This tutorial will show you how to create useful semantic actions for the meta_metadata object urban_spoon_restaurant.

All semantic actions must be placed within the meta_metadata tag of the object they are intended for.

In general there are three types of semantic actions: variable declarations, control flow statements, and bridge function statements.

Variable Declarations:

• get_field - Puts a meta_metadata_field in context, needed before that field can be passed as an argument.
• check - Decide the value of a flag.

Control Flow Statements:

• for_each - Iterates through a collection meta_metadata_field/
• if - Checks the conditions of a flag_check.
• flag_check - Used in conjunction with an if statement.

Bridge Function Statements:

• create_and_visualize_img_surrogate - Used to signal to visualization application to display an image, the image's URL is included as an argument.
• create_and_visualize_text_surrogate - Same as the previous function except
  the argument passed is text.
• parse_document - Signals the information crawler to download and parse a
  resource, either immediately or later, taking a URL as an argument.

The actions we want to take for an Urban Spoon restaurant information page might include:

• Displaying a picture of the restaurant
• If that picture is not available display a picture of the restaurant on the map
• Queue for download the pages containing lists of similar restaurants

The semantic_actions for displaying the image of the restaurant (as stored in the meta_metadata_field pic) are:
Not all restaurant pages have a picture. In the case that it doesn't have a picture this action will do nothing. We can add a check to see if pic is null and if so then we can display the image map. Now we will add the semantic actions for adding each food genre page to the download queue. To do this we will have to use a for_each statement as well as parse_document.

This is the corresponding action set that will be added to the end of our existing semantic actions:

Tip: The default behavior of parse_document is to queue that URL for downloading and parsing in a waiting list. However, it has an attribute now that you can set to true if you want meta-metadata to immediately download and parse that document.

The next part of this tutorial talks about some advanced topics about the meta-language.