The Oneway Programming Language

Oneway is a small, opinionated language that transpiles to Rust. It inherits Rust's ownership model and zero-cost abstractions while presenting a much smaller surface area to the programmer.

The guiding rule: wherever ordering is discretionary, the compiler enforces alphabetical order. Components of product types, variants of unions, method declarations, match arms, imports — all alphabetical. Reordering is never a meaningful change.

What It Looks Like

Bool = False | True

main = (Stdout) -> Noop {
    List(1, 2, 3)
        .map((Int) -> Int { Int.mul(2) })
        .length()
        .print(Stdout)
}

A few things to notice:

• There is no let, no local variables, no if/else, no comments.
• Every function is implemented on a type: Type.name = (params) -> Ret { ... }.
• The exception is main, the program's entry point.
• Branching is match on a union.
• Side effects are passed in as capabilities (Stdout, Filesystem, …).
• Imports are file-based: use Foo imports the type declared in foo.ow.

Status

Oneway is an experimental design exploration. The compiler exists, examples run, and the design is stable enough to write about — but every detail is subject to change.

The reference implementation lives in the same repository as this book. The authoritative design spec is DESIGN.md.

How to Read This Book

• Getting Started — install the toolchain and run your first program.
• A Tour of Oneway — every feature, one short chapter each.
• Reference — sort-order rules, operator table, Rust comparison.

The chapters are short on purpose. Read straight through, or skip to whatever you need.