SWI-Prolog -- require_prolog

Documentation
- Reference manual
  - The SWI-Prolog library
    - library(prolog_versions): Demand specific (Prolog) versions
- Packages

Availability::- use_module(library(prolog_versions)).(can be autoloaded)

[det]require_prolog_version(+Required, +Features:list)

Claim that the running Prolog version is at least version Required and provides the requested Features. Required is an expression of versions. At the lowest level, a version is an atom or string that provides the version as

Major.Minor[[.Patch][[-GitRev],-GitHash]]]

Example strings are’8.5’,’8.5.0’,’8.5.0-50’,’8.5.0-69-gad38e8ad8`. The last two require fetching the sources from git or using the Windows daily builds.

Versions may be embedded in a comparison operator (<, =<, =, >= or >), e.g., =<('9.1'). Versions are considered to compare equal only on the components of the Required version. I.e., '9.1' compares equal to '9.1.2'.

Version expressions can be constructed from the Prolog operators’,’/2,’;’/2 and’\+’/1. An example of a complicated expression is below, which demands major version 9, but considers 9.1.2 not suitable.

(>=('9'), \+(=('9.1.2')))

Features is a list of required or preferred features. Individual features are:

warning(Feature): Only print a warning instead of throwing an error.
library(Lib): Demand library(Lib) to be present. Thde library not being there may indicate an incomplete installation. For example library(pce) to demand xpce graphics support.
Flag: Demand current_prolog_flag(Flag, true) to be true.
FlagValue: If FlagValue is Flag(Value), demand current_prolog_flag(Flag, Value) to be true.

Errors: - version_error('SWI-Prolog', PrologVersion, Cmp, Required)
- existence_error(prolog_feature, Feature)