Hello, world¶

This page walks through the smallest candela program there is. If you have not installed the toolchain yet, start with Install.

Write it¶

Put this in a file called hello.cdl:

fn main() {
    print("Hello, world!");
}

Run it¶

candela hello.cdl

Hello, world!

What happened¶

Four things are worth pulling out of those three lines.

Execution starts at main. Every program declares fn main() in the file you name on the command line, and that function is what runs. A file without a main is a compile error.

The top level holds declarations, not statements. let x = 1; outside a function does not compile. Only functions, imports, structs, enums, impl blocks and library declarations live at the top level; the work goes inside a function body.

print is built in. A handful of functions are available with no import at all, print among them. It writes its argument followed by a newline. The built-in functions page lists the rest.

Nothing runs until it type-checks. candela reads the whole program, parses it and checks the types before the first line executes. A misspelt name or a number where a string belongs is reported up front rather than partway through a run.

Statements end with a semicolon, and blocks are delimited with braces.

A little further¶

Programs get more interesting once values have names:

fn main() {
    let name = "world";
    print("Hello, " + name + "!");
}

let introduces a variable. You do not write a type: the compiler works out that name holds a string, and + on two strings joins them. Try + between a string and a number and the program does not compile.

Next¶

Running programs covers the REPL, compiled artifacts, and the rest of the command line. To carry on with the language itself, start at Variables.