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# Builder blocks

A **builder block** is `@ctx { ... }`, an expression that builds a value
declaratively. It is the one piece of dedicated DSL syntax in C+, and it was the
**final** language feature: as of v0.0.22 the language surface is frozen, and
everything else grows in packages. `@` was previously an invalid character, so no
existing source changes meaning.

A block holds **item expressions**, **leading-dot modifier lines** that apply to
the item above them, `let` setup bindings, and nested blocks:

```cplus
import "ui/view" as view;

let panel = @view {
    let title = "Settings";

    view::text(title)
        .font = bigger          // modifier: assign a field on the item above
        .color(blue)            // modifier: call a method on it

    view::spacer(8)
};
```

## It is not magic — it lowers to a protocol

There are no macros and no compiler-blessed UI types. A builder block lowers to
ordinary calls against a **fixed protocol** the context package provides:

- `ctx::Builder::new()` starts the block.
- each item becomes a temporary with its modifiers applied (`__i.font = v`,
  `__i.color(args)`), then `builder.add(__i)`.
- `builder.finish()` is the block's value.

So **any package that ships a `Builder` (`new` / `add` / `finish`), an item type,
and constructor functions becomes a construction DSL.** Here is a complete one:

```cplus
// ui/view.cplus — a context package
pub struct Node { pub value: i32, pub weight: i32 }

pub fn text(v: i32) -> Node { return Node { value: v, weight: 1 }; }

impl Node {
    pub fn boost(mut self, by: i32) { self.weight = self.weight + by; return; }
}

pub struct Builder { sum: i32 }

impl Builder {
    pub fn new() -> Builder { return Builder { sum: 0 }; }
    pub fn add(mut self, item: Node) { self.sum = self.sum + item.value * item.weight; return; }
    pub fn finish(move self) -> Node { return Node { value: self.sum, weight: 1 }; }
}
```

```cplus
let tree = @view {
    view::text(8)
        .weight = 2          // field assignment modifier
    view::text(3)
        .boost(1)            // method modifier
};
// lowers to: new(); t1 = text(8); t1.weight = 2; add(t1);
//            t2 = text(3); t2.boost(1); add(t2); finish()
```

## Contextual name lookup

Inside `@view { ... }`, a bare item name resolves through the context, so
`text(...)` means `view::text(...)` with no qualification. Precedence is
**locals → same-file top-level → contextual**: a `let` or a same-file function of
the same name shadows the package member. Item field and method names in
modifiers (`.font`, `.boost(...)`) are never contextual. Because the rewrite
produces real `view::text` references before the graph is built,
[code-graph](/docs/tooling) and LSP navigation resolve them automatically.

## Containers and control flow

A bare `name { ... }` inside a block is a **container element of the same
context**: `vstack { ... }` builds `view::vstack`, and its children resolve in
`view` too. A container takes a *filled* `Builder` (`fn vstack(b: Builder) -> Node`),
so the whole feature lowers to `new`/`add` plus a finisher — the compiler's output
never names a collection type, which is why DSL packages work even on targets
where `Vec` is gated.

`if`/`for` are **collection control flow**, Flutter-style: their items add into
the same builder. `if` needs no `else`.

```cplus
let menu = @view {
    view::text(header)

    vstack {
        for item in items {
            view::row(item)
        }
        if logged_in {
            view::button(logout)
        }
    }
};
```

## What the block rejects

Modifier lines are line-oriented: a `.name` that *starts* a line attaches to the
current item, while a same-line `.name` is ordinary postfix access. Inside call
arguments, indexing, grouping parentheses, and nested expression blocks the rule
is off, so wrapped subexpressions are unaffected.

A few shapes are parse-time errors, reported on the offending DSL line: a
modifier with no current item (including right after a `let`), and `return` /
`break` / `continue` / `yield` / `await` / loops / `defer` / `guard` inside a
block. A nested *different* `@`-DSL is rejected too — use a same-context
container instead. Because the desugar reuses your spans, ordinary sema
diagnostics land where you wrote them: a wrong item type at the item line, an
unknown modifier field at the modifier line, a context without a `Builder` at the
`@ctx` line.

`cpc fmt` keeps `@ctx` glued to its block and round-trips the whole thing —
containers and `if`/`for` included.

See the worked example, [Build a construction DSL](/examples/builder-block-dsl),
for a package and a block you can compile and run.