Dear all,
According to O'Keefe [1,2], char_coversion/2 was a mistake.
Ciao Prolog does not implement char_conversion/2 at all. Support in other traditional systems is mixed (implemented for a small range 0-255, limited, or just drafted). More recent systems like Scryer or Trealla (if I am not wrong) seem to expose no implementation at all. There seems to be little incentive to implement a feature narrowly conceived, scarcely used, and dangerous in effect.
I would like to propose flagging char_conversion/2 as obsolescent (still conforming, but marked as removal in any future revision, with existing implementations explicitly free to retain it as an extension). The C language had a similar issue with trigraphs, which have been removed (C23), and Fortran maintains a formal obsolescent-features list; so there is ISO precedent for retiring features.
Cheers,
[1] https://www.cs.otago.ac.nz/staffpriv/ok/pllib.htm [2] https://www.complang.tuwien.ac.at/ulrich/iso-prolog/pllib-2015-02-11.htm