________________________________________________________________________

This file is part of Logtalk <https://logtalk.org/>  
SPDX-FileCopyrightText: 1998-2026 Paulo Moura <pmoura@logtalk.org>  
SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
________________________________________________________________________


`format`
========

The `format` object provides a portable abstraction over how the de
facto standard `format/2-3` predicates are made available by the
supported backend Prolog systems. Some systems provide these predicates
as built-in predicates, while others make them available using a library
that must be explicitly loaded.

Calls to the library predicates are inlined when compiled with the
`optimize` flag turned on for most of the backends. When that's the
case, there is no overhead compared with calling the abstracted
predicates directly.

This library provides linter checks for calls to the `format/2-3` predicates.
Given the differences between the implementations of these predicates among
Prolog systems, the linter checks focus on detecting common errors such as
missing arguments and too many arguments. The linter warnings are printed
when the `suspicious_calls` flag is set to `warning` (its usual default).


API documentation
-----------------

Open the [../../apis/library_index.html#format](../../apis/library_index.html#format)
link in a web browser.


Loading
-------

To load all entities in this library, load the `loader.lgt` file:

	| ?- logtalk_load(format(loader)).


Testing
-------

Minimal tests for this library predicates can be run by loading the
`tester.lgt` file:

	| ?- logtalk_load(format(tester)).

Detailed tests for the `format/2-3` predicates are available in the
`tests/prolog/predicates` directory as part of the Prolog standards
conformance test suite. Use those tests to confirm the portability
of the format specifiers that you want to use.


Usage
-----

Load this library from your application loader file. To call the `format/2-3`
predicates using implicit message-sending, add the following directive to any
object or category calling the predicates:

	:- uses(format, [
		format/2, format/3
	]).


Portability
-----------

Some Prolog systems provide only a subset of the expected format specifiers.
Notably, table-related format specifiers are only fully supported by a few
systems. See the section below on testing.

Only some of the supported Prolog backends provide implementations of the
`format/2-3` predicates that allow using not only an atom or a list of
character codes for the format string (as de facto standard) but also using
a list of characters. These currently include ECLiPSe, GNU Prolog, SICStus
Prolog, SWI-Prolog, Trealla Prolog, XVM, and YAP. Therefore, when wide
portability is sought, atoms must be used for the format specifier argument
as they bypass any dependency on the `double_quotes` standard Prolog flag.
Some systems, like Tau Prolog, only accept a list of characters for the
format string. In this case, this library will convert the atom format
string before calling these systems native implementations.
