Getting started

This page makes a package, builds and runs it, and shows the commands you will use every day. It assumes ceesafec is installed and on your PATH.

A new package

ceesafec new hello
cd hello

ceesafec new writes a manifest, a first source file, and instructions for AI coding agents:

hello/
├── ceesafe.toml
├── src/
│   └── main.cee
├── AGENTS.md
└── .gitignore

AGENTS.md tells an agent working in the package what CeeSafe is and how to check its work. With ceesafec new hello --agent claude (or gemini, copilot, cursor) it also writes that agent's own file and configures CeeSafe's MCP server for it; --no-agent writes none. See CeeSafe for AI agents.

ceesafe.toml names the package and says what it builds:

[package]
name = "hello"
version = "0.1.0"
kind = "executable"

[dependencies]

and src/main.cee is a program:

fn main() {
    Console.writeLine("Hello from hello");
}

Hello from hello

Build and run

Inside a package, the commands need no file names:

ceesafec build            # compiles to C, then to out/hello
./out/hello
ceesafec run --interp     # runs the program in the reference interpreter instead
ceesafec check            # type and borrow checks only: the fastest way to find errors
ceesafec fmt              # formats the package's sources in the canonical style

Every .cee file under src/ belongs to the package, and all of them share one scope: a function declared in one file can be called from another without an import.

A slightly larger program

Replace src/main.cee with:

/// The number of steps the Collatz sequence takes from `n` to 1.
fn steps(n: u64) -> u32 {
    var value = n;
    var count: u32 = 0;
    while (value != 1) {
        value = if (value % 2 == 0) value / 2 else 3 * value + 1;
        count += 1;
    }
    return count;
}

fn main() {
    var longest: u64 = 1;
    for (n in 1..1000u64) {
        if (steps(n) > steps(longest)) {
            longest = n;
        }
    }
    Console.write("longest below 1000: ");
    Console.write(longest);
    Console.write(", ");
    Console.write(steps(longest));
    Console.writeLine(" steps");
}

longest below 1000: 871, 178 steps

3 * value + 1 is checked: if it overflowed a u64, the program would stop with a message naming the line, in every build. CeeSafe never wraps silently.

When the compiler says no

Errors name a code, the place, and usually the fix. This program keeps a reference to a vector's element and then grows the vector, which could move the element:

fn main() {
    var numbers = Vector<i32>.create();
    numbers.push(1)!;
    let first = borrow numbers[0];
    numbers.push(2)!;
    Console.writeLine(first);
}

main.cee:5:5: error[N0241]: cannot borrow 'numbers' as mutable while 'numbers[0]' is borrowed
  |
5 |     numbers.push(2)!;
  |     ^^^^^^^
  = note: 'numbers[0]' is borrowed here (4:5)
  = note: the borrow is used later here (6:5)

The fix is to finish with first before the push, or to copy the value (let first = numbers[0];). Every code is explained in Diagnostics.

Files without a package

For a quick experiment, a package is not needed: give the files to the command.

ceesafec build tool.cee helpers.cee -o tool
ceesafec run --interp tool.cee

Next

• Basics: values, types, functions and control flow.
• Packages: dependencies, the lock file, and sharing code.
• The ceesafec command: every command and option.