Add NOP Instruction

[Tristano Ajmone]

REQUEST

Add a new NOP instruction to do nothing.

Possible syntax: NOP, DO NOTHING, NOTHING.

RATIONALE

In various occasions I've come across the need of a NOP instructions, e.g. when I'm forced to rely on the ELSE part of a condition containing an expression that can't be NOT-evaluated, or because I needed a temporary placeholder while drafting some code changes.

As a practical example, class expression can only be positively evaluated, so it's not possible to use If X NOT IsA someclass (e.g. when trying to affect only instances of the ancestor class, when targeting an attribute that is inherited along the line).

The closest thing to a NOP right now is printing an empty string, but this is not possible inside RULEs:

WHEN location OF hero IS lit
  THEN
    IF location OF hero IsA cave
      THEN "" -- do nothing! (doesn't work)
      ELSE
        -- Whatever needs to be done on `lit` locations that
        -- are not cave instances...

The above example (possibly not 100% correct) roughly illustrates where a NOP instruction would be useful.

Hopefully, implementing a NOP shouldn't be too complex, nor have unexpected side effects (since it does nothing), and wouldn't affect backward compatibility.

[Thomas Nilefalk]

For the example you gave the natural solution would be to switch the IF statement around:

...
    IF location Of hero Not IsA cave ...

This would make more sense since the only action you want to do is when that expression is true.

I think that if you can find a situation where a NOP is actually needed I'd prefer to fix that rather than introduce a NOP statement.

EDIT: Just saw that you said that it's not possible to write "Not Isa", it should be according to the grammar. I'll have to investigate this.

[Thomas Nilefalk]

Ok, so thank you for exploring the edges of the Alan language ;-)

It is not possible to use not in a general way to negate the logical expression. It surely should. I'll get right on it.

[Tristano Ajmone]

Thanks for the prompt reply.

Definitely, being allowed to negate any expression would be useful — after all, they should evaluate to boolean, so there's is no reason why the NOT operator shouldn't be usable here.

But I still think that a NOP could always come handy — even if as a temp placeholder, or a quick fix when editing code. Since it does nothing, it shouldn't cause harm either (except for the few bytes overhead it might occupy).