fx-lang

fx-lang is an experimental language for exploring scoped effects, named scopes, and coeffects.

The repository contains a Rust implementation of the parser, formatter, type checker, JavaScript compiler path, CLI, and language server experiments. It is research code first: the examples are organized by implementation phase, and some language features are intentionally narrower than the long-term design.

Example

This example combines the main ideas:

• State is a scoped effect family.
• Counter is a named scope for one State Int cell.
• within Counter lets operations omit their explicit scope argument.
• given { clock: Clock } is a coeffect: the function needs contextual data, but it is not an ordinary parameter.

Copyable source

export scoped effect State s a
  get() : a
  put(value: a) : Unit

export type Clock = { now: Text }

scope Counter with <State Int>

increment-and-label() : Text with <State Counter Int> given { clock: Clock }
  within Counter
    n = get()
    put(n + 1)
    clock.now ++ " count=" ++ show(n + 1)

The scope name distinguishes one state cell from another at the type level, while given tracks required context separately from ordinary function arguments and effect rows.

Try It

Check the promoted Phase 5 examples:

cargo run -q -p fx-cli -- check examples/phase-5

The full examples tree also contains intentional diagnostics fixtures.

Format sources:

cargo run -q -p fx-cli -- fmt --check examples

Compile the currently supported JavaScript subset:

cargo run -q -p fx-cli -- compile --target js --out-dir /tmp/fxl examples/phase-5
node /tmp/fxl/with-clock.mjs

Repository Layout

• crates/fx-syntax: lexer, parser, AST, diagnostics, and formatter
• crates/fx-core: module loading, resolution, and type checking
• crates/fx-compiler: lowering and JavaScript emission
• crates/fx-cli: command-line interface
• crates/fx-lsp: language server prototype
• examples: phase-oriented language examples
• notes: design notes, implementation plans, and phase learnings

Status

fx-lang is pre-release and changing quickly. It is useful for experimenting with typed effect systems, named effect scopes, contextual requirements, and compiler/runtime design, but it is not intended for production use.