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|The Old SICStus CHR implementation|
There are small differences between the current K.U.Leuven CHR system in SWI-Prolog, older versions of the same system, and SICStus' CHR system.
The current system maps old syntactic elements onto new ones and ignores a number of no longer required elements. However, for each a deprecated warning is issued. You are strongly urged to replace or remove deprecated features.
Besides differences in available options and pragmas, the following differences should be noted:
check_guard_bindingsoption only turns invalid calls to unification into failure. In SICStus this option does more: it intercepts instantiation errors from Prolog built-ins such as is/2 and turns them into failure. In SWI-Prolog, we do not go this far, as we like to separate concerns more. The CHR compiler is aware of the CHR code, the Prolog system, and the programmer should be aware of the appropriate meaning of the Prolog goals used in guards and bodies of CHR rules.