Query Classes

Queries posed by the system can be classified according to the kind of input they expect, the way the input is processed, etc. Queries of the same kind form a query class.

For example, queries requiring a yes/no answer form a query class with the following characteristics:

There are built-in query classes for reading in yes/no answers, toplevel queries, debugger commands, etc.

A query class is characterized by a ground Prolog term, which is supplied as the first argument to the query processing predicate ask_query/4. The characteristics of a query class are normally described by the extendible predicate

'SU_messages':query_class(QueryClass, Prompt, InputMethod,
                          MapMethod, FailureMode).

The arguments of the query_class predicate have the following meaning:


an atom to be used for prompting the user.


a non-variable term, which specifies how to obtain input from the user.

For example, a built-in input method is described by the atom line. This requests that a line is input from the user, and the code-list is returned. Another built-in input method is term(Options); here, a Prolog term is read and returned.

The input obtained using InputMethod is called raw input, as it may undergo further processing.

In addition to the built-in input methods, the user can define his/her own extensions.


a non-variable term, which specifies how to process the raw input to get the abstract answer to the query.

For example, the built-in map method char([yes-"yY", no-"nN"]) expects a code-list as raw input, and gives the answer term yes or no depending on the first non-whitespace character of the input. For another example, the built-in map method = requests that the raw input itself be returned as the answer term—this is often used in conjunction with the input method term(Options).

In addition to the built-in map methods the user can define his/her own extensions.


This is used only when the mapping of raw input fails, and the query must be repeated. This happens for example if the user typed a character other than y or n in case of the yes_or_no query class. FailureMode determines what to print before re-querying the user. Possible values are:


print a help message, then print the text of the query again


only print the help message


only print the text of the query


don’t print anything

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