Tech behind-the-scenes question re: Understand "BLAH/BLAH..." as BLAH when BLAH

Authoring Inform 7
1

For people who know more about how Inform runs behind-the-scenes, I have a question.

When Inform is dealing with code that adds a pile of grammar tokens to an object and has ‘when’ qualifications on it -

e.g. Understand "a/b/c/d/e/f/g/h/i/j" as part-of-the-alphabet when alphabet-time is true;

– Does it check the ‘when’ qualification first before bothering to try to match any of the tokens? I’m guessing it does because that seems like a more efficient way to run things.

-Wade

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2

Yes, it does. If the when/while condition is not met, then the Understand line is considered to be inapplicable.

The relevant I6 logic is compiled into the applicable parse_name() routine(s). For example, with code:

A foo is a kind of thing.

Bar is a scene.

Understand "baz" as a foo when bar is happening.

the resulting I6 (in 6M62) is:

[ Parse_Name_GV93 
	...
	            if (Cond_Token_161() == GPR_FAIL) jump Fail_1;
	            if (NextWordStopped() ~= 'baz') jump Fail_1;
	...
];

[ Cond_Token_161 ;
	if (((((scene_status-->(I126_bar-1)==1))))) return GPR_PREPOSITION; ! tests whether scene is active
	return GPR_FAIL;
];

As you can see, the check for the when/while causes the parse_name() routine to decide on failure before the actual input is inspected for a match to the given word.

4 Likes
3

Thanks for the explanation otis.

-Wade