# PBS - Game Profile Syntax Specification v1 (Draft)

Status: Draft (Normative for Game Profile parser/lowering work)
Scope: Syntax delta over `PBS - Language Syntax Specification v0.md`
Language: English

## 1. Purpose

This document defines the parser-facing syntax additions for the PBS Game Profile.

Goals:

- keep beginner usage fluid for 2D game construction,
- preserve deterministic/frame-oriented execution,
- remain compatible with closed VM + bytecode/runtime authority,
- avoid introducing hidden runtime behavior in language syntax.

## 2. Authority and precedence

Normative precedence order:

1. Runtime authority (`docs/specs/hardware/topics/chapter-2.md`, `chapter-3.md`, `chapter-9.md`, `chapter-12.md`, `chapter-16.md`)
2. Bytecode authority (`docs/specs/bytecode/ISA_CORE.md`)
3. `PBS - Language Syntax Specification v0.md`
4. This document
5. Legacy references (`docs/specs/pbs_old/*`)

If a rule here conflicts with higher authority, the rule is invalid.

## 3. Profile activation

Game Profile syntax is enabled by toolchain profile selection (`game` profile).
Without this profile, v0 Core syntax rules remain in effect.

## 4. Lexical delta (keywords)

### 4.1 Newly active keywords in Game Profile

- `host`
- `while`
- `break`
- `continue`

### 4.2 Still reserved in this draft

- `alloc`, `borrow`, `mutate`, `peek`, `take`, `weak`
- `spawn`, `yield`, `sleep`
- `handle`

This draft does not activate heap-specialized syntax or coroutine syntax.

## 5. Top-level grammar delta

Game Profile extends top-level declarations with host function declarations.

```ebnf
TopDecl ::= TypeDecl | ServiceDecl | FunctionDecl | HostFnDecl

HostFnDecl ::= 'host' 'fn' Identifier ParamList ReturnType? ';'
```

Rules:

- `host fn` declares a host-provided call surface only (no body).
- Calling `host fn` lowers to syscall-linked call sites.
- Host functions are callable entities, never first-class function values.
- Service-shaped facades over host functions belong to stdlib design, not to host syntax.

Examples:

```pbs
host fn input_state(): (int, int, int);
host fn gfx_present();
```

## 6. Statement grammar delta

Game Profile adds imperative loop/control and assignment statements.

```ebnf
Stmt ::= LetStmt
       | AssignStmt
       | ReturnStmt
       | IfStmt
       | ForStmt
       | WhileStmt
       | BreakStmt
       | ContinueStmt
       | ExprStmt

WhileStmt ::= 'while' Expr Block
BreakStmt ::= 'break' ';'
ContinueStmt ::= 'continue' ';'

AssignStmt ::= LValue AssignOp Expr ';'
AssignOp ::= '=' | '+=' | '-=' | '*=' | '/=' | '%='
LValue ::= Identifier LValueSuffix*
LValueSuffix ::= '.' Identifier | '[' Expr ']'
```

Rules:

- `break` and `continue` are valid only inside `for`/`while`.
- Assignment is a statement, not an expression.
- `LValue` cannot contain function calls.
- Compound assignments are semantic sugar over read/compute/write.

## 7. Expression grammar delta

Game Profile adds postfix member/index access chains for fluent gameplay code.

```ebnf
UnaryExpr ::= ('!' | '-') UnaryExpr | PostfixExpr
PostfixExpr ::= PrimaryExpr PostfixSuffix*
PostfixSuffix ::= CallSuffix | MemberSuffix | IndexSuffix
CallSuffix ::= '(' ArgList? ')'
MemberSuffix ::= '.' Identifier
IndexSuffix ::= '[' Expr ']'
```

This replaces the v0 Core `CallExpr` chain and enables expressions such as:

```pbs
gfx_present();
player.transform.position.x;
tiles[i].id;
```

## 8. Determinism and lowering constraints

Game Profile syntax additions MUST preserve:

- deterministic control flow,
- verifier-friendly stack effects,
- explicit host call boundaries.

Lowering requirements:

- `host` call sites map to resolved syscall identities.
- `while`/`break`/`continue` lower to structured jumps compatible with verifier rules.
- Member/index read-write operations must lower to deterministic, type-checked access paths.

## 9. Diagnostics requirements (parser + early semantic phase)

Implementations MUST produce deterministic diagnostics for:

- `break`/`continue` outside loops,
- invalid `LValue` targets,
- duplicate host signatures in the same scope,
- unresolved host declarations at link/resolve stage,
- use of still-reserved keywords as active syntax.

## 10. Minimal canonical examples

### 10.1 Frame loop style with while

```pbs
host fn gfx_present();

fn run(): int {
  let running = true;
  while running {
    update_world();
    render_world();
    gfx_present();
  }
  return 0;
}
```

### 10.2 Assignment and member/index access

```pbs
fn integrate(p: Vec2, v: Vec2): Vec2 {
  let out = p;
  out.x += v.x;
  out.y += v.y;
  return out;
}
```

## 11. Non-goals of this syntax draft

- Defining pool/arena memory syntax.
- Enabling `alloc/borrow/mutate` in surface syntax.
- Defining coroutine syntax (`spawn/yield/sleep`) before bytecode/runtime profile activation.