Tagged grammars

Tagged grammars¹ are a controllable text-generation tool that has been used in both research and commercial games.  The idea is simple: it's a relatively normal grammar, where some of the rules can be “tagged” with strings.  When you generate an output, you tell it only to produce strings that include a given tag.

Returning to our greeting example:

[randomly][predicate]
Greet ?who: Hello, ?who.
Greet ?who: Hey, ?who.
Greet ?who: Good morning, ?who.

We might tag various rules as “formal”, “informal”, “rude”, etc. (you may disagree with some of these classifications):

[randomly][predicate]
Greet ?who: Hello, ?who. 
Greet ?who: Hey, ?who. [Tag informal]
Greet ?who: Yo! [Tag informal]
Greet ?who: Good morning, ?who. [Tag formal]
Greet ?who: Good day, ?who.  [Tag formal]
Greet ?who: ?who [Tag rude]
Greet ?who: Drop dead, ?who [Tag rude]
...

Here, the Tag task doesn't need to do anything:

Tag ?.

Its only purpose is to be in the execution history so that if we want a rude greeting we can say:

[Greet fred] [PreviousCall [Tag rude]]

This will generate a greeting, but specifically a rude one.  Go ahead and try it:

# Try: [Greet fred] [PreviousCall [Tag rude]]
[randomly] [predicate]
Greet ?who: Hello, ?who. 
Greet ?who: Hey, ?who. [Tag informal]
Greet ?who: Yo! [Tag informal]
Greet ?who: Good morning, ?who. [Tag formal]
Greet ?who: Good day, ?who.  [Tag formal]
Greet ?who: ?who [Tag rude]
Greet ?who: Drop dead, ?who [Tag rude]

Tag ?.

You can add as many tags to a given rule as you like.  And you can use it with complicated grammars with lots of phrase types.

Notes