let io = import! std.io
io.println "Hello world!"

There are a number of things going on in the hello world example so lets break them down one step at a time.

let

Gluon uses the keyword let to bind values for later use.

import! std.io

import! is a builtin macro which loads the contents of another module. In the example we use it to get access to the standard library's io module. Since it appeared on the right side of let io = we thus bound the std.io module to the io binding.

io.println

Here we access the println function from the io module we bound earlier which is a function that lets us write strings to stdout.

"Hello world!"

Finally we create a string literal which gets passed to println to get printed.