Context managers for SWI Prolog

Synopsis

?- use_module(library(with)).

% IO streams automatically closed
?- with(open('/tmp/foo.txt', write, Stream),
        format(Stream, 'My context was managed!', [])).


% Asserted clauses automatically retracted.
?- dynamic(foo/1),
   with(assertz(foo(1) :- true),
        (foo(X),
         writeln(X))),
   foo(Y),
   writeln(Y).
1
false.

% Changed setting automatically reverted
?- [user].
:- setting(mymod:foo, boolean, true, '').
|: true.
with(setting(mymod:foo, false), setting(mymod:foo, V)), setting(mymod:foo, V1).
V = false,
V1 = true.



% Automatically revert changes to environment
?- with(setenv('FOO', 'BAR'),
       (getenv('FOO', V),
        writeln(V))),
   getenv('FOO', V),
   writeln(V).
BAR
false.

% And you can define your own!
?- [user].
with:manage_context(my_ctx(X), assert(foo(X)), retract(foo(X))).
|: true.

?- with(my_ctx(1), listing(foo)), listing(foo).
:- dynamic foo/1.

foo(1).

:- dynamic foo/1.

true

Description

This module provides context management for various types of Prolog objects. Builtin support includes IO Streams, environment variables, dynamic clauses, prolog flags, library(setting), and library(debug). User defined contexts can be implemented using the multifile predicate manage_context/3. manage_context(Term, Setup, Cleanup) defines a setup/cleanup pair for a specific type of term. `with(Term, Goal)` calls Goal using setup_call_cleanup/3, with the corresponding setup and cleanup goals. If List is a list of terms then, `with(List, Goal)` nests the corresponding context managers.

There is one difference between the semantics of setup_call_cleanup/3 and the corresponding goal using this library. Setup and Cleanup goals must succeed. If they fail, the error `error(mode_error(must_succeed, FailingGoal))` is thrown.

For example, the provided manage_context/3 clause for opening files could be defined:

with:manage_context(open(File, Mode, Stream),
                    open(File, Mode, Stream),
                    close(Stream)).

The result is that the following are equivalent:

?- setup_call_cleanup(open(File, read, Stream),
   is_stream(Stream),
   close(Stream)).

?- use_module(library(with)),
   with(open(File, read, Stream),
        is_stream(Stream)).

To show defined context managers, using `listing/1`. E.g, the context managers packaged with this module are:

?- use_module(library(with)), listing(with:manage_context/3).

manage_context(open(A, C, D),  (absolute_file_name(A, B), open(B, C, D)), close(D)).
manage_context(assertz(A), assertz(A, B), erase(B)).
manage_context(setting(A, B),  (setting(A, C), set_setting(A, B)), set_setting(A, C)).

Installation

Using SWI-Prolog 6.3 or later:

?- pack_install(with).

Author: Eyal Dechter