Single-line comments

The sharp (#) character sequence marks the following text as a single-line comment. Single-line comments end at the end-of-line.
## The hello-world program...
say "Hello, world!"; # with single-line comments.

Multiple-line comments

Comments can span multiple lines by using the multiple-line comment style. Such comments start with /* and end with */. The text between those multi-line comment markers is the comment.
This is another style of a comment.
It allows multiple lines.

Embedded comments

This kind of comments are a little bit weird at the first glance, but they are useful sometimes.
var speed = (distance #`(in meters) / time #`(in seconds));
is equivalent with:
var speed = (distance / time);
A common use of embedded comments is to specify a shell evaluation code to execute the actual Sidef script when it's executed by a shell program. However, the eval statement is completely ignored by Sidef since the block in which it is defined, it's never executed.
#`(if running under some shell) {
eval 'exec /usr/bin/sidef $0 ${1+"[email protected]"}'