Object capabilities and Main

Finally, we have discussed all of the knowledge needed to make our first Fearless program.

package test
alias base.Main as Main,
alias base.caps.UnrestrictedIO as UnrestrictedIO,

Test:Main {sys -> UnrestrictedIO#sys.println(`Hello, World!`)}
//prints Hello, World!

This program will print Hello, World!.

That is, if we run Fearless test.Test over a folder containing just that file, we will see Hello, World! on the console.

Any type implementing base.Main can be the starting point for the execution. Main is declared in base as follows:

Main:{ .main(sys: mut System): Void }

The parameter sys refers to an object capability: an object able to do external side effects. base.UnrestrictedIO is a function creating a restricted object capability from the system capability. The result of UnrestrictedIO#sys is an object of type mut base.IO: an object that can do input output side effects, like writing on the console or reading/writing files.

That is, Object capabilities and Reference capabilities are two different concepts.

• Reference capabilities are a type system feature, while
• Object capabilities are just a programming style that is embraced by the standard library.