### 8.18 Logic, control and exceptions

- `abort/0`, `stop/0`, `top_level/0`, `break/0`, `halt/1`, `halt/0`
- `false/0`, `once/1`, `(``\``+)/1` - not provable, `call/2-11`, `call_with_args/1-11`, `call_det/2`, `forall/2`
- `repeat/0`
- `for/3`

#### 8.18.1 `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 7.1.14), `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.

#### 8.18.2 `false/0`, `once/1`, `(\+)/1` - not provable, `call/2-11`, `call_with_args/1-11`, `call_det/2`, `forall/2`

Templates

  `false
  once(+callable_term)
  \+(+callable_term)
  call(+callable_term, +term,…, +term)
  call_with_args(+atom, +term,…, +term)
  call_det(+callable_term, ?boolean)
  forall(+callable_term, +callable_term)`

Description

`false` always fails and enforces backtracking. It is equivalent to the `fail/0` control construct (section 7.2.1).

`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.

`forall(Condition, Action)` succeeds if for all alternative bindings of `Condition`, `Action` can be proven. It is equivalent to `\+ (Condition, \+ Action)`.

`\+` is a predefined prefix operator (section 8.14.10).

Errors

`Goal` (or `Condition` or `Action`) is a variable |  | `instantiation_error`
`Goal` (or `Condition` or `Action`) 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 8.22.1) |  | `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 8.22.1) |  | `representation_error(max_arity)`

Portability

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

#### 8.18.3 `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.

#### 8.18.4 `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.

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