/* Part of SWI-Prolog Author: Jan Wielemaker E-mail: jan@swi-prolog.org WWW: http://www.swi-prolog.org Copyright (c) 2024, SWI-Prolog Solutions b.v. All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */ :- module(exceptions, [ catch/4, % :Goal, :ErrorType, ?Ball, :Recover exception/2, % :ExceptionType, ?Ball exception_type/2 % +Type, +Term ]). :- use_module(library(error)). :- set_prolog_flag(generate_debug_info, false). :- meta_predicate catch(0, :, ?, 0), exception(:,?). :- multifile error_term/2, % Type, Formal exception_term/2. % Type, Exception /** Exception classification Prolog catch/3 selects errors based on unification. This is problematic for two reasons. First, one typically wants the exception term to be more specific than the term passed to the 2nd (`Ball`) argument of catch/3. Second, in many situations one wishes to select multiple errors that may be raised by some operations, but let the others pass. Unification is often not suitable for this. For example, open/3 can raise an _existence_error_ or a _permission_error_ (and a couple more), but _existence_error_ are also raised on, for example, undefined procedures. This is very hard to specify, Below is an attempt that still assumes nothing throws error(_,_). ``` catch(open(...), error(Formal,ImplDefined), ( ( Formal = existence_error(source_sink,_) ; Formal = permission_error(open, source_sink, _) ) -> ; throw(Formal, ImplDefined) )), ... ``` Besides being hard to specify, actual Prolog systems define a large number of additional error terms because there is no reasonable ISO exception defined. For example, SWI-Prolog open/3 may raise resource_error(max_files) if the maximum number of file handles of the OS is exceeded. As a result, we see a lot of Prolog code in the wild that simply uses the construct below to simply fail. But, this may fail for lack of stack space, a programmer error that causes a type error, etc. This both makes it much harder to debug the code and provide meaningful feedback to the user of the application. ``` catch(Goal, _, fail) ``` Many programing languages have their exceptions organised by a (class) hierarchy. Prolog has no hierarchy of terms. We introduce exception/2 as exception(+Type, ?Term), which can both be used as a type test for an exception term and as a _constraint_ for the `Ball` of catch/3. Using a predicate we can express abstractions over concrete exception terms with more flexibility than a hierarchy. Using a _multifile_ predicate, libraries can add their exceptions to defined types or introduce new types. The predicate catch/4 completes the interface. */ %! catch(:Goal, +ExceptionType, ?Ball, :Recover) % % As catch/3, only catching exceptions for which % exception(ErrorType,Ball) is true. See error/2. For example, the % code below properly informs the user some file could not be % processed due do some issue with `File`, while propagating on all % other reasons while process/1 could not be executed. % % ``` % catch(process(File), file_error, Ball, % file_not_processed(File, Ball)) % % file_not_processed(File, Ball) :- % message_to_string(Ball, Msg), % format(user_error, 'Could not process ~p: ~s', [File, Msg]). % ``` :- noprofile(catch/4). catch(Goal, ErrorType, Ball, Recover) :- exception(ErrorType, Ball), catch(Goal, Ball, Recover), del_attr(Ball, freeze). %! exception(:Type, --Ball) is det. %! exception(:Type, +Ball) is semidet. % % If Ball is unbound, adds a delayed goal that tests the error belongs % to Type when Ball is instantiated (by catch/3). Else succeed is % error is of the specified Type. % % Note that the delayed goal is added using freeze/2 and therefore the % stepwise instantiation of Ball does not work, e.g. % exception(file_error, error(Formal,_)) immediately fails. % % Error types may be defined or extended (e.g., by libraries) by % adding clauses to the multifile predicates error_term/2 and % exception_term/2. _Modules_ may (re-)define local error types using % the exception_type/2 directive. exception(Type, Ball) :- freeze(Ball, is_exception(Type, Ball)). is_exception(M:Type, Ball) :- is_exception(Type, M, Ball). is_exception((A;B), M, Ball) => ( is_exception(A, M, Ball) -> true ; is_exception(B, M, Ball) ). is_exception(\+A, M, Ball) => \+ is_exception(A, M, Ball). is_exception(Type, M, Ball) => ( ex_term(Type, M, Pattern) *-> subsumes_term(Pattern, Ball), ! ; existence_error(exception_type, Type) ). %! ex_term(+Type, +Module, -Term) is nondet. ex_term(Type, Module, error(Term,_)) :- ( current_predicate(Module:'$error_term'/2), Module:'$error_term'(Type, Term) *-> true ; error_term(Type, Term) ). ex_term(Type, Module, Term) :- ( current_predicate(Module:'$exception_term'/2), Module:'$exception_term'(Type, Term) *-> true ; exception_term(Type, Term) ). %! error_term(?Type, ?Term) is nondet. % % Describe the formal part of error(Formal,ImplDefined) exceptions. error_term(file_error, existence_error(source_sink, _Culprit)). error_term(file_error, permission_error(open, source_sink, _Culprit)). error_term(file_error, resource_error(max_files)). error_term(file_error, representation_error(max_symbolic_links)). error_term(file_error, representation_error(max_path_length)). error_term(network_error, socket_error(_Code, _Message)). error_term(network_error, timeout_error(_Operation, _Culprit)). error_term(network_error, io_error(_Operation, _Culprit)). error_term(timeout, timeout_error(_Operation, _Culprit)). error_term(evaluation_error, evaluation_error(_)). %! exception_term(?Type, ?Term) is nondet. % % Describe exceptions that are not error(Formal, _) terms. exception_term(timeout, time_limit_exceeded). exception_term(timeout, time_limit_exceeded(_TimeLimit)). %! exception_type(+Type, +Term) % % Declare all exceptions subsumed by Term to be an exception of Type. % This declaration is module specific. exception_type(Type, Term) :- throw(error(context_error(nodirective, exception_type(Type, Term)), _)). exception_type_clause(Type, error(Formal, Var), Clause), ground(Type), var(Var) => Clause = '$error_term'(Type, Formal). exception_type_clause(Type, Exception, Clause), ground(Type) => Clause = '$exception_term'(Type, Exception). add_decl(Clause, Clauses) :- prolog_load_context(module, Module), pi_head(PI, Clause), ( current_predicate(Module:PI) -> Clauses = Clause ; Module == user -> Clauses = [(:- multifile(PI)), Clause] ; Clauses = [(:- discontiguous(PI)), Clause] ). system:term_expansion((:-exception_type(Type, Term)), Clauses) :- exception_type_clause(Type, Term, Clause), add_decl(Clause, Clauses).