feat(lang): L11 views and templates - the UI is markup, not code

A `view Name { field = x; function q(..); on e(..) }` declaration is the one bridge between a
program and its UI: it writes view_<name>() -> UiView, whose model is a Value of every field and
whose call runs a query or an event by name.

ludic.ui is a template runtime:
- HTML-shaped XML screens and components, loaded at run time;
- {expression} bindings, if/else/each, props, slots, per-instance state;
- on-press / onclick actions (event, set, emit);
- component libraries (export="true", <import src as>).

Styling:
- stylesheets in <style> or importable .lss files (@import);
- CSS selectors (#id, .class, [attr=v], descendant and > combinators, :hover, :disabled,
  :first-child, :last-child, :nth-child, :not) weighed by specificity;
- the box model and flex under CSS's property names.

Also:
- default parameters, and positional-then-named calls;
- Value gains a float kind;
- a function shadowing a runtime one is refused;
- an index is evaluated before the slice is read;
- runtime errors name the right file.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

selfhost/frontend/aliases.ludic

@@ -64,6 +64,7 @@ function ns_alias_labels(i: int) -> []pointer {
   return out
 }
 function emit_alias_call(i: int, e: Node) -> Val {
+  call_mixed_to_named(e, ns_alias_labels(i))
   reorder_named(e, ns_alias_labels(i))
   let id = node(E_ID)
   id.s = g_al_target[i]

selfhost/frontend/defaults.ludic

@@ -0,0 +1,96 @@
+# defaults.ludic — L11: a parameter may have a default, `pad: float = 8.0`, and a call may leave it
+# out - positionally from the end, or by not naming it. That is what makes a component's props
+# usable: `button(label: "Buy", on_press: fn buy)` rather than every prop of a button spelt out.
+# The default is an expression written where the function is and evaluated for each call.
+# `f(a, b, size: 2)`: the leading arguments are the first parameters, by position, and the rest
+# are named - the natural call of a component, whose first prop is the one it is about
+function call_mixed_to_named(e: Node, labels: []pointer) -> void {
+  var first_named = -1
+  var i = 0
+  while i < len(e.kids) {
+    if e.kids[i].kind == E_FINIT and first_named < 0 { first_named = i }
+    if e.kids[i].kind != E_FINIT and first_named >= 0 {
+      g_err_file = e.file
+      g_err_line = e.line
+      perr("a positional argument after a named one")
+    }
+    i += 1
+  }
+  if first_named <= 0 { return }
+  let out = new []Node
+  var k = 0
+  while k < len(e.kids) {
+    var a = e.kids[k]
+    if k < first_named and k < len(labels) {
+      let fi = node(E_FINIT)
+      fi.s = labels[k]
+      fi.a = a
+      fi.line = a.line
+      fi.file = a.file
+      a = fi
+    }
+    push(out, a)
+    k += 1
+  }
+  e.kids = out
+}
+function call_fill_defaults(e: Node, f: Node) -> void {
+  if f == null { return }
+  let labels = new []pointer
+  let defs = new []Node
+  var any = false
+  var i = 0
+  while i < len(f.kids) {
+    let p = f.kids[i]
+    if p.kind == N_PARAM {
+      push(labels, p.s)
+      push(defs, p.a)
+      if p.a != null { any = true }
+    }
+    i += 1
+  }
+  call_mixed_to_named(e, labels)
+  if not any { return }
+  if len(e.kids) > 0 and e.kids[0].kind == E_FINIT {
+    let out = new []Node
+    var li = 0
+    while li < len(labels) {
+      var found: Node = null
+      var k = 0
+      while k < len(e.kids) {
+        if (e.kids[k].s == labels[li]) { found = e.kids[k] }
+        k += 1
+      }
+      if found == null and defs[li] != null {
+        found = node(E_FINIT)
+        found.s = labels[li]
+        found.a = default_copy(defs[li], e)
+      }
+      if found != null { push(out, found) }
+      li += 1
+    }
+    # a label the function does not have stays, so the call is refused for it
+    var k2 = 0
+    while k2 < len(e.kids) {
+      if not default_label_in(labels, e.kids[k2].s) { push(out, e.kids[k2]) }
+      k2 += 1
+    }
+    e.kids = out
+    return
+  }
+  while len(e.kids) < len(labels) and defs[len(e.kids)] != null { push(e.kids, default_copy(defs[len(e.kids)], e)) }
+}
+function default_label_in(labels: []pointer, s: pointer) -> bool {
+  var i = 0
+  while i < len(labels) {
+    if (labels[i] == s) { return true }
+    i += 1
+  }
+  return false
+}
+# each call gets its own copy of the default, at the call's line for its errors
+function default_copy(d: Node, at: Node) -> Node {
+  let c = gen_clone(d, new []pointer, new []pointer)
+  c.line = at.line
+  return c
+}

selfhost/frontend/parse.ludic

@@ -577,7 +577,9 @@ function parse_fn() -> Node {
   eat_op("(")
   if is_reserved_word(n.s) { perr(`'{n.s}' is a reserved word and cannot name a function`) }
   while not is_op(")") {
-    let p = node(N_PARAM); p.s = eat_id(); eat_op(":"); p.ty = ptype(); push(n.kids, p)
+    let p = node(N_PARAM); p.s = eat_id(); eat_op(":"); p.ty = ptype()
+    if is_op("=") { pi += 1;  p.a = expr() }       # L11: a default, for a call that leaves it out
+    push(n.kids, p)
     if is_op(",") { pi += 1 }
   }
   eat_op(")")
@@ -1014,6 +1016,7 @@ function parse_one_decl() -> void {
   if is_id("namespace") { parse_namespace(); return }   # #76 namespace block
   if is_id("registry") and (toks[pi + 1].kind == TK_ID) and (toks[pi + 2].text == "of") { parse_registry(); return }   # L8
   if is_id("def") and (toks[pi + 1].kind == TK_ID) and (toks[pi + 2].kind == TK_ID) { parse_def(); return }            # L8
+  if is_id("view") and (toks[pi + 1].kind == TK_ID) and (toks[pi + 2].text == "{") { parse_view(); return }            # L11
   if is_id("var")    { push(prog, parse_var()); return }
   if is_id("const")  { push(prog, parse_const()); return }
   var is_unsafe = false
@@ -1510,4 +1513,5 @@ function parse_program() -> void {
     parse_one_decl()
   }
   registries_finish()                              # L8: each registry gets its defs
+  views_finish()                                   # L11: each view gets its model and call
 }

selfhost/frontend/view.ludic

@@ -0,0 +1,113 @@
+# view.ludic — L11: a view is the one bridge between a program and its UI templates. The
+# program says what a template may read and what it may do, and nothing else crosses:
+#
+#   view Yard {
+#     purse = hm_purse                      # a field: the global's own type
+#     spots: int = yard_spots()             # any expression, with its type said
+#     function placed(hf: int) -> int { return yard_placed(hf) }   # a query a template may ask
+#     on buy(hf: int) { yard_buy(hf) }      # an event a template may send
+#   }
+#
+# gives view_yard() -> UiView, whose model() is a Value object of every field (a record is an
+# object of its fields, a list and a registry a list) and whose call(name, args) runs a query or
+# an event by name with its arguments out of a Value list. Everything generated belongs to the
+# view's own file, so it is in the view's module and exported only when the view is.
+var g_vw_name: []pointer = new []pointer
+var g_vw_node: []Node = new []Node          # view_<name>(), which carries the file, line and export
+var g_vw_fields: [][]Node = new [][]Node    # the getters, one per field
+var g_vw_fnames: [][]pointer = new [][]pointer
+var g_vw_calls: [][]Node = new [][]Node     # the queries and the events
+var g_vw_cnames: [][]pointer = new [][]pointer
+var g_vw_helpers: []pointer = new []pointer # conversion helpers already written, program-wide
+
+function vw_prefix(name: pointer) -> pointer { return `view_{reg_lower(name)}_` }
+
+# view NAME { members }
+function parse_view() -> void {
+  pi += 1
+  let name = eat_id()
+  let px = vw_prefix(name)
+  g_uses_value = true
+  let fields = new []Node
+  let fnames = new []pointer
+  let calls = new []Node
+  let cnames = new []pointer
+  skipnl()
+  eat_op("{")
+  while true {
+    skipnl()
+    if is_op("}") { break }
+    if is_id("function") or is_id("on") {
+      let ev = is_id("on")
+      if ev { toks[pi].text = "function" }
+      let f = parse_fn()
+      push(cnames, f.s)
+      if ev { f.s = px + "on_" + f.s } else { f.s = px + f.s }
+      push(calls, f)
+      push(prog, f)
+      continue
+    }
+    push(fnames, eat_id())
+    push(fields, vw_field(px, fnames[len(fnames) - 1]))
+  }
+  eat_op("}")
+  let head = vw_head(name, px)
+  push(g_vw_name, name)
+  push(g_vw_node, head)
+  push(g_vw_fields, fields)
+  push(g_vw_fnames, fnames)
+  push(g_vw_calls, calls)
+  push(g_vw_cnames, cnames)
+}
+# one field: NAME [: TYPE] = EXPR, as a getter function whose type the finish fills in if unsaid
+function vw_field(px: pointer, fname: pointer) -> Node {
+  let g = node(N_FN)
+  g.s = px + "get_" + fname
+  g.ty = null
+  if is_op(":") {
+    pi += 1
+    g.ty = ptype()
+  }
+  if not is_op("=") { perr(`view field {fname}: say what it is - {fname} = some_global`) }
+  pi += 1
+  let r = node(S_RETURN)
+  r.a = expr()
+  let body = node(N_BLOCK)
+  push(body.kids, r)
+  g.a = body
+  push(prog, g)
+  return g
+}
+# view_<name>() -> UiView, written now so it takes the export the declaration is under
+function vw_head(name: pointer, px: pointer) -> Node {
+  let src = `function view_{reg_lower(name)}() -> UiView {{\n  let v = new UiView\n  v.name = "{name}"\n  v.model = fn {px}model\n  v.call = fn {px}call\n  return v\n}}\n`
+  let at = len(prog)
+  vw_parse(src, g_parse_file, toks[pi].line, 0)
+  return prog[at]
+}
+# parse generated source into the program, as if it were written in `file` at `line`
+function vw_parse(src: pointer, file: pointer, line: int, vis: int) -> void {
+  let saved_toks = toks
+  let saved_pi = pi
+  let saved_file = g_parse_file
+  let saved_parsing = g_parsing
+  g_parse_file = file
+  g_parsing = true
+  lex_at(src, line)
+  pi = 0
+  let at = len(prog)
+  while true {
+    skipnl()
+    if toks[pi].kind == TK_EOF { break }
+    parse_one_decl()
+  }
+  var k = at
+  while k < len(prog) {
+    prog[k].vis = vis
+    k += 1
+  }
+  toks = saved_toks
+  pi = saved_pi
+  g_parse_file = saved_file
+  g_parsing = saved_parsing
+}

selfhost/frontend/view_conv.ludic

@@ -0,0 +1,68 @@
+# view_conv.ludic — L11: a value of the program as a Value a template reads. A number, a bool
+# and a string are themselves; a list is a list of its items; a record is an object of every
+# field a template could read (a field it could not - a function, a buffer - is left out).
+# Anything else answers null, and the caller says so.
+function vw_conv(v: int, ty: pointer, x: pointer, head: Node) -> pointer {
+  if (ty == "int") { return `value_int({x})` }
+  if (ty == "float") { return `value_float({x})` }
+  if (ty == "string") { return `value_str({x})` }
+  if (ty == "bool") {
+    vw_helper(`function view_val_bool(b: bool) -> Val {{\n  if b {{ return value_bool(1) }}\n  return value_bool(0)\n}}\n`, "view_val_bool", head)
+    return `view_val_bool({x})`
+  }
+  if len(ty) > 2 and ty[0] == '[' and ty[1] == ']' {
+    let et: pointer = ty[2..len(ty)]
+    let name = `view_val_list_{vw_mangle(et)}`
+    if vw_have(name) { return `{name}({x})` }
+    let item = vw_conv(v, et, "xs[i]", head)
+    if item == null { return null }
+    push(g_vw_helpers, name)
+    vw_parse(`function {name}(xs: {ty}) -> Val {{\n  let out = value_list()\n  var i = 0\n  while i < len(xs) {{\n    value_add(out, {item})\n    i += 1\n  }}\n  return out\n}}\n`, head.file, head.line, 1)
+    return `{name}({x})`
+  }
+  let comp = find_comp(ty)
+  if comp == null { return null }
+  let name = `view_val_{vw_mangle(ty)}`
+  if vw_have(name) { return `{name}({x})` }
+  push(g_vw_helpers, name)
+  var src = `function {name}(r: {ty}) -> Val {{\n  if r == null {{ return value_null() }}\n  let o = value_object()\n`
+  var i = 0
+  while i < len(comp.kids) {
+    let f = comp.kids[i]
+    if f.ty != null {
+      let c = vw_conv(v, f.ty, `r.{f.s}`, head)
+      if c != null { src = src + `  value_put(o, "{f.s}", {c})\n` }
+    }
+    i += 1
+  }
+  vw_parse(src + "  return o\n}\n", head.file, head.line, 1)
+  return `{name}({x})`
+}
+function vw_have(name: pointer) -> bool {
+  var i = 0
+  while i < len(g_vw_helpers) {
+    if (g_vw_helpers[i] == name) { return true }
+    i += 1
+  }
+  return false
+}
+function vw_helper(src: pointer, name: pointer, head: Node) -> void {
+  if vw_have(name) { return }
+  push(g_vw_helpers, name)
+  vw_parse(src, head.file, head.line, 1)
+}
+# a type as part of a name: []int -> list_int, a.B -> a_B
+function vw_mangle(ty: pointer) -> pointer {
+  let n = len(ty)
+  let out = bytes(n + 1)
+  var i = 0
+  while i < n {
+    var c = ty[i]
+    let ok = (c >= 'a' and c <= 'z') or (c >= 'A' and c <= 'Z') or (c >= '0' and c <= '9')
+    if not ok { c = '_' }
+    out[i] = c
+    i += 1
+  }
+  out[n] = 0
+  return out
+}

selfhost/frontend/view_finish.ludic

@@ -0,0 +1,84 @@
+# view_finish.ludic — L11: when the program is read, each view gets its model and its call.
+# A field's type is the one it was given or its global's; converting it to a Value is a helper
+# written once for the whole program per type (view_val_<type>), so two views of one record
+# share it.
+function views_finish() -> void {
+  var v = 0
+  while v < len(g_vw_name) {
+    vw_finish(v)
+    v += 1
+  }
+}
+function vw_err(v: int, msg: pointer) -> void {
+  g_err_file = g_vw_node[v].file
+  g_err_line = g_vw_node[v].line
+  g_parsing = false
+  perr(`view {g_vw_name[v]}: {msg}`)
+}
+function vw_finish(v: int) -> void {
+  let head = g_vw_node[v]
+  if find_comp("UiView") == null { vw_err(v, `a view is shown by ludic.ui - import "ludic.ui"`) }
+  let px = vw_prefix(g_vw_name[v])
+  var model = `function {px}model() -> Val {{\n  let o = value_object()\n`
+  let fields = g_vw_fields[v]
+  var i = 0
+  while i < len(fields) {
+    let g = fields[i]
+    let fname = g_vw_fnames[v][i]
+    if g.ty == null { g.ty = vw_infer(v, g, fname) }
+    let conv = vw_conv(v, g.ty, `{g.s}()`, head)
+    if conv == null { vw_err(v, `{fname} is a {g.ty}, which a template cannot read`) }
+    model = model + `  value_put(o, "{fname}", {conv})\n`
+    i += 1
+  }
+  model = model + "  return o\n}\n"
+  vw_parse(model, head.file, head.line, head.vis)
+  vw_parse(vw_call_src(v, px, head), head.file, head.line, head.vis)
+}
+# a field written `name = some_global` has that global's declared type
+function vw_infer(v: int, g: Node, fname: pointer) -> pointer {
+  let e = g.a.kids[0].a
+  if e.kind == E_ID {
+    let d = find_global(e.s)
+    if d != null and d.ty != null { return d.ty }
+  }
+  g_vw_node[v].line = g.line
+  vw_err(v, `say what type {fname} is - {fname}: int = ...`)
+  return "int"
+}
+# call(name, args): each query and event by its name, its arguments out of the list
+function vw_call_src(v: int, px: pointer, head: Node) -> pointer {
+  var src = `function {px}call(name: string, args: Val) -> Val {{\n`
+  let calls = g_vw_calls[v]
+  var i = 0
+  while i < len(calls) {
+    let f = calls[i]
+    var args = ""
+    var p = 0
+    while p < len(f.kids) {
+      if p > 0 { args = args + ", " }
+      args = args + vw_arg(v, f.kids[p], p)
+      p += 1
+    }
+    let call = `{f.s}({args})`
+    src = src + `  if name == "{g_vw_cnames[v][i]}" {{\n`
+    if f.ty == "void" { src = src + `    {call}\n    return value_null()\n` } else {
+      let conv = vw_conv(v, f.ty, call, head)
+      if conv == null { vw_err(v, `{g_vw_cnames[v][i]} answers a {f.ty}, which a template cannot read`) }
+      src = src + `    return {conv}\n`
+    }
+    src = src + "  }\n"
+    i += 1
+  }
+  return src + "  return value_null()\n}\n"
+}
+# a parameter out of the template's argument list
+function vw_arg(v: int, p: Node, i: int) -> pointer {
+  let at = `value_at(args, {i})`
+  if (p.ty == "int") { return `value_as_int({at})` }
+  if (p.ty == "float") { return `value_as_float({at})` }
+  if (p.ty == "bool") { return `value_as_int({at}) != 0` }
+  if (p.ty == "string") { return `value_as_str({at})` }
+  vw_err(v, `{p.s} is a {p.ty}; a template can pass an int, a float, a bool or a string`)
+  return ""
+}