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)