captured raw
+      i = i + 1
+      let out = mem_alloc(n)
+      var j = 0
+      while i < n and peek8(src, i) != 96 { poke8(out, j, peek8(src, i)); j = j + 1; i = i + 1 }
+      i = i + 1
+      poke8(out, j, 0)
+      tok_push(TK_INTERP, out, 0, line)
+      continue
+    }
     if c == 39 {                                   # 'c' char literal -> int
       i = i + 1
       var v = 0

selfhost/parse.ludic

@@ -54,8 +54,64 @@ fn args_call(call: Node) -> void {
   eat_op(")")
 }
 
+# ---- string interpolation --------------------------------------------------
+# `text {expr} text` desugars to a `+` chain of string literals and `str(expr)`
+# holes, so it reuses the string-concat operator and needs no new runtime.
+fn interp_lit(buf: ptr, len: int) -> Node { let n = node(E_STR); n.s = substr(buf, 0, len); return n }
+fn interp_add(acc: Node, part: Node) -> Node {
+  if acc == null { return part }
+  return mkbin("+", acc, part)
+}
+fn interp_str(e: Node) -> Node {                  # wrap a hole in str(...)
+  let c = node(E_CALL); let id = node(E_ID); id.s = "str"; c.a = id; push(c.kids, e); return c
+}
+fn parse_hole(inner: ptr) -> Node {               # re-lex+parse an embedded expression
+  let saved_toks = toks;  let saved_pi = pi
+  lex(inner);  pi = 0;  skipnl()
+  let e = expr()
+  toks = saved_toks;  pi = saved_pi
+  return e
+}
+fn parse_interp(raw: ptr) -> Node {
+  let n = slen(raw)
+  var acc: Node = null
+  let lit = mem_alloc(n + 1)
+  var lj = 0
+  var i = 0
+  while i < n {
+    let c = peek8(raw, i)
+    if c == 123 {                                 # '{'
+      if peek8(raw, i + 1) == 123 { poke8(lit, lj, 123); lj = lj + 1; i = i + 2; continue }   # {{ -> {
+      if lj > 0 { acc = interp_add(acc, interp_lit(lit, lj));  lj = 0 }
+      i = i + 1
+      let hs = i
+      var depth = 1
+      while i < n and depth > 0 {
+        let d = peek8(raw, i)
+        if d == 123 { depth = depth + 1 }
+        else { if d == 125 { depth = depth - 1;  if depth == 0 { break } } }
+        i = i + 1
+      }
+      acc = interp_add(acc, interp_str(parse_hole(substr(raw, hs, i - hs))))
+      i = i + 1                                   # skip the closing '}'
+    } else {
+      if c == 125 and peek8(raw, i + 1) == 125 { poke8(lit, lj, 125); lj = lj + 1; i = i + 2; continue }  # }} -> }
+      if c == 92 {                                # backslash escape in literal text
+        let e = peek8(raw, i + 1);  var r = e
+        if e == 110 { r = 10 }
+        if e == 116 { r = 9 }
+        poke8(lit, lj, r); lj = lj + 1; i = i + 2; continue
+      }
+      poke8(lit, lj, c); lj = lj + 1; i = i + 1
+    }
+  }
+  if lj > 0 or (acc == null) { acc = interp_add(acc, interp_lit(lit, lj)) }
+  return acc
+}
+
 fn p_primary() -> Node {
   let t = toks[pi]
+  if t.kind == TK_INTERP { pi = pi + 1; return parse_interp(t.text) }
   if t.kind == TK_INT { let n = node(E_INT); n.ival = t.ival; pi = pi + 1; return n }
   if t.kind == TK_FLOAT { let n = node(E_FLOAT); n.ival = t.ival; pi = pi + 1; return n }
   if t.kind == TK_STR { let n = node(E_STR); n.s = t.text; pi = pi + 1; return n }