3.40 User Toplevel Manipulation

break

Recursively start a new Prolog top level. This Prolog top level has its own stacks, but shares the heap with all break environments and the top level. Debugging is switched off on entering a break and restored on leaving one. The break environment is terminated by typing the system's end-of-file character (control-D). If the -t toplevel command line option is given this goal is started instead of entering the default interactive top level (prolog/0).

abort

Abort the Prolog execution and restart the top level. If the -t toplevel command line options is given this goal is started instead of entering the default interactive top level.

There are two implementations of abort/0. The default one uses the exception mechanism (see throw/1), throwing the exception $aborted. The other one uses the C-construct longjmp() to discard the entire environment and rebuild a new one. Using exceptions allows for proper recovery of predicates exploiting exceptions. Rebuilding the environment is safer if the Prolog stacks are corrupt. Therefore the system will use the rebuild-strategy if the abort was generated by an internal consistency check and the exception mechanism otherwise. Prolog can be forced to use the rebuild-strategy setting the prolog flag abort_with_exception to false.

halt

Terminate Prolog execution. Open files are closed and if the command line option -tty is not active the terminal status (see Unix stty(1)) is restored. Hooks may be registered both in Prolog and in foreign code. Prolog hooks are registered using at_halt/1. halt/0 is equivalent to halt(0).

halt(+Status)

Terminate Prolog execution with given status. Status is an integer. See also halt/0.

prolog

This goal starts the default interactive top level. Queries are read from the stream user_input. See also the history prolog_flag (current_prolog_flag/2). The prolog/0 predicate is terminated (succeeds) by typing the end-of-file character (On most systems control-D).

The following two hooks allow for expanding queries and handling the result of a query. These hooks are used by the toplevel variable expansion mechanism described in section 2.8.

expand_query(+Query, -Expanded, +Bindings, -ExpandedBindings)

Hook in module user, normally not defined. Query and Bindings represents the query read from the user and the names of the free variables as obtained using read_term/3. If this predicate succeeds, it should bind Expanded and ExpandedBindings to the query and bindings to be executed by the toplevel. This predicate is used by the toplevel (prolog/0). See also expand_answer/2 and term_expansion/2.

expand_answer(+Bindings, -ExpandedBindings)

Hook in module user, normally not defined. Expand the result of a successfully executed toplevel query. Bindings is the query <Name>=<Value> binding list from the query. ExpandedBindings must be unified with the bindings the toplevel should print.