MeowScript

A small scripting language where the keywords are cat puns. purrhaps is if, pawction is function, tail is return, and every program announces itself with a meow. The interpreter is written in Rust and also runs in the browser as WebAssembly.

Try it at meowscript.vercel.app.

pawckage "nya:furrball";

scratch cats = ["Tom", "Luna", "Salem"];

pawction introduce(cat) {
    purrhaps cat == "Salem" {
        tail cat + " (a witch's familiar)";
    }
    tail cat;
}

fur cat ~ cats {
    meow("This is", introduce(cat));
}

Meow! This is Tom
Meow! This is Luna
Meow! This is Salem (a witch's familiar)

The language in one table

Cat	Human
scratch x = 1;	declare
amew x = 2;	assign (also amew cat's age = 4; and amew arr[0] = 1;)
pawction f(a, b) { tail a + b; }	function and return
purrhaps … { } meowtually { }	if / else (an expression)
furrever { }, furrever cond { }	loop forever, loop while
fur item ~ things { }	for each (furrballs, whiskers, object keys, or 0..n)
hiss; / continue;	break / continue
purrfect, clawful, mew	true, false, null
furreal x	typeof: "number", "whiskers", "furrball", …
cat's name, cat["name"]	property access
x ~ things	membership
pawckage "nya:furrball";	import a built-in library, or "./file.meow"

Strings are whiskers, arrays are furrballs. Operators follow Python's precedence (6 & 3 == 2 is (6 & 3) == 2), only mew and clawful are falsy, and dividing by zero is an error. The reference is in the docs, or at site/content/docs.md.

Errors point at the problem:

Meow-sterious! name error: `cuont` hasn't been `scratch`ed yet; did you mean `count`?
  --> toys.meow:3:6
   |
 3 | meow(cuont);
   |      ^^^^^

Install

Prebuilt binaries for Windows x64, macOS (Apple silicon and Intel), and Linux (x64 and arm64) are on the releases page, with a SHA256SUMS.txt. The download page picks the right file for each platform and explains the unsigned-binary warnings on Windows and macOS. Unpack the archive and put meowscript on your PATH.

With a Rust toolchain (1.88 or newer):

cargo install --git https://github.com/AlenVelocity/MeowScript --tag v0.1.0 meowscript-cli

meowscript                    # the REPL
meowscript hello.meow         # run a file
meowscript eval "1 + 2"       # evaluate a snippet
meowscript packages           # what the nya: pawckages contain

Changes between versions are in CHANGELOG.md.

Layout

crates/meowscript        the language: lexer, parser, interpreter, standard library
crates/meowscript-cli    the `meowscript` binary (runner and REPL)
crates/meowscript-wasm   the wasm-bindgen entry point for the browser
examples/                programs; those with a .out file are snapshot-tested
site/                    the playground and docs (Next.js)

The interpreter reaches the outside world only through a Host trait. The CLI gives it a terminal, the tests give it a buffer, and the browser build gives it a callback that posts lines to the page.

Development

cargo test --workspace                       # unit tests, doctests, and every example
cargo run -p meowscript-cli -- examples/yarn.meow
cargo clippy --workspace --all-targets -- -D warnings
cargo fmt --all

For the site, see site/README.md. After changing the Rust code, run pnpm build:wasm && pnpm sync there to refresh the WebAssembly and the generated tables.

The first version of this interpreter is in the git history before the rewrite. This one replaces it with a byte-indexed lexer that records spans, a Pratt parser that reports where it got stuck, an evaluator that returns Result instead of error values, and a browser build on stable Rust with wasm-bindgen instead of Emscripten.

Contributing

See CONTRIBUTING.md. Cat puns welcome.