# Lux Comprehensive Roadmap

*A complete design document covering modules, behavioral types, schema evolution, C backend optimizations, LSP, package manager, REPL, HTTP server, async/concurrency, documentation, and error messages.*

---

## Table of Contents

1. [Modules](#1-modules)
2. [Behavioral Types](#2-behavioral-types)
3. [Schema Evolution](#3-schema-evolution)
4. [C Backend Optimizations](#4-c-backend-optimizations)
5. [LSP Improvement Plan](#5-lsp-improvement-plan)
6. [Package Manager](#6-package-manager)
7. [REPL](#7-repl)
8. [HTTP Server](#8-http-server)
9. [Async & Concurrency](#9-async--concurrency)
10. [Documentation](#10-documentation)
11. [Error Messages](#11-error-messages)

---

## 1. Modules

### Current State: COMPLETE

The module system is production-ready with comprehensive features.

**Implemented:**
- Module imports and exports (`import foo.bar`)
- Visibility modifiers (`pub` keyword)
- Module aliases (`import foo as bar`)
- Selective imports (`import foo.{a, b, c}`)
- Wildcard imports (`import foo.*`)
- Circular dependency detection
- Module caching and lazy loading
- Package entry points (`lib.lux`, `src/lib.lux`)

**Code Locations:**
- `src/modules.rs` (797 lines) - Module loading and resolution
- `src/ast.rs` - ImportDecl structure
- `src/parser.rs` - Import parsing
- `docs/guide/07-modules.md` - User documentation

### Roadmap

| Task | Priority | Status |
|------|----------|--------|
| Lock file support (`lux.lock`) | P1 | Missing |
| Version resolution algorithm | P1 | Missing |
| Re-exports (`pub import foo`) | P2 | Missing |
| Private module directories | P3 | Missing |
| Type-only imports | P3 | Missing |

### Lock File Design

```toml
# lux.lock - generated, don't edit
[[package]]
name = "http-client"
version = "1.2.3"
source = "registry"
checksum = "sha256:abc123..."

[[package]]
name = "json-parser"
version = "0.5.0"
source = { git = "https://github.com/...", rev = "abc123" }
dependencies = ["utf8-utils"]
```

---

## 2. Behavioral Types

### Current State: COMPLETE

All five behavioral properties are fully implemented with verification.

**Properties:**

| Property | Verification | Compiler Checks |
|----------|--------------|-----------------|
| `is pure` | Effect analysis | Empty effect set |
| `is total` | Structural recursion | No Fail, decreasing args |
| `is deterministic` | Effect analysis | No Random/Time effects |
| `is idempotent` | Pattern recognition | Constants, identity, clamping, abs |
| `is commutative` | Operator analysis | 2 params + commutative op |

**Code Locations:**
- `src/ast.rs:123-150` - BehavioralProperty enum
- `src/types.rs:361-462` - PropertySet
- `src/typechecker.rs:1290-1403` - Verification logic
- `docs/guide/12-behavioral-types.md` - Documentation

### Roadmap

| Task | Priority | Status |
|------|----------|--------|
| Property inference | P2 | Missing |
| Property testing integration | P2 | Missing |
| Effect-aware optimization hints | P1 | Missing |
| `is associative` property | P3 | Missing |
| `is monotonic` property | P3 | Missing |

### Property Testing Integration

```lux
// Compiler-generated property tests
fn add(a: Int, b: Int): Int is commutative = a + b

// Auto-generates:
test "add is commutative" = {
    forAll(fn(a: Int, b: Int): Bool => add(a, b) == add(b, a))
}
```

---

## 3. Schema Evolution

### Current State: COMPLETE

Full schema evolution with versioned types and migrations.

**Implemented:**
- Version annotations (`@v1`, `@v2`, `@latest`)
- Migration declarations (`from @v1 = { ... }`)
- Schema registry and compatibility checking
- Auto-migration generation for simple changes
- Migration chain execution (v1→v2→v3)
- Runtime Schema module (versioned, migrate, getVersion)

**Code Locations:**
- `src/schema.rs` - Schema registry
- `src/interpreter.rs:784-851` - Migration execution
- `src/typechecker.rs:1045-1120` - Validation
- `docs/guide/13-schema-evolution.md` - Documentation

### Roadmap

| Task | Priority | Status |
|------|----------|--------|
| JSON codec generation | P1 | Missing |
| Version-aware serialization | P1 | Missing |
| Binary format support | P2 | Missing |
| Avro/Protobuf interop | P3 | Missing |
| Migration visualization | P3 | Missing |

### Codec Generation Design

```lux
type User @v2 {
    name: String,
    email: String,
    createdAt: Timestamp
} deriving (JsonCodec, BinaryCodec)

// Auto-generates:
// User.toJson(user: User@v2): String
// User.fromJson(json: String): Result<User@v2, ParseError>
// User.toBinary(user: User@v2): Bytes
// User.fromBinary(bytes: Bytes): Result<User@v2, ParseError>
```

---

## 4. C Backend Optimizations

### Current State: PRODUCTION-READY

Sophisticated C backend with Perceus-style reference counting.

**Implemented:**
- Reference counting with scope-based cleanup
- FBIP (Functional But In-Place) optimizations
- Evidence passing for zero-cost effects
- All 8 built-in effects (Console, File, Http, Random, Time, State, Reader, Process)
- Drop specialization by type
- Ownership transfer analysis

**Code Locations:**
- `src/codegen/c_backend.rs` (4,749 lines)
- `docs/C_BACKEND.md`, `docs/REFERENCE_COUNTING.md`
- `docs/COMPILER_OPTIMIZATIONS.md`

### Behavioral Type Optimizations

The C backend is ready to leverage behavioral types:

| Property | Optimization | Implementation |
|----------|--------------|----------------|
| `is pure` | Memoization | Cache results by args hash |
| `is pure` | CSE | Eliminate redundant calls |
| `is pure` | Dead code elimination | Remove unused pure calls |
| `is pure` | Auto-parallelization | `#pragma omp parallel` |
| `is total` | No exception overhead | Skip try/catch codegen |
| `is total` | Aggressive inlining | No stack overflow risk |
| `is deterministic` | Result caching | Global cache for expensive ops |
| `is idempotent` | Duplicate elimination | `f(x); f(x)` → `f(x)` |
| `is idempotent` | Retry optimization | No state reset needed |
| `is commutative` | Argument canonicalization | `f(b,a)` → `f(a,b)` for cache hits |
| `is commutative` | Parallel reduction | Tree reduction pattern |

### Roadmap

| Task | Priority | Effort | Impact |
|------|----------|--------|--------|
| Pure function memoization | P1 | 2 weeks | High |
| Idempotent call deduplication | P1 | 1 week | Medium |
| Commutative parallel reduction | P2 | 2 weeks | High |
| Deterministic result caching | P2 | 1 week | Medium |
| Total function inlining | P2 | 1 week | Medium |
| Drop fusion | P3 | 3 days | Low |
| LLVM backend | P3 | 8 weeks | High |

### Memoization Implementation

```c
// For: fn fib(n: Int): Int is pure = ...

typedef struct {
    int64_t key;
    int64_t value;
    bool valid;
} MemoEntry_fib;

static MemoEntry_fib memo_fib[1024];  // Power of 2 for fast modulo

int64_t fib_lux(int64_t n) {
    size_t idx = (size_t)n & 1023;  // Fast modulo
    if (memo_fib[idx].valid && memo_fib[idx].key == n) {
        return memo_fib[idx].value;  // Cache hit
    }

    int64_t result = (n <= 1) ? n : fib_lux(n-1) + fib_lux(n-2);

    memo_fib[idx].key = n;
    memo_fib[idx].value = result;
    memo_fib[idx].valid = true;
    return result;
}
```

---

## 5. LSP Improvement Plan

### Current State: FUNCTIONAL

Working LSP with diagnostics, hover, completion, go-to-definition, and references.

**Implemented:**
- Diagnostics (parse + type errors)
- Hover (type signatures for functions, variables, etc.)
- Completion (context-aware, module-specific, trigger on '.')
- Go-to-definition (AST-based symbol table)
- Find references (symbol table lookup)
- Document symbols (functions, types, effects)
- Document synchronization
- Proper symbol table infrastructure (`src/symbol_table.rs`)

**Code Locations:**
- `src/lsp.rs` (~900 lines) - LSP server
- `src/symbol_table.rs` (~660 lines) - Semantic analysis infrastructure

### Gaps vs Full LSP

| Feature | Status | Priority |
|---------|--------|----------|
| Diagnostics | Complete | - |
| Hover | Complete (via symbol table) | - |
| Completion | Mostly complete | P2 |
| Go-to-definition | Complete (symbol table) | - |
| References | Complete (symbol table) | - |
| Document symbols | Complete | - |
| Rename | Missing | P2 |
| Workspace symbols | Missing | P2 |
| Signature help | Missing | P2 |
| Code actions | Missing | P3 |
| Formatting | Missing (integration) | P2 |
| Semantic tokens | Missing | P3 |
| Inlay hints | Missing | P3 |

### Architecture

**Implemented:** AST-based symbol table with scope resolution

The `SymbolTable` (`src/symbol_table.rs`) provides:
- `SymbolId` - Unique identifiers for symbols
- `Symbol` - Definitions with name, kind, span, type signature, documentation
- `Reference` - Usages of symbols with position tracking
- `Scope` - Nested scopes with parent references
- AST visitors for building the table from a parsed program

```rust
// New architecture
struct SymbolTable {
    scopes: Vec<Scope>,
    definitions: HashMap<SymbolId, Definition>,
    references: HashMap<Position, SymbolId>,
}

struct Definition {
    name: String,
    kind: SymbolKind,  // Function, Variable, Type, Effect
    typ: Type,
    span: Span,
    doc: Option<String>,
}

impl LspServer {
    fn on_did_change(&mut self, uri: &Url, text: &str) {
        let ast = self.parser.parse(text);
        let symbols = self.build_symbol_table(&ast);
        self.symbol_tables.insert(uri.clone(), symbols);
    }

    fn goto_definition(&self, uri: &Url, pos: Position) -> Option<Location> {
        let table = self.symbol_tables.get(uri)?;
        let sym_id = table.references.get(&pos)?;
        let def = table.definitions.get(sym_id)?;
        Some(Location { uri: uri.clone(), range: def.span.to_range() })
    }
}
```

### Roadmap

| Phase | Tasks | Effort |
|-------|-------|--------|
| Phase 1 | ~~Build symbol table, fix goto-def~~ | ✅ Complete |
| Phase 2 | ~~References, document symbols~~ | ✅ Complete |
| Phase 3 | Rename, signature help | 2 weeks |
| Phase 4 | Workspace symbols, formatting integration | 1 week |
| Phase 5 | Code actions, inlay hints, semantic tokens | 2 weeks |

---

## 6. Package Manager

### Current State: NEARLY COMPLETE

Full-featured package manager with manifest, lock files, version resolution, and registry integration.

**Implemented:**
- `lux pkg init` - Create lux.toml
- `lux pkg add/remove` - Manage dependencies
- `lux pkg install` - Install from manifest with lock file
- `lux pkg list` - Show dependencies
- `lux pkg update` - Update and reinstall
- `lux pkg clean` - Remove installed packages
- `lux pkg search` - Search registry
- `lux pkg publish` - Publish to registry
- Git, local path, and registry dependencies
- `.lux_packages/` directory resolution
- Lock file generation (`lux.lock`)
- Version constraint parsing (^, ~, >=, <, *, ranges)
- Semantic versioning with prerelease support

**Registry Server:**
- Full HTTP server (`src/registry.rs`)
- Package metadata and tarball storage
- Search API
- Publish endpoint

**Code Locations:**
- `src/package.rs` (~1000 lines) - Package manager and resolver
- `src/registry.rs` (~637 lines) - Registry server

### Comparison with Other Package Managers

| Feature | Cargo (Rust) | npm (JS) | pip (Python) | Lux |
|---------|--------------|----------|--------------|-----|
| Manifest | Cargo.toml | package.json | pyproject.toml | lux.toml |
| Lock file | Cargo.lock | package-lock.json | requirements.txt | lux.lock ✅ |
| Version constraints | Yes | Yes | Yes | ✅ ^, ~, >=, <, * |
| Registry | crates.io | npmjs.com | PyPI | ✅ Built-in server |
| Publish | `cargo publish` | `npm publish` | `twine upload` | ✅ `lux pkg publish` |
| Search | `cargo search` | `npm search` | `pip search` | ✅ `lux pkg search` |
| Transitive deps | Yes | Yes | Yes | ⚠️ Direct only |
| Scripts | build.rs | scripts | setup.py | Missing |
| Workspaces | Yes | Yes | No | Missing |

### Roadmap

| Task | Priority | Effort | Status |
|------|----------|--------|--------|
| Lock file generation | P0 | 1 week | ✅ Complete |
| Version constraint parsing | P0 | 3 days | ✅ Complete |
| Registry server | P1 | 3 weeks | ✅ Complete |
| `lux pkg publish` | P1 | 1 week | ✅ Complete |
| Package search CLI | P2 | 3 days | ✅ Complete |
| Transitive dependency resolution | P1 | 2 weeks | Partial |
| Workspaces support | P3 | 2 weeks | Missing |
| HTTPS support | P2 | 1 week | Missing |

### Version Resolution Design

```rust
// Semantic versioning constraints
enum VersionConstraint {
    Exact(Version),           // "1.2.3"
    Caret(Version),           // "^1.2.3" - compatible updates
    Tilde(Version),           // "~1.2.3" - patch updates only
    Range(Version, Version),  // ">=1.0, <2.0"
    Any,                      // "*"
}

// Resolution algorithm (PubGrub-inspired)
fn resolve(root: &Manifest) -> Result<LockFile, ResolutionError> {
    let mut solution = PartialSolution::new();
    let mut incompatibilities = Vec::new();

    loop {
        match solution.next_undecided() {
            Some(package) => {
                let versions = fetch_versions(&package);
                match select_version(&package, &versions, &solution) {
                    Some(v) => solution.decide(package, v),
                    None => {
                        // Conflict - backtrack or fail
                        let conflict = analyze_conflict(&solution);
                        incompatibilities.push(conflict);
                        if !solution.backtrack(&conflict) {
                            return Err(ResolutionError::Unsatisfiable);
                        }
                    }
                }
            }
            None => return Ok(solution.to_lock_file()),
        }
    }
}
```

### Registry Design

```
registry.lux-lang.dev/
├── api/
│   ├── v1/
│   │   ├── packages/          GET list, POST publish
│   │   ├── packages/{name}/   GET metadata
│   │   ├── packages/{name}/{version}/  GET specific version
│   │   └── search?q=...       GET search
│   └── auth/                  Authentication
├── storage/
│   └── packages/
│       └── {name}/
│           └── {version}/
│               ├── package.tar.gz
│               └── checksum.sha256
```

---

## 7. REPL

### Current State: FUNCTIONAL

Interactive REPL with history, completions, and commands.

**Implemented:**
- Expression evaluation
- Multi-line input (brace continuation)
- History (persisted to ~/.lux_history)
- Completions (keywords, types, user definitions)
- Commands: `:help`, `:quit`, `:type`, `:info`, `:clear`, `:load`, `:env`, `:effects`
- User-defined function tracking

**Code Location:** `src/main.rs:1698-1847`

### Comparison with Other REPLs

| Feature | GHCi (Haskell) | utop (OCaml) | iex (Elixir) | Lux |
|---------|----------------|--------------|--------------|-----|
| Tab completion | Yes | Yes | Yes | Yes |
| Multi-line | Yes | Yes | Yes | Yes |
| History | Yes | Yes | Yes | Yes |
| Type query | `:t` | `#show` | `i` | `:type` |
| Load file | `:l` | `#use` | `c` | `:load` |
| Reload | `:r` | `#reload` | `r` | Missing |
| Debug/trace | Yes | Yes | `IEx.pry` | Missing |
| Step eval | Partial | No | No | Missing |
| Effect viz | N/A | N/A | N/A | Missing |
| Hot reload | No | No | Yes | Missing |

### Roadmap

| Task | Priority | Effort |
|------|----------|--------|
| `:reload` command | P1 | 2 days |
| Step-by-step evaluation | P2 | 2 weeks |
| Effect visualization | P2 | 2 weeks |
| Type hole support (`_`) | P2 | 1 week |
| Syntax highlighting | P2 | 1 week |
| Pretty-printed output | P2 | 3 days |
| `:bench` command | P3 | 3 days |
| WebSocket REPL server | P3 | 1 week |

### Effect Visualization Design

```
lux> :trace
Effect tracing enabled.

lux> run processOrder(order) with {}
[00:00.001] Console.print("Processing order...")
[00:00.015] Database.query("SELECT * FROM products WHERE id = 42")
            → [{ id: 42, name: "Widget", price: 9.99 }]
[00:00.023] Database.query("SELECT balance FROM accounts WHERE user = 'alice'")
            → [{ balance: 150.00 }]
[00:00.031] Database.execute("UPDATE accounts SET balance = 140.01 WHERE user = 'alice'")
            → 1 row affected
[00:00.045] Email.send({ to: "alice@example.com", subject: "Order Confirmed" })
            → Ok(())
[00:00.046] Console.print("Done!")

Result: Ok({ orderId: "ORD-123", total: 9.99 })
Effects: 4 Database ops, 1 Email, 2 Console
```

---

## 8. HTTP Server

### Current State: FUNCTIONAL

Built-in HTTP server effect with stdlib helpers.

**Implemented:**
- HttpServer effect: `listen`, `accept`, `respond`, `respondWithHeaders`, `stop`
- Http client effect: `get`, `post`, `put`, `delete`
- Stdlib helpers: Response builders, path matching, JSON utilities
- Examples: Basic server, router, REST API

**Code Locations:**
- `src/interpreter.rs` - tiny_http integration
- `stdlib/http.lux` (161 lines)
- `examples/http_server.lux`, `examples/http_api.lux`

### Comparison with Other Languages

| Feature | Express (Node) | Actix (Rust) | Phoenix (Elixir) | Gin (Go) | Lux |
|---------|----------------|--------------|------------------|----------|-----|
| Routing DSL | Yes | Yes | Yes | Yes | Basic |
| Middleware | Yes | Yes | Yes | Yes | Missing |
| WebSockets | Plugin | Yes | Yes | Plugin | Missing |
| Static files | Plugin | Yes | Yes | Yes | Missing |
| Templates | Plugin | Yes | Yes | Plugin | Missing |
| Sessions | Plugin | Yes | Yes | Plugin | Missing |
| CORS | Plugin | Yes | Plugin | Plugin | Missing |
| Rate limiting | Plugin | Plugin | Plugin | Plugin | Missing |
| Request validation | Plugin | Yes | Yes | Plugin | Missing |
| Async handling | Yes | Yes | Yes | Yes | Single-threaded |

### Is the Stdlib Too Big?

**Current stdlib size:** ~1,000 lines total
- `stdlib/http.lux`: 161 lines (16%)
- `stdlib/html.lux`: 384 lines (38%)
- `stdlib/testing.lux`: 192 lines (19%)
- `stdlib/browser.lux`: 89 lines (9%)
- `std/prelude.lux`: 38 lines (4%)
- Other: ~136 lines (14%)

**Analysis:** The stdlib is **lean and focused**. Compare:
- Go stdlib: ~1.5M lines
- Rust stdlib: ~500K lines
- Python stdlib: ~700K lines

Lux's ~1K lines is minimal. The question is whether HTTP/HTML belong in stdlib or should be external packages.

**Recommendation:** Keep HTTP/HTML in stdlib because:
1. Web is the primary use case
2. Effect integration requires compiler support
3. 550 lines is negligible
4. Ensures consistent patterns

### HTTP Server Roadmap

| Task | Priority | Effort |
|------|----------|--------|
| Middleware pattern | P1 | 1 week |
| Routing DSL | P1 | 1 week |
| Request validation | P1 | 1 week |
| Static file serving | P2 | 3 days |
| CORS handler | P2 | 2 days |
| WebSocket effect | P2 | 2 weeks |
| Sessions/cookies | P2 | 1 week |
| Rate limiting | P3 | 3 days |
| HTTP/2 support | P3 | 2 weeks |

### Middleware Design

```lux
// Middleware type
type Middleware = fn(Request, fn(Request): Response): Response

// Built-in middleware
fn logging(): Middleware = fn(req, next) => {
    let start = Time.now()
    let res = next(req)
    let duration = Time.now() - start
    Console.print("{req.method} {req.path} - {res.status} ({duration}ms)")
    res
}

fn cors(origins: List<String>): Middleware = fn(req, next) => {
    let res = next(req)
    res with {
        headers: res.headers ++ [
            ("Access-Control-Allow-Origin", String.join(origins, ", ")),
            ("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE")
        ]
    }
}

// Usage
let app = compose([
    logging(),
    cors(["https://example.com"]),
    authenticate(),
    router
])

run HttpServer.listen(8080, app) with {}
```

---

## 9. Async & Concurrency

### Current State: SINGLE-THREADED

Lux currently has no async/concurrency primitives. The interpreter runs single-threaded, and HTTP requests are processed sequentially.

### Design Considerations

**Option A: Green Threads (Go-style)**
```lux
// Spawn lightweight threads
fn main(): Unit with {Spawn} = {
    spawn { downloadFile("a.txt") }
    spawn { downloadFile("b.txt") }
    spawn { processData() }
}
```

**Option B: Async/Await (JS-style)**
```lux
// Explicit async functions
fn fetchData(): Async<Data> with {Http} = {
    let a = await Http.get("api/a")
    let b = await Http.get("api/b")
    combine(a, b)
}
```

**Option C: Effect-based (Recommended)**
```lux
// Concurrency as an effect
effect Concurrent {
    fn spawn<T>(f: fn(): T): Future<T>
    fn await<T>(f: Future<T>): T
    fn parallel<T>(tasks: List<fn(): T>): List<T>
}

fn main(): Unit with {Http, Concurrent} = {
    let results = Concurrent.parallel([
        fn() => Http.get("api/a"),
        fn() => Http.get("api/b"),
        fn() => Http.get("api/c")
    ])
    // results: List<Response>
}
```

### Recommended Design: Effect-based Concurrency

**Why effects?**
1. Consistent with Lux's philosophy
2. Handlers can control scheduling
3. Testable (swap for sequential execution)
4. No colored functions (async/sync split)

**Primitives:**

```lux
effect Concurrent {
    // Spawn a task
    fn spawn<T>(f: fn(): T with {E}): Task<T> with {E}

    // Wait for task completion
    fn await<T>(task: Task<T>): T

    // Run tasks in parallel, collect results
    fn parallel<T>(tasks: List<fn(): T with {E}>): List<T> with {E}

    // Race: return first to complete
    fn race<T>(tasks: List<fn(): T with {E}>): T with {E}

    // Yield execution
    fn yield(): Unit
}

// Channel for communication
effect Channel<T> {
    fn send(value: T): Unit
    fn receive(): T
    fn tryReceive(): Option<T>
}
```

**Example:**

```lux
fn fetchAllUsers(): List<User> with {Http, Concurrent} = {
    let userIds = [1, 2, 3, 4, 5]

    // Fetch all in parallel
    Concurrent.parallel(
        List.map(userIds, fn(id) => fn() => {
            let response = Http.get("/users/{id}")
            Json.parse(response.body)
        })
    )
}

// Test with sequential execution
run fetchAllUsers() with {
    Http = mockHttp,
    Concurrent = sequentialHandler  // No actual parallelism
}
```

### Roadmap

| Task | Priority | Effort |
|------|----------|--------|
| Concurrent effect definition | P1 | 1 week |
| Task/Future type | P1 | 1 week |
| Work-stealing scheduler | P1 | 3 weeks |
| Channel effect | P2 | 2 weeks |
| Select/race primitives | P2 | 1 week |
| Structured concurrency | P2 | 2 weeks |
| Cancellation tokens | P3 | 1 week |

---

## 10. Documentation

### Current State: GOOD FOUNDATION

Documentation exists but needs organization and expansion.

**Existing:**
- `docs/guide/` - 14 chapters (introduction through property testing)
- `docs/tutorials/` - Calculator, dependency injection, project ideas
- `docs/reference/` - Syntax reference
- `docs/*.md` - Design documents (C backend, evidence passing, etc.)
- `examples/` - 57 example files

### Documentation Structure Plan

```
docs/
├── guide/                    # Learning path (beginner → advanced)
│   ├── 01-introduction.md
│   ├── 02-basic-types.md
│   ├── 03-functions.md
│   ├── 04-data-types.md
│   ├── 05-effects.md
│   ├── 06-handlers.md
│   ├── 07-modules.md
│   ├── 08-errors.md
│   ├── 09-stdlib.md
│   ├── 10-advanced.md
│   ├── 11-databases.md
│   ├── 12-behavioral-types.md
│   ├── 13-schema-evolution.md
│   └── 14-property-testing.md
│
├── reference/                # API reference (generated)
│   ├── syntax.md            # Complete syntax reference
│   ├── types.md             # Type system reference
│   ├── effects.md           # Built-in effects
│   ├── stdlib/              # Stdlib API docs
│   │   ├── list.md
│   │   ├── string.md
│   │   ├── option.md
│   │   ├── result.md
│   │   ├── json.md
│   │   └── ...
│   └── cli.md               # CLI reference
│
├── tutorials/               # Task-oriented guides
│   ├── web-api.md           # Build a REST API
│   ├── cli-tool.md          # Build a CLI application
│   ├── testing.md           # Testing guide
│   ├── deployment.md        # Deployment guide
│   └── migration.md         # Migrating from X to Lux
│
├── cookbook/                # Copy-paste recipes
│   ├── http-patterns.md
│   ├── database-patterns.md
│   ├── error-handling.md
│   └── testing-patterns.md
│
└── internals/               # Implementation docs
    ├── architecture.md
    ├── type-system.md
    ├── codegen.md
    └── contributing.md
```

### Website Integration

```
website/
├── index.html               # Landing page
├── playground/              # Online REPL
│   └── index.html
├── docs/                    # Rendered documentation
│   ├── guide/
│   ├── reference/
│   ├── tutorials/
│   └── cookbook/
├── examples/                # Interactive examples
│   └── index.html
└── blog/                    # News and updates
    └── index.html
```

### Roadmap

| Task | Priority | Effort |
|------|----------|--------|
| Auto-generate stdlib API docs | P1 | 2 weeks |
| Build REST API tutorial | P1 | 1 week |
| Build CLI tool tutorial | P1 | 3 days |
| Create cookbook section | P1 | 1 week |
| Online playground | P2 | 3 weeks |
| Interactive examples | P2 | 2 weeks |
| Video tutorials | P3 | 4 weeks |

### API Documentation Generator

```lux
// Source file with doc comments
/// Transforms each element of a list using the given function.
///
/// ## Example
/// ```lux
/// List.map([1, 2, 3], fn(x) => x * 2)
/// // => [2, 4, 6]
/// ```
///
/// ## Complexity
/// O(n) where n is the length of the list.
pub fn map<T, U>(list: List<T>, f: fn(T): U): List<U> = ...
```

Generated output:
```markdown
### List.map

```lux
fn map<T, U>(list: List<T>, f: fn(T): U): List<U>
```

Transforms each element of a list using the given function.

**Example:**
```lux
List.map([1, 2, 3], fn(x) => x * 2)
// => [2, 4, 6]
```

**Complexity:** O(n) where n is the length of the list.
```

---

## 11. Error Messages

### Current State: GOOD

Elm-inspired diagnostic system with categorized errors and suggestions.

**Implemented:**
- 26 error codes (E01xx parse, E02xx type, E03xx name, etc.)
- Context lines with highlighting
- Levenshtein-based suggestions ("did you mean?")
- Color support with fallback
- Type diff visualization

**Code Location:** `src/diagnostics.rs` (1,033 lines)

### Current Error Quality

**Good:**
```
── TYPE MISMATCH ─────────────────────────────────── src/main.lux

   14│   let total = calculateTotal(order.quantity)
                                    ^^^^^^^^^^^^^^

This function expects an `Int` but got a `String`.

Hint: Maybe parse the string first?
    let qty = Int.parse(order.quantity)?
```

**Needs Improvement:**
- LSP only uses message field (not codes, hints, related spans)
- Some errors are generic ("Type error")
- No example-based hints

### Error Message Roadmap

| Task | Priority | Effort |
|------|----------|--------|
| LSP rich diagnostic support | P1 | 1 week |
| Example-based hints | P1 | 2 weeks |
| Effect mismatch explanations | P1 | 1 week |
| Missing pattern suggestions | P2 | 3 days |
| Error code documentation | P2 | 1 week |
| Beginner-friendly mode | P3 | 1 week |
| Localization infrastructure | P3 | 2 weeks |

### Enhanced Error Examples

**Effect mismatch (improved):**
```
── MISSING EFFECT ─────────────────────────────────── src/api.lux

   23│   fn getUser(id: Int): User = {
   24│       Database.query("SELECT * FROM users WHERE id = ?", id)
         ^^^^^^^^
   25│   }

The function `getUser` uses the `Database` effect but doesn't declare it.

Add `with {Database}` to the function signature:

   fn getUser(id: Int): User with {Database} = {
       Database.query("SELECT * FROM users WHERE id = ?", id)
   }

Or, if this should be a pure function, consider passing the user as
a parameter instead of querying the database.

See: https://lux-lang.dev/errors/E0401
```

**Pattern match (improved):**
```
── INEXHAUSTIVE PATTERN ──────────────────────────── src/parser.lux

   45│   match token {
   46│       Token.Number(n) => Expr.Lit(n),
   47│       Token.Ident(s) => Expr.Var(s)
   48│   }

This match doesn't cover all cases. Missing patterns:

    Token.String(_)
    Token.Operator(_)
    Token.Eof

Add the missing cases:

   match token {
       Token.Number(n) => Expr.Lit(n),
       Token.Ident(s) => Expr.Var(s),
       Token.String(s) => ...,
       Token.Operator(op) => ...,
       Token.Eof => ...
   }

Or use a catch-all pattern if appropriate:

   _ => Expr.Error("unexpected token")

See: https://lux-lang.dev/errors/E0501
```

---

## Implementation Priority Matrix

### Phase 1: Foundation (Next 3 months)

| Area | Task | Priority | Effort |
|------|------|----------|--------|
| Package | Lock file + version resolution | P0 | 3 weeks |
| LSP | Symbol table architecture | P0 | 2 weeks |
| C Backend | Pure function memoization | P1 | 2 weeks |
| HTTP | Middleware + routing DSL | P1 | 2 weeks |
| Docs | API documentation generator | P1 | 2 weeks |

### Phase 2: Polish (Months 4-6)

| Area | Task | Priority | Effort |
|------|------|----------|--------|
| Package | Registry server | P1 | 3 weeks |
| LSP | References + rename | P1 | 2 weeks |
| REPL | Step evaluation + effect viz | P2 | 4 weeks |
| Async | Concurrent effect | P1 | 5 weeks |
| Errors | Rich LSP diagnostics | P1 | 1 week |

### Phase 3: Advanced (Months 7-12)

| Area | Task | Priority | Effort |
|------|------|----------|--------|
| C Backend | Full behavioral optimizations | P2 | 8 weeks |
| Schema | JSON codec generation | P1 | 3 weeks |
| HTTP | WebSocket effect | P2 | 2 weeks |
| Docs | Online playground | P2 | 3 weeks |
| Modules | Workspaces | P3 | 2 weeks |

---

## Summary

Lux has a **solid foundation** with most core features complete:

| Area | Status | Next Steps |
|------|--------|------------|
| Modules | Complete | Lock files, registry |
| Behavioral Types | Complete | Optimization integration |
| Schema Evolution | Complete | Codec generation |
| C Backend | Production-ready | Behavioral optimizations |
| LSP | Basic | Symbol table rewrite |
| Package Manager | Functional | Version resolution |
| REPL | Functional | Effect visualization |
| HTTP Server | Functional | Middleware, routing |
| Async/Concurrency | Missing | Effect-based design |
| Documentation | Good | API generator, tutorials |
| Error Messages | Good | LSP integration |

The highest-impact work is:
1. **Package manager version resolution** - Essential for real projects
2. **LSP symbol table** - Unlocks modern IDE experience
3. **Behavioral type optimizations** - Unique value proposition
4. **Concurrent effect** - Required for production web services