ON GOSUB may be slightly off.

[flagxor]

ON GOSUB unconditionally pushes a return address onto the stack, even when out of bounds.
Unsure if this matches canonical behavior.
We should confirm and add a test.

[pauwell]

I found the following article really interesting:
https://www.atarimagazines.com/compute/issue87/The_Power_Of_ON-GOTO_And_ON-GOSUB.php

It says the following about on..gosubs behaviour in this case:

10 PRINT "ENTER NUMBER : "; : INPUT D
20 PRINT "D = "; D; " : ";
30 ON D GOSUB 40, 50, 60, 70 : GOSUB 80 : GOTO 10
40 PRINT "CAME TO LINE 40 : "; : RETURN
50 PRINT "CAME TO LINE 50 : " : RETURN
60 PRINT "CAME TO LINE 60 : "; : RETURN
70 PRINT "CAME TO LINE 70 : "; : RETURN
80 PRINT "CAME TO LINE 80 : "; : RETURN

When you run this program, it asks you to enter a number. If you enter 1, the program displays the message Came to line 40. After it returns from the subroutine at line 40, notice that the program does not proceed to the routines in lines 40–70. Instead, it proceeds to the next BASIC statement in the line, GOSUB 80. This statement is always performed, allowing you to note the difference between ON-GOSUB and a normal GOSUB command.

Now try entering a number smaller than 1 or greater than 4. Notice how ON-GOSUB responds. In this case, the program skips all of the possible destination lines listed in the ON-GOSUB statement. ON-GOSUB does not cause any branches, and the program proceeds to the next statement (GOSUB 80). For example, if you enter 0 at the prompt, the program prints Came to line 80.

Is this how it should handled here? I read that in some versions of BASIC it results in an error but in most versions it just gets skipped if it is out of bounds.

[flagxor]

My offhand reaction is that until we have a more unified place to put compat flags / modes of operation that we'd want to support the most permissive thing (fallthru) as working programs are unlikely to meaningfully rely on erroring in this case.

But this might be a good place for a TODO: Make this behavior configurable once we have BASIC version flags.

In terms of the return behavior, also note that:

10 PRINT "ENTER NUMBER : "; : INPUT D
20 PRINT "D = "; D; " : ";
30 ON D GOSUB 40, 50, 60, 70 : GOSUB 80 : GOTO 10
40 PRINT "CAME TO LINE 40 : "; : RETURN
50 PRINT "CAME TO LINE 50 : " : RETURN
60 PRINT "CAME TO LINE 60 : "; : RETURN
70 PRINT "CAME TO LINE 70 : "; : RETURN
80 PRINT "CAME TO LINE 80 : "; : RETURN

Emits:

hi
hi
there

Where one might have expected:

hi
there

[flagxor]

Resolved in:
#39

Thanks pauwell!