Exception handling

Version: 1.5#118 (2000/4/19, 18:13:43 CEST)

This module includes predicates related to exceptions, which alter the normal flow of Prolog.

• Usage and interface (exceptions)
• Documentation on exports (exceptions)

Usage and interface (exceptions)

Documentation on exports (exceptions)

PREDICATE: catch/3:

catch(Goal,Error,Handler)

Executes Goal. If an exception is raised during its execution, Error is unified with the exception, and if the unification succeeds, the entire execution derived from Goal is aborted, and Handler is executed. The execution resumes with the continuation of the catch/3 call. For example, given the code

p(X) :- throw(error), display('---').
p(X) :- display(X).

the execution of "catch(p(0), E, display(E)), display(.), fail." results in the output "error.".

Meta-predicate with arguments: catch(goal,?,goal).

Usage 2: * ISO *

• Calls should, and exit will be compatible with: Error is any term. (basic_props:term/1)
• The following properties should hold at call time: Goal is a term which represents a goal, i.e., an atom or a structure. (basic_props:callable/1) Handler is a term which represents a goal, i.e., an atom or a structure. (basic_props:callable/1)
• The following properties hold upon exit: Error is any term. (basic_props:term/1)

PREDICATE: intercept/3:

intercept(Goal,Error,Handler)

Executes Goal. If an exception is raised during its execution, Error is unified with the exception, and if the unification succeeds, Handler is executed and then the execution resumes after the predicate which produced the exception. Note the difference with builtin catch/3, given the same code defined there, the execution of "intercept(p(0), E, display(E)), display(.), fail." results in the output "error---.0.".

Meta-predicate with arguments: intercept(goal,?,goal).

Usage 2:

• Calls should, and exit will be compatible with: Error is any term. (basic_props:term/1)
• The following properties should hold at call time: Goal is a term which represents a goal, i.e., an atom or a structure. (basic_props:callable/1) Handler is a term which represents a goal, i.e., an atom or a structure. (basic_props:callable/1)
• The following properties hold upon exit: Error is any term. (basic_props:term/1)

PREDICATE: throw/1:

throw(Ball)

Raises an error, throwing the exception Ball, to be caught by an ancestor catch/3 or intercept/3. The closest matching ancestor is chosen. Exceptions are also thrown by other builtins in case of error.

Usage 1: * ISO *

• Calls should, and exit will be compatible with: Ball is currently a term which is not a free variable. (term_typing:nonvar/1)

Usage 2: * ISO *

• Calls should, and exit will be compatible with: Ball is currently a term which is not a free variable. (term_typing:nonvar/1)

PREDICATE: halt/0:

halt

Halt the system, exiting to the invoking shell.

PREDICATE: halt/1:

halt(Code)

Halt the system, exiting to the invoking shell, returning exit code Code.

Usage 2: * ISO *

• The following properties should hold at call time: Code is an integer. (basic_props:int/1)

PREDICATE: abort/0:

abort

Abort the current execution.

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