Milo

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Milo Roadmap ​

Completed ​

Core Language ​

The foundation is complete: primitive types, let/var bindings, if/else, while/for loops, functions, structs, enums with pattern matching (exhaustiveness checked), generics with monomorphization and type inference, move semantics with use-after-move detection, second-class references (&T/&mut T in params only), closures (including escaping/move closures), traits with static dispatch and @derive(Eq), operator overloading via traits, Heap<T>, Option<T>, Result<T,E> with !/?/?? operators, string interpolation, bitwise operators, hex/binary literals, type casts, for-in loops over ranges/Vec/array/string/HashMap, and HIR-based typed IR.

Type System & Safety ​

  • Ownership: single-owner move semantics, compiler-tracked drops, no GC
  • Null safety: Option<T> — no null pointers in safe code
  • Race safety: Send/Sync traits — compiler rejects data races at spawn() boundaries
  • Overflow safety: compile-time range checks + debug-mode traps via LLVM overflow intrinsics
  • No implicit coercion: explicit as casts only
  • Ranged integers (L1+L2): type Altitude = i32(0..50000) with range propagation through arithmetic

Concurrency ​

Full concurrency stack, from OS threads to lightweight green threads to structured promises:

  • OS threads (std/thread): Thread.spawn() with move closures, Thread.join()
  • Synchronization (std/sync): Mutex, RwLock, Channel<T> (bounded FIFO, multi-producer, blocking + non-blocking), AtomicI64, AtomicBool
  • Green threads (std/runtime): stackful coroutines via ucontext (64KB stacks, guard pages, kqueue/epoll), cooperative scheduling, transparent async I/O — stream.recv()/stream.send() auto-yield on EAGAIN
  • Promises (std/runtime): Promise<T>.run(), .await(), Promise.all(), Promise.race() — structured concurrency over green threads
  • Task API (std/runtime): Task.spawn() for fire-and-forget lightweight concurrency
  • No async/await: write normal blocking code — it yields automatically in green thread context

Standard Library (44 modules) ​

I/O & system: io, fs, path, env, args, process, signal Networking: net, http, url Data: json, csv, toml, base64, hex, sqlite, arena, set Concurrency: thread, sync, runtime, event Strings: string, fmt, strconv, unicode, regex Math: math, random, sort CLI: argparse, color, log Crypto: crypto Time: time, datetime, uuid Testing: testing Memory: mem

Developer Experience ​

  • LSP server: diagnostics, hover, go-to-definition, completions, code lens
  • VS Code extension: syntax highlighting + LSP client
  • Formatter (milo-fmt): context-sensitive formatting, LSP integration (written in Milo)
  • Package manager (milo-pkg): init, new, add, install, git-based cache with lockfile (written in Milo)
  • Test framework: @expect:/@error: annotations, milo test runner
  • Example apps: web servers (7), CLI tools (jq, grep, cat, wc, tree, calc, hex)
  • GitHub Actions CI: build + test on push/PR, release pipeline
  • Playground: browser-based compiler via JS backend (in progress)

Self-Hosting (Stage-0 Complete) ​

milo0 — a Milo compiler written in Milo — compiles a substantial subset: primitives, functions, let/var, if/else/while, structs (construct + field access + mutation), enums (payload-free + single/multi-field payloads + match), Heap<T> with deref, strings (literals, slice, index, concat, clone, len, eq), closures, as casts, bitwise ops. Verified on examples/fib.milo, examples/fizzbuzz.milo, examples/hello.milo.

In Progress ​

Self-Hosting — Stage-1 ​

Blocking full milo0-on-milo0:

  • [ ] Vec<T> in milo0 — 84 use sites, biggest blocker
  • [ ] String.push in milo0 — needs mutation-through-self + realloc
  • [ ] Port type checker, HIR, lower, codegen to Milo

End goal: compiler compiles itself, producing equivalent IR for the full Milo source set.

Planned ​

Runtime Maturity — node-milo ​

node-milo is the current stress test for Milo as an implementation language. If a missing feature keeps runtime logic in C++ or forces raw pointer arithmetic in Milo, it moves up the roadmap.

  • [ ] Move more internalBinding() implementations out of C++ and into Milo
  • [ ] Track success by Node compat %, shrinking C++ glue, and keeping unsafe contained to binding seams

Language ​

Runtime pressure from node-milo changes the order here: binary data and FFI safety land before more expressive abstractions.

  • [ ] Borrowed slices / byte views — &[T] / &mut [T], with slicing generalized beyond string. Unblocks offset/length I/O, Buffer/ArrayBuffer interop, and zero-copy protocol parsing.

  • [ ] Opaque foreign handle types — extern type sqlite3, extern type SSL, extern type NmRuntime instead of raw *u8. Keeps FFI zero-cost while preventing handle mixups.

  • [ ] C ABI / layout control — repr(c), packed structs, sizeof, offsetof, extern struct. Needed for epoll_event, kevent, stat, addrinfo, V8 glue, and other platform/FFI structs.

  • [ ] Structured OS / syscall errors — OsError/SysError carrying errno/code plus syscall/path context. Needed for runtime bindings, better diagnostics, and Node-compatible error surfacing.

  • [x] Interfaces (runtime polymorphism) — Go-style interfaces with structural typing and vtable dispatch. interface Shape { fn area(self: &Self): f64 } — any type with matching methods satisfies the interface. Separate from traits (which remain compile-time only).

  • [ ] Heap<Interface> + heterogeneous collections — Vec<Heap<Shape>> for mixed-type collections via heap-allocated interface values.

  • [ ] Iterators — iterator trait, .map().filter().collect() chains, lazy evaluation. Needs associated types.

  • [ ] Error conversion — From trait for automatic error conversion in ?, anyhow-style boxing.

  • [ ] Ranged integers L3 — branch narrowing: after if x < 50, x is known (min..49) in the then-branch.

  • [ ] Safety profiles — --strict-ranges (require ranged types on all integers), --no-unwrap (ban ! — force exhaustive error handling). Aircraft-grade opt-in.

  • [ ] MIR — lower-level IR for optimization passes (post self-hosting)

Tooling ​

  • [ ] LSP: rename + find references
  • [ ] Doc comments + generation — /// comments, milo doc to generate HTML/markdown
  • [ ] Cross-compilation — --target aarch64-linux etc. (infrastructure exists in target.ts, needs CLI flag + sysroot handling)
  • [ ] Benchmarking — @bench annotations, milo bench runner
  • [ ] Documentation / tutorials / "the book"