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7.18  Logic, control and exceptions

7.18.1  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 6.1.13), 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.

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

Templates
once(+callable_term)
\+(+callable_term)
call(+callable_term, +term,..., +term)
call_with_args(+atom, +term,..., +term)
call_det(+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(Closure, Arg1,..., ArgN) calls the goal call(Goal) where Goal is constructed by appending Arg1,..., ArgN (1 ≤ N ≤ 10) additional arguments to the arguments (if any) of Closure.

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

call_det(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 7.14.10).

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 7.22.1)    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)
for call/2-11 the resulting arity of Goal (arity of Closure + N) is an integer > max_arity flag (section 7.22.1)    representation_error(max_arity)

Portability

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

7.18.3  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.

7.18.4  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) 1999-2007 Daniel Diaz

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