gosub and return

If you ever coded in old basic languages back in the 80's, you might know the gosub command already. If not, here's what this does:

In contrast to a goto, which simply jumps to a label and continues from there, a gosub (read: "goto sub routine") also jumps, but it remembers, where it came from and when a return statement is encountered, script execution jumps back to the line after the original gosub.

This is a simple mechanism to create kind of sub routines, which do things, and then return and continue in the main path of the script.

Syntax

Similar to the goto command:

gosub jumplabel
continues_here_after_return

jumplabel:
//... some code
return

...but with a twist: return may return a value, just like a function does!

The $res variable

Due to the single line architecture of scriptor, you can not do an assignment statement with a gosub. Instead, the returned value of a gosub can be retrieved through the scriptor variable $res, which is created when you run a gosub command:

gosub subroutine
result = $res
// "result" contains 6 now

subroutine:
return 4 + 2

Here is an example, how it looks like in a real script, taken from one of the unit tests of scriptor:

test = 0
returned = 0

// now enter the sub routine, which sets "test" to 1
gosub add_it
// script continues here after the "return" in add_it
returned = 1
// skip/jump over the sub routine to the end of the script
goto end

add_it:
	test++
	return

never_reached = true

end:
	returned++

The line never_reached will never be reached, because return leaves the path and jumps back, and the main path jumps over it to the end: label.

At the end of the script, the variables have these values:

• returned = 2 (Can you figure out/understand, why it is 2?)
• test = 1 (The subroutine was entered only once)
• never_reached does not exist, as this line is never hit

Another example can be seen at the page for $ Variables, at $res.