Important note: this article is outdated! Go to http://zsiciarz.github.io/24daysofrust/ for a recent version of all of 24 days of Rust articles. The blogpost here is kept as it is for historical reasons.

A few days ago the Rust subreddit got excited about the rebirth of rusti - an interactive shell for Rust. Such interpreters (sometimes called REPLs - read-eval-print loop) are the bread and butter of dynamic languages like Lisp, Python or Ruby. But that is not a requirement - as Haskell demonstrates with its GHCi, you can have a REPL for a statically typed, compiled language too. rusti is an attempt to build such an interpreter for Rust.

rusti is still under development and currently installable only from source checkout. Clone the repository and start the interpreter with cargo run:

$ cargo run
    Running `target/rusti`
rusti=> 1u + 4
5

You can use it as a calculator but remember about type annotations, just like in regular Rust code. Otherwise you'll get an error like:

rusti=> 1 + 4
<anon>:13:16: 13:25 error: unable to infer enough type information to locate the impl of the trait `core::fmt::Show` for the type `_`; type annotations required
<anon>:13 println!("{}", { 1 + 4 });

rusti can evaluate most of Rust code and display results. You can use crates and modules from the standard library as shown in a few examples below.

rusti=> use std::os;
rusti=> os::args()
[target/rusti]
rusti=> use std::iter::AdditiveIterator;
rusti=> range(1, 1000u).filter(|x| *x % 19 == 3).sum()
26341
rusti=> #![feature(phase)]
rusti=> #[phase(plugin)] extern crate regex_macros;
rusti=> extern crate regex;
rusti=> let re = regex!(r"\b[a-z]{6}:[0-9]{3}\b"); re.is_match("qwerty:123")
true

One great thing about rusti is the .type command which shows the type of an expression. (It's quite similar to :type in GHCi.)

rusti=> .type Some("Hello world!".to_string())
Some("Hello world!".to_string()) = core::option::Option<collections::string::String>
rusti=> .type vec![1u, 2, 3]
vec![1u, 2, 3] = collections::vec::Vec<uint>
rusti=> .type std::io::stdin
std::io::stdin = fn() -> std::io::stdio::StdinReader

Unfortunately there is no code completion yet, but at least readline capabilities (such as line editing and history) are available.

There are a few other limitations, some of which are rather inconvenient at the moment (such as let declarations being very local), but the authors are aware of them. The real issue for me today is that it can't reference external crates. However this is already being discussed and I hope it will happen.

Code examples in this article were built with rustc 0.13.0-nightly.

Photo by photonate.com and shared under the Creative Commons Attribution 2.0 Generic License. See https://www.flickr.com/photos/themacguy2k/3468131941