Logic, control and exceptions

6.18

Logic, control and exceptions

6.18.1 abort/0, stop/0, top_level/0, break/0, halt/1, halt/0

Templates

: abort stop top_level break halt(+integer) halt

Description

abort aborts the current execution. If this execution was initiated under a top-level the control is given back to the top-level and the message {execution aborted} is displayed. Otherwise, e.g. execution started by a initialization/1 directive (section 5.1.13), abort/0 is equivalent to halt(1) (see below).

stop stops the current execution. If this execution was initiated under a top-level the control is given back to the top-level. Otherwise, stop/0 is equivalent to halt(0) (see below).

top_level starts a new recursive top-level (including the banner display). To end this new top-level simply type the end-of-file key sequence (Ctl-D) or its term representation: end_of_file.

break invokes a recursive top-level (no banner is displayed). To end this new level simply type the end-of-file key sequence (Ctl-D) or its term representation: end_of_file.

halt(Status) causes the GNU Prolog process to immediately exit back to the shell with the return code Status.

halt is equivalent to halt(0).

Errors

Status is a variable instantiation_error

Status is neither a variable nor an integer type_error(integer, Status)

Portability

halt/1 and halt/0 are ISO predicates. abort/0, stop/0, top_level/0 and break/0 are GNU Prolog predicates.

6.18.2 once/1, (\+)/1 - not provable, call_with_args/1-11, call/2

Templates

: once(+callable_term)\+(+callable_term) call_with_args(+atom, +term,..., +term) call(+callable_term, ?boolean)

Description

once(Goal) succeeds if call(Goal) succeeds. However once/1 is not re-executable on backtracking since all alternatives of Goal are cut. once(Goal) is equivalent to call(Goal), !.

\+ Goal succeeds if call(Goal) fails and fails otherwise. This built-in predicate gives negation by failure.

call_with_args(Functor, Arg1,..., ArgN) calls the goal whose functor is Functor and whose arguments are Arg1,..., ArgN (0 £ N £ 10).

call(Goal, Deterministic) succeeds if call(Goal) succeeds and unifies Deterministic with true if Goal has not created any choice-points, with false otherwise.

\+ is a predefined prefix operator (section 6.14.10).

Errors

Goal is a variable instantiation_error

Goal is neither a variable nor a callable term type_error(callable, Goal)

The predicate indicator Pred of Goal does not correspond to an existing procedure and the value of the unknown Prolog flag is error (section 6.22.1) existence_error(procedure, Pred)

Functor is a variable instantiation_error

Functor is neither a variable nor an atom type_error(atom, Functor)

Deterministic is neither a variable nor a boolean type_error(boolean, Deterministic)

Portability

once/1 and (\+)/1 are ISO predicates, call_with_args/1-11 and call/2 are GNU Prolog predicates.

6.18.3 repeat/0

Templates

: repeat

Description

repeat generates an infinite sequence of backtracking choices. The purpose is to repeatedly perform some action on elements which are somehow generated, e.g. by reading them from a stream, until some test becomes true. Repeat loops cannot contribute to the logic of the program. They are only meaningful if the action involves side-effects. The only reason for using repeat loops instead of a more natural tail-recursive formulation is efficiency: when the test fails back, the Prolog engine immediately reclaims any working storage consumed since the call to repeat/0.

Errors

None.

Portability

ISO predicate.

6.18.4 for/3

Templates

: for(?integer, +integer, +integer)

Description

for(Counter, Lower, Upper) generates an sequence of backtracking choices instantiating Counter to the values Lower, Lower+1,..., Upper.

Errors

Counter is neither a variable nor an integer type_error(integer, Counter)

Lower is a variable instantiation_error

Lower is neither a variable nor an integer type_error(integer, Lower)

Upper is a variable instantiation_error

Upper is neither a variable nor an integer type_error(integer, Upper)

Portability

GNU Prolog predicate.

Copyright (C) 1999,2000 Daniel Diaz

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