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 system 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 implementation 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 ../../docs/library_index.html#format link in a web browser.


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

| ?- logtalk_load(format(loader)).


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.


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


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, XVM, SICStus Prolog, SWI-Prolog, Trealla Prolog, 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.