writeonce

A small compiled language with a database built in. You write .wo files; one command turns them into a single native binary that carries its own storage engine — a typed, WAL-durable, crash-recoverable database — with no server to install, no ORM, and no query strings. Tables are just classes, queries are written in the language and checked by the compiler, and the whole program ships as one file that depends only on the system C library.

Status: early, honest. Everything documented on this page compiles and runs today and is exercised by the acceptance tests in this repository. Features that are planned but not yet available are listed separately under Roadmap — they are not described as if they work. Nothing here is API-stable yet.

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Why writeonce

• The database is part of the language. A class marked @table is a table. Its rows persist through a write-ahead log, survive a restart, and are reached by navigating typed relations — not by assembling SQL text.
• Queries are compiled, not interpreted. from e in Employee where e.salary > 90000 select e lowers to bytecode loops over the engine. A mistyped field name is a compile error, not a runtime surprise. There is no SQL string anywhere in the shipped binary.
• One binary, no runtime dependencies. woc . produces a self-contained executable (~100 KB for the sample programs) that links only libc. Copy it to a server and run it.
• Small on purpose. No FFI, no package manager, no framework. The standard library is a handful of OS modules. The language is designed to be read.

writeonce is not a web framework and does not (yet) serve HTTP, WebSockets, or a UI. It is a systems language whose distinguishing feature is the embedded database. If you have seen an older "writeonce" that served REST from cargo run, that was a separate, earlier runtime; this page documents the current woc/wovm toolchain.

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System requirements

To run a compiled writeonce program:

• Linux on x86-64. The produced binary is a native executable that links only the system C library (libc); nothing else is required at runtime.

To build programs from source (the toolchain), you need:

Tool	Version tested	Purpose
OCaml	4.14+	builds woc, the compiler front end
dune	3.14+	OCaml build driver
A C11 compiler	gcc 13 / clang	builds wovm, the runtime VM
just	1.x	task runner for the build/test recipes
make	any	drives the runtime build

Other POSIX platforms (macOS, BSD) are untested. The toolchain itself has no network or package-download step — it builds entirely from the checked-in source.

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Getting the toolchain

Two artifacts make up the toolchain:

• woc — the compiler (OCaml). Reads .wo source, type-checks it, runs the ownership pass, and emits a .wob image or a standalone binary.
• wovm — the runtime (C11). Loads a .wob image and executes it. When woc builds a standalone binary, it embeds the image into a copy of wovm.

Build both from the repository root:

just woc-build      # builds compiler/_build/default/bin/woc
just wovm-build     # builds runtime/wovm

# gate them (optional but recommended)
just woc-test       # compiler unit + golden suites
just wovm-test      # runtime unit suites, both dispatch flavors, ASan-clean

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Your first program

A writeonce project is a directory with a wo.toml manifest and one or more .wo files. Every program has an entry point:

-- hello/main.wo
fn main(args: multi Text) -> Int {
  print("hello, writeonce");
  return 0;
}

# hello/wo.toml
name    = "hello"
version = "0.1.0"

[runtime]
wo = ">= 0.1"

Compile the directory into a single binary and run it:

woc hello/           # produces hello/target/hello
./hello/target/hello
# hello, writeonce

main returns an Int — that value is the process exit code. args is the command-line arguments (the program name is not included).

The two build paths

# 1. standalone binary (what you ship): woc reads wo.toml, emits target/<name>
woc myproject/

# 2. image + VM (handy while developing): emit a .wob, run it with wovm
woc --emit myproject/ -o app.wob
wovm app.wob arg1 arg2

Both paths run the same program. The standalone binary is the release artifact; the image path lets you inspect or move the image around.

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Language at a glance

writeonce is statically typed with a compile-time ownership model — every value has a known owner, memory is freed deterministically, and values that form cycles are collected by an inferred garbage collector (you never annotate GC- ness; the compiler infers it). The surface will look familiar:

• Types: Int, Text, Bool, and user class types. ?T marks an optional (nullable) value; nil is the empty case.
• Containers: multi T (a growable list) and map<K, V>. Literals: [], [a, b], {}.
• Classes & records: classes with fields and methods, static const / static fn members, module-scoped across files.
• Control flow: if/else, for x in xs, for k, v in m, switch expressions, and try { … } catch (e) { … } (also an expression form).
• Strings: interpolation with ${expr} inside a "…" literal.
• Functions: free functions and methods; arguments and returns are typed.

fn classify(n: Int) -> Text {
  if n < 0 { return "negative"; }
  return switch n {
    case 0: "zero";
    default: "positive";
  };
}

Standard library

A compact set of OS modules, reached by their reserved names — no imports:

Module	What it does
fs	exists, list, stat, read_all, read_at, append
time	sleep, now, local, iso
env	get, stopping (a cooperative shutdown flag)
net	TCP listen / accept / read / write / close (host + port)
proc	run a child process, capture stdout/stderr/exit
json	encode / decode (json.decode(t) as T yields ?T)

These are deliberately minimal — the surface a real program needs, and no more.

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The database

This is the point of the language. Declaring storage is declaring a class:

@table(name: "departments", index: [name])
class Department {
  name:  Text @unique
  staff: backlink Employee.dept    -- reverse relation, not a stored column
}

@table(name: "employees", index: [dept], index: [dept, salary])
class Employee {
  name:   Text
  salary: Int
  hired:  Int
  dept:   ref Department           -- foreign key: stored as the row id
}

• @table makes a class persistent — named storage plus declared secondary indexes. Every instance you insert is written to a write-ahead log before it is acknowledged, so an acked write survives a crash; on the next start the log is replayed.
• ref T is a typed foreign key (a forward relation). backlink T.f is its inverse — a virtual field, no stored column, resolved by an index scan.
• @unique enforces uniqueness at insert/update; a violation is a catchable trap.
• Foreign keys restrict deletes: deleting a row that another row still references traps rather than orphaning it.

Writing and reading data

Mutation is direct; queries are a comprehension the compiler lowers to engine operations:

-- insert (WAL-durable); @unique makes a re-insert trap, and try/catch it:
let eng = try insert Department { name: "Engineering" } catch (e) nil;
insert Employee { name: "Asha", salary: 9200000, hired: 1704067200000, dept: eng };

-- query: filter, order, limit, project — checked at compile time
for e in from s in Employee where s.salary > 8000000 order by s.salary desc select s {
  print("${e.name} ${e.salary} (${e.dept.name})");   -- ref navigation
}

-- navigate a backlink (the department's staff), update through the result
for e in from s in dept.staff select s {
  e.salary = e.salary + e.salary * 5 / 100;           -- update-through-row
}

-- delete (restricted if still referenced)
let ok = try delete row catch (e) nil;

The query surface available today is from v in <table | relation> where … [order by k [desc]] [take n] select v | v.field, plus insert, delete, and update-through-a-row. It is proven end to end by the employee sample, whose data survives a process restart via log replay.

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Project layout & the manifest

myproject/
├── wo.toml         # manifest: name, version, [runtime], [build]
├── main.wo         # entry point (fn main)
├── types.wo        # your @table classes, other types
└── target/         # build output (the standalone binary lands here)

name    = "myproject"
version = "0.1.0"

[runtime]
wo = ">= 0.1"

[build]
runtime = "../../../runtime/wovm"   # path to the wovm the binary is built from

woc myproject/ compiles every .wo file under the directory as one program.

Programs that create tables read their data directory from the WO_DATA environment variable at run time:

WO_DATA=./data ./target/myproject seed
WO_DATA=./data ./target/myproject report     # a fresh process still sees the data

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Worked examples

Two complete sample programs live in the repository and double as the language's acceptance tests:

• docs/examples/employee/ — departments and employees related by ref/backlink, @unique, foreign-key restrict on delete, per-department reports, and persistence across a restart. Run it:

  just employee            # compile + run every mode against a durable database
  

• docs/examples/log-watcher/ — a long-running daemon that watches log files for silent death, using the fs/time/net/proc stdlib. Run it:

  just log-watcher
  

Read either program's main.wo for idiomatic, working writeonce.

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Roadmap

Planned, not yet available — listed so the shipped surface above stays honest. These exist as design iterations and/or work-in-progress branches, not as features you can use today:

• Query aggregates — group … by … into g with count/avg/min/max and projection records. (Today the same result is written by hand from the shipped primitives.)
• HTTP service layer — service blocks that route requests to methods.
• Concurrency — a shard-actor runtime and green-threaded fibers.
• Cross-program database access — one program attaching to another's database over a local channel, with keypair authentication and per-client rights.
• Blue-green deployment — in-process recompile and atomic version switch.
• Compile-time metaprogramming — @derive(Json/Csv/Eq/…) generated from a class's own metadata, no reflection.

Known current limits worth naming: net is TCP host+port only; proc.run has no timeout or signal control; there is no stdin/stdout byte I/O and no FFI.

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writeonce is a work in progress. Interfaces will change. If you build something with it, pin to a commit.