end_of_file.can be entered to return from the break environment. If the -t toplevel command line option is given, this goal is started instead of entering the default interactive top level (prolog/0).
Notably the gui based versions (swipl-win on Windows and MacOS) provide the menu Run/New thread that opens a new toplevel that runs concurrently with the initial toplevel. The concurrent toplevel can be used to examine the program, in particular global dynamic predicates. It can not access global variables or thread-local dynamic predicates (see thread_local/1) of the main thread.
Aborting is implemented by throwing the reserved exception
'$aborted'. This exception can be caught using catch/3,
but the recovery goal is wrapped with a predicate that prunes the choice
points of the recovery goal (i.e., as once/1)
and re-throws the exception. This is illustrated in the example below,
where we press control-C and `a'.
?- catch((repeat,fail), E, true). ^CAction (h for help) ? abort % Execution Aborted
halt(0). See halt/1 for details.
user_input. See also the Prolog flag history. The prolog/0 predicate is terminated (succeeds) by typing the end-of-file character (typically control-D).
The following two hooks allow for expanding queries and handling the result of a query. These hooks are used by the top level variable expansion mechanism described in section 2.8.
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 top level. This predicate is used by the top level (prolog/0). See also expand_answer/2 and term_expansion/2.
user, normally not defined. Expand the result of a successfully executed top-level query. Bindings is the query <Name>=<Value> binding list from the query. ExpandedBindings must be unified with the bindings the top level should print.