I6 Compiler: The compiler does not detect the absence of a semicolon at the end of the instruction

Authoring Inform 6
1

There is no semicolon at the end of the @set_cursor instruction, but the compiler compiles without complaint. And the program does not run properly.

[ Main   key;
   @set_cursor 3 3
   print "Hello, World!";
   @read_char 1 ->key;
];
2

The compiler is not very strict about Z-machine assembly syntax. For EXT/VAR opcodes, it just accepts “0 to 4” arguments and leaves it up to the author to make sure the result is valid Z-code.

By leaving off the semicolon, you’re writing a four-argument opcode:

@set_cursor 3 3 print "Hello, World!";

I’m not sure how the keyword print is getting transformed into an argument value. The compiler should catch this as an error, so that’s a bug. But I don’t see it as worth getting into at this point.

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3

Now I know that, I’ll be more careful; it’s a difficult error to detect. I had put a comment at the end of the instruction, so I didn’t see that the semicolon was missing.

4

I looked a little farther and it turns out it’s not even invalid code!

I6 defines a “print” property, although the manual admits it’s pointless. See §3.12.

…provided because it logically ought to be, rather than because it’s useful.

You can write, for example:

val = "This is a string.";
val.print();

So print is a valid constant – it’s a property ID. In Z-code it happens to always be 70. Since I6 is not strongly typed, you can use it in any expression, not just val.print. Opcode arguments can be any constant or variable. So there you are.

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5

Ah, I6, one of the few C-style languages that has no qualms about having a built-in property with the same name as a built-in keyword and no type checking to tell the difference…

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