17.33. Printing a parser error

| Chapter 17: Activities |

| 17.33. Printing a parser error

1. When it happens. The parser is the part of the run-time software, included in all works produced by Inform, which tries to match the player's command against the grammar provided by the work. When it is unable to make a valid match, the parser prints an error to the player: for instance,

> > BIFURCATE TREE
> That's not a verb I recognise.

There are some 19 possible messages. The one which the parser wants to say is stored in the variable "latest parser error", which has the convenient kind "command parser error". This has the following possible values:

> didn't understand error
> only understood as far as error
> didn't understand that number error
> can't see any such thing error
> said too little error
> aren't holding that error
> can't use multiple objects error
> can only use multiple objects error
> not sure what it refers to error
> excepted something not included error
> can only do that to something animate error
> not a verb I recognise error
> not something you need to refer to error
> can't see it at the moment error
> didn't understand the way that finished error
> not enough of those available error
> nothing to do error
> I beg your pardon error
> noun did not make sense in that context error

2. The default behaviour. Prints the message in question.

3. Examples. (a) Perhaps for newcomers:

> After printing a parser error:
> say "If you are new to interactive fiction, you may like to try typing HELP."

(b) Or to give the parser a certain amount of character:

> Rule for printing a parser error when the latest parser error is the I beg your pardon error:
> say "What's that? Speak up, speak up." instead.

> Rule for printing a parser error:
> say "That's a rum thing to say, and no mistake." instead.

366
| Example WXPQ Creating a more sensible parser error than "that noun did not make sense in this context". |
RB

The parser error "That noun did not make sense in this context" arises instead of "You can't see any such thing" when the player uses a command that could apply to any item in the game -- that is, a command such as Understand "go to [any room]" as going directly to. Understand "talk about [any subject]" as discussing. ...and so on. The idea here is that "You can't see any such thing" isn't a sensible rejoinder when the player doesn't really need to be able to see the object. Nonetheless, "That noun did not make sense..." is itself a fairly dry and uninformative response, and we may want to override it to something more appropriate for the specific kind of context in which it might appear. For instance: "WXPQ" WXPQ Studio is a room. "After about 2 AM, no one is listening anyway, so you can more or less make up whatever you like to fill the airwaves." John F Kennedy, Elvis, Ralph Nader, Tony Blair, and single-origin chocolate are things. Understand "talk about [any thing]" or "discuss [any thing]" as discussing. Discussing is an action applying to one visible thing. Carry out discussing: say "You babble for a while about your [one of]interest in[or]hatred of[or]passionate devotion to[or]conspiracy theory concerning[or]mother's secret love affair with[as decreasingly likely outcomes] [the noun]." Rule for printing a parser error when the latest parser error is the noun did not make sense in that context error: say "For once, you're at a loss for anything to say." Test me with "discuss Elvis / discuss Kennedy / discuss chocolate / discuss narratology vs ludology debate". Note that this solution works as simply as it does because we only have one command in the game that can apply to an "[any]" token. If we had several, we'd need to distinguish between the parser error attached to "discuss" and the parser error attached to "go to" (for instance). In that case, we might instead write something like Rule for printing a parser error when the latest parser error is the noun did not make sense in that context error: if the player's command includes "go": say "There's no such place you know how to get to."; otherwise: say "For once, you're at a loss for anything to say."

367
| Example Xot Storing an invalid command to be repeated as text later in the game. |
RB

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