Getting Started

Requirements

macOS on Apple Silicon (M1/M2/M3/M4)
Zig compiler (0.13+)

fy generates ARM64 machine code and uses macOS-specific JIT APIs (MAP_JIT, pthread_jit_write_protect_np). It does not run on Linux or x86.

Building

git clone <repo-url>
cd fy
zig build

The binary lands at ./zig-out/bin/fy (~1.8MB). The build system handles code signing for JIT execution automatically.

Running

Interactive REPL

./zig-out/bin/fy

If no files or -e flags are given, fy drops into the REPL:

fy! v0.0.1
fy> 2 3 + .
5
    0
fy> : square dup * ;
    0
fy> 7 square .
49
    0

The REPL preserves the data stack between lines. The 0 printed after each result is the return value of the session expression.

Running Files

./zig-out/bin/fy examples/fib.fy

Evaluating Expressions

./zig-out/bin/fy -e "2 3 + ."

Shebang Scripts

fy files can start with a shebang:

#!/path/to/fy
10 fib .

The #! line is automatically stripped before compilation.

Command-Line Flags

Flag	Description
`-e`, `--eval <expr>`	Evaluate an expression
`-r`, `--repl`	Launch interactive REPL
`-i`, `--image`	Dump JIT image to `fy.out`
`-v`, `--version`	Print version
`-s`, `--serve`	Start the hot-patching TCP listener
`-p`, `--port <n>`	Set the listener port (default: OS-assigned)
`-h`, `--help`	Show help

Editor Support

A VSCode extension with syntax highlighting and live hot-patching is available in editors/vscode/fy-lang/. See the Editor Support page for installation and usage.

Running Tests

The examples/golden/ directory contains golden tests (.fy files paired with .expected output files). Run them with:

zig build test

Next Steps

Language Guide — learn the syntax and core concepts
Examples — see annotated real-world programs

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